Re: Evaluation of variable as f-string
On Wed, 8 Feb 2023 at 05:30, Stefan Ram wrote:
>
> Rob Cliffe writes:
> >Does that mean that it is not possible to have a (built-in) function
> >that would construct and return a dictionary of all available variables
> >and their values? If it were possible, it could be useful, and there
> >would be no impact on Python run-time speed if it were only constructed
> >on demand.
>
> Here's a quick attempt to get local and non-local names as a set.
> It might only work under some implementations of Python.
>
> main.py
>
> set_class = set
>
> def f():
> x = 0
> def g():
> set = set_class()
> for n in g.__qualname__.split( '.' ):
> if n != '':
> set = set.union( set_class( eval( n ).__code__.co_varnames ))
> print( set )
> g()
> f()
>
> output
>
> {'n', 'set', 'x'}
>
Breaks on wrapped functions. Also, how are you going to get the values
of those variables?
ChrisA
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Re: Evaluation of variable as f-string
On Wed, 8 Feb 2023 at 02:12, Rob Cliffe wrote: > > [re-sending this to both the list and to Chris, as a prior send to the > list only was bounced back] > On 31/01/2023 22:33, Chris Angelico wrote: > > > >> Thanks for clarifying. > >> Hm. So 'x' is neither in locals() nor in globals(). Which starts me > >> wondering (to go off on a tangent): Should there be a nonlocals() > >> dictionary? > > I don't think so, but there might be some value in a dictionary > > containing all available variables. It would have the same "don't > > depend on writing" caveats that locals() has (or would be specifically > > defined as a copy and thus disconnected), so its value would be > > limited. And it would probably STILL be imperfect, because perfection > > would require that it be a compiler construct, due to the way that > > nonlocals are implemented. > Does that mean that it is not possible to have a (built-in) function > that would construct and return a dictionary of all available variables > and their values? If it were possible, it could be useful, and there > would be no impact on Python run-time speed if it were only constructed > on demand. > It can't be a built-in function and also be 100% reliable; and if it's a special compiler construct, its presence in your code would have semantic impact on all nested functions - even if you never call it: def func(): x = Obj() def inner(): if False: get_vars return inner x can no longer be disposed of, just in case you call inner and get the vars. And quite frankly, I don't think this functionality justifies a magic compiler construct and consequent keyword. But a builtin won't be reliable. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
[re-sending this to both the list and to Chris, as a prior send to the list only was bounced back] On 31/01/2023 22:33, Chris Angelico wrote: Thanks for clarifying. Hm. So 'x' is neither in locals() nor in globals(). Which starts me wondering (to go off on a tangent): Should there be a nonlocals() dictionary? I don't think so, but there might be some value in a dictionary containing all available variables. It would have the same "don't depend on writing" caveats that locals() has (or would be specifically defined as a copy and thus disconnected), so its value would be limited. And it would probably STILL be imperfect, because perfection would require that it be a compiler construct, due to the way that nonlocals are implemented. Does that mean that it is not possible to have a (built-in) function that would construct and return a dictionary of all available variables and their values? If it were possible, it could be useful, and there would be no impact on Python run-time speed if it were only constructed on demand. Best wishes Rob -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On Wed, 1 Feb 2023 at 09:14, Rob Cliffe via Python-list
wrote:
> With great respect, Chris, isn't it for the OP (or anyone else) to
> decide - having been warned of the various drawbacks and limitations -
> to decide if it's a terrible idea *for him*? He's entitled to decide
> that it's just what *he* needs, and that the drawbacks don't matter *for
> him". Just as you're entitled to disagree.
It's an objectively bad idea. If the OP wants to do it, well, it's a
free world, but that doesn't mean I'm going to sugarcoat it and say
"oh yes, yes, you are totally right to do that".
> Thanks for clarifying.
> Hm. So 'x' is neither in locals() nor in globals(). Which starts me
> wondering (to go off on a tangent): Should there be a nonlocals()
> dictionary?
I don't think so, but there might be some value in a dictionary
containing all available variables. It would have the same "don't
depend on writing" caveats that locals() has (or would be specifically
defined as a copy and thus disconnected), so its value would be
limited. And it would probably STILL be imperfect, because perfection
would require that it be a compiler construct, due to the way that
nonlocals are implemented.
>>> class Destructible:
... def __init__(self, name): self.name = name
... def __del__(self): print("Deleting", self.name)
...
>>> def func():
... x = Destructible("x")
... y = Destructible("y")
... return lambda: x
...
>>> func()
Deleting y
. at 0x7ff8c9897ce0>
The compiler is free to dispose of y as soon as func ends, but x has
to be retained for the inner function. So if there were any function
that could return every readable variable, it would have to force both
x and y to be retained; as such, it would have to be a compiler
construct. And given what happened with star imports in functions as
of Python 3, I am highly dubious that such a pessimisation would ever
be implemented.
> > Maybe you don't care. Maybe you do. But locals() is not the same as
> > "all names currently available in this scope". And, this example is
> > definitely not something I would recommend, but good luck making this
> > work with eval:
> >
> def func():
> > ... x = 1
> > ... print(f"{(x:=2)}")
> > ... print(x)
> > ...
> func()
> > 2
> > 2
> > ... x = 1
> > ... print(eval("(x:=2)", globals(), locals()))
> > ... print(x)
> > ...
> func()
> > 2
> > 1
> Now that, I have to admit, IS a challenge!
Exactly. This sort of thing is why the OP's idea as written is so bad:
it will cause many unnecessary corner cases, where the much simpler
idea of working it around format_map will actually behave sanely.
So I do not apologize for calling it a bad idea. It is a bad idea.
Lying about it won't change anything and won't help anyone.
ChrisA
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Re: Evaluation of variable as f-string
On 27/01/2023 23:41, Chris Angelico wrote:
On Sat, 28 Jan 2023 at 10:08, Rob Cliffe via Python-list
wrote:
Whoa! Whoa! Whoa!
I appreciate the points you are making, Chris, but I am a bit taken
aback by such forceful language.
The exact same points have already been made, but not listened to.
Sometimes, forceful language is required in order to get people to
listen.
If it's addressed to me: How about if I wanted a program (a learning
tool) to allow the user to play with f-strings?
I.e. to type in a string, and then see what the result would be if it
had been an f-string?
I suspect there are other use cases, but I confess I can't think of one
right now.
Use the REPL, which will happily evaluate f-strings in their original
context, just like any other code would. You're already eval'ing, so
it's exactly what you'd expect. This is not the same thing as "typing
in a string", though - it's typing in code and seeing what the result
would be. (Except to the extent that source code can be considered a
string.)
This is hypothetical, but I might want to work on a platform where the
REPL was not available.
If it's addressed to me: "it" means a function that will take a string
and evaluate it at runtime as if it were an f-string. Sure, with
caveats and limitations.
And that's what I am saying is a terrible terrible idea. It will
evaluate things in the wrong context, it has all the normal problems
of eval, and then it introduces its own unique problems with quote
characters.
With great respect, Chris, isn't it for the OP (or anyone else) to
decide - having been warned of the various drawbacks and limitations -
to decide if it's a terrible idea *for him*? He's entitled to decide
that it's just what *he* needs, and that the drawbacks don't matter *for
him". Just as you're entitled to disagree.
And indeed Thomas Passim found this partial
solution on Stack Overflow:
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
You can find anything on Stack Overflow. Just because you found it
there doesn't mean it's any good - even if it's been massively
upvoted.
Addressing your points specifically:
1) I believe the quote character limitation could be overcome. It
would need a fair amount of work, for which I haven't (yet) the time or
inclination.
No problem. Here, solve it for this string:
eval_me = ' f"""{f\'\'\'{f"{f\'{1+2}\'}"}\'\'\'}""" '
F-strings can be nested, remember.
I remember it well.
As far as I can see (and I may well be wrong; thinking about this
example made my head hurt ) this could be solved if PEP 701 were
implemented (so that f-string expressions can contain backslashes) but
not otherwise.
2) Yes in general you would have to pass it one dictionary, maybe
two. I don't see this as an insuperable obstacle. I am not sure what
you mean by "can't be done with full generality" and perhaps that's not
important.
def func():
... x = 1
... class cls:
... y = 2
... print(f"{x=} {y=}")
... print(locals())
...
func()
x=1 y=2
{'__module__': '__main__', '__qualname__': 'func..cls', 'y': 2}
Thanks for clarifying.
Hm. So 'x' is neither in locals() nor in globals(). Which starts me
wondering (to go off on a tangent): Should there be a nonlocals()
dictionary?
Maybe you don't care. Maybe you do. But locals() is not the same as
"all names currently available in this scope". And, this example is
definitely not something I would recommend, but good luck making this
work with eval:
def func():
... x = 1
... print(f"{(x:=2)}")
... print(x)
...
func()
2
2
... x = 1
... print(eval("(x:=2)", globals(), locals()))
... print(x)
...
func()
2
1
Now that, I have to admit, IS a challenge!
3) Not sure I understand this.
Before f-strings existed, one of the big problems with "just use
str.format_map" was that you can't just pass it locals() to get all
the available names. You also can't eval arbitrary code and expect to
get the same results, even if you pass it globals and locals. And
various other considerations here - the exact issues seen here, but
flipped on their heads. So the obvious question is: why not just use
str.format_map?
What this underlines to me is what a good thing f-strings are. And with
PEP 701 they will be IMO even better.
Just as when you were working on PEP 463 (Exception-catching
expressions) - which I still think would be a Good Thing - some research
I did made me realise how good the existing try/except/else/finally
mechanism actually is. There's lots of Good Stuff in Python.
Best wishes
Rob
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Re: Evaluation of variable as f-string
On 2023-01-29 10:18:00 +0100, Johannes Bauer wrote: > Am 29.01.23 um 05:27 schrieb Thomas Passin: > > IOW, perhaps there is a more practical way to accomplish what you want. > > Except that we don't know what that is. > > Well, I don't know. I pretty much want a generic Python mechanism that > allows for exactly what f-strings do: execute arbitrary Python snippets of > code That exists. Use eval (or exec). > and format them in one go. Include an f-string in the code you eval. > In other words, I want to be able to do things like that, given an > *arbitrary* dictionary x and a string s As I wrote before: An f-string isn't a string. It's a grammatical construct. So you want to execute Python code which is what eval and exec do. hp -- _ | Peter J. Holzer| Story must make more sense than reality. |_|_) || | | | h...@hjp.at |-- Charles Stross, "Creative writing __/ | http://www.hjp.at/ | challenge!" signature.asc Description: PGP signature -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On 1/29/2023 6:09 AM, Christian Gollwitzer wrote:
Am 28.01.23 um 02:56 schrieb Thomas Passin:
On 1/27/2023 5:10 PM, Christian Gollwitzer wrote:
Am 27.01.23 um 21:43 schrieb Johannes Bauer:
I don't understand why you fully ignore literally the FIRST example
I gave in my original post and angrily claim that you solution works
when it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
It's because "you're holding it wrong!". Notice the error message; it
says that the key 'y' does not exist.
(base) Apfelkiste:Abschlussmeeting chris$ ipython
Python 3.8.8 (default, Apr 13 2021, 12:59:45)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: x = { "y": "z" }
In [2]: s = "-> {x[y]}"
In [3]: print(s.format(x = x))
-> z
In [4]:
Christian
Oops, that's not quite what he wrote.
You: s = "-> {x[y]}" # Works
Him: s = "-> {x['y']}" # Fails
You might want to reconsider why I could have possibly written this
message
I might .. or I might wish you had actually said what you wanted to
convey ...
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Re: Evaluation of variable as f-string
Am 29.01.23 um 05:27 schrieb Thomas Passin:
Well, yes, we do see that. What we don't see is what you want to
accomplish by doing it, and why you don't seem willing to accept some
restrictions on the string fragments so that they will evaluate correctly.
I'll have to accept the restrictions. That's a good enough answer for
me, actually. I was just thinking that possibly there's something like
(made-up code):
x = { "foo": "bar" }
fstr = string.fstring_compile(s)
fstr.eval(x = x)
Which I didn't know about. It would make sense to me, but possibly not
enough of a usecase to make it into Python. The format() flavors do not
IOW, perhaps there is a more practical way to accomplish what you want.
Except that we don't know what that is.
Well, I don't know. I pretty much want a generic Python mechanism that
allows for exactly what f-strings do: execute arbitrary Python snippets
of code and format them in one go. In other words, I want to be able to
do things like that, given an *arbitrary* dictionary x and a string s
(which has the only restriction that its content needs to be vald
f-string grammar):
x = {
"d": 12,
"t": 12345,
"dt": datetime.datetime,
"td": datetime.timedelta
}
s = "{x['d']:09b} {'->' * (x['d'] // 3)} {(x['dt'](2000, 1, x['d']) +
x['td'](120)).strftime('%y.%m.%d')} {'<-' * (x['d'] // 4)}"
q = magic_function(s, x = x)
and have "q" then be
'01100 ->->->-> 00.05.11 <-<-<-'
I believe the closest solution would be using a templating mechanism
(like Mako), but that has slightly different syntax and doesn't do
string formatting as nice as f-strings do. f-strings really are the
perfect syntax for what I want to do.
Cheers,
Johannes
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Re: Evaluation of variable as f-string
Am 28.01.23 um 02:56 schrieb Thomas Passin:
On 1/27/2023 5:10 PM, Christian Gollwitzer wrote:
Am 27.01.23 um 21:43 schrieb Johannes Bauer:
I don't understand why you fully ignore literally the FIRST example I
gave in my original post and angrily claim that you solution works
when it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
It's because "you're holding it wrong!". Notice the error message; it
says that the key 'y' does not exist.
(base) Apfelkiste:Abschlussmeeting chris$ ipython
Python 3.8.8 (default, Apr 13 2021, 12:59:45)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: x = { "y": "z" }
In [2]: s = "-> {x[y]}"
In [3]: print(s.format(x = x))
-> z
In [4]:
Christian
Oops, that's not quite what he wrote.
You: s = "-> {x[y]}" # Works
Him: s = "-> {x['y']}" # Fails
You might want to reconsider why I could have possibly written this
message
Christian
--
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Re: Evaluation of variable as f-string
Am 29.01.23 um 02:09 schrieb Chris Angelico: The exact same points have already been made, but not listened to. Sometimes, forceful language is required in order to get people to listen. An arrogant bully's rationale. Personally, I'm fine with it. I've been to Usenet for a long time, in which this way of "educating" people was considered normal. But I do think it creates a deterring, toxic environment and reflects back to you as a person negatively. Arrogant bully? Or someone who has tried *multiple times* to explain to you that what you're asking for is IMPOSSIBLE, and you need to ask a better question if you want a better answer? In literally your first answer you resorted to aggressive language and implied that what I asked wasn't what I actually wanted. It was. Also note that in your first answer you did not answer "sorry, this is not possible", which would have been completely sufficient as an answer. Instead you tried your best at guesswork, implying I didn't know what I was doing. So, yes, absolutely toxic behavior. I fully stand by that judgement of mine. I'll go a step further and again repeat that THIS sort of behavior is what gives open source forums a bad rep. There's always a Lennart Poettering, an Ulrich Drepper or maybe a Chris Angelico around who may have great technical skill but think they can treat people like shit. If that's "bullying", then fine, ban me for bullying, and go find somewhere else where you'll be coddled and told that your question is fine, it's awesome, and yes, wouldn't it be nice if magic were a thing. LOL, "ban you"? What the heck are you talking about, my friend? I don't need to be coddled by you. I'm trying to give you the favor of honest feedback, which is that you sound like an utter bully. If you don't care, that is totally fine by me. They're not different things, because what you asked for is NOT POSSIBLE without the caveats that I gave. It is *fundamentally not possible* to "evaluate a string as if it were an f-string", other than by wrapping it in an f-string and evaluating it - with the consequences of that. Yeah that sucks, unfortunately. But I'll live. In other words, if there were a magic function: evalfstring(s, x = x) That would have been the ideal answer. There does not seem to be one, however. So I'm back to silly workarounds. Right. Exactly. Now if you'd asked for what you REALLY need, maybe there'd be a solution involving format_map, but no, you're so utterly intransigent that you cannot adjust your question to fit reality. Does format_map do exactly what f-strings can do? Can I pass arbitrary functions and Python expressions insode a format_map? No? Of course not. Then it does not answer the question. If that makes me a bad guy, then fine. I'll be the bad guy. Awww, it's adorable how you're trying to frame yourself as the victim. I'll be here if you need a hug, buddy. But you're not going to change the laws of physics. Yeah we'll have to disagree about the fact that it's the "laws of physics" preventing a specific implementation of a Python function. Cheers, Johannes -- https://mail.python.org/mailman/listinfo/python-list
To gateway or not to gateway a specific person (was: Evaluation of variable as f-string)
On 2023-01-29 15:47:47 +1100, Chris Angelico wrote: > On Sun, 29 Jan 2023 at 14:36, Stefan Ram wrote: > > (This message was written for Usenet. If you read it in a > > mailing list or the Web, it has been stolen from Usenet.) > > I'm curious as to the copyright protections available to you, but if > you're going to threaten python-list's owners with legal action for > daring to rebroadcast a public post, I would have to recommend that > you get promptly banned from the list in order to reduce liability. Stefan had been banned for years because of this issue. In mid-2021 (wow, that long ago? I thought that was a lot more recently) his posts started to appear again. I don't know if this was a concious decision of the moderators, a technical error or whether Stefan changed something which caused him to escape the filter. I don't think it matters. If someone posts to a Usenet group with the full knowledge that it is gatewayed to a mailing list it is unreasonable to expect the moderators of the list to jump through hoops to comply with wishes hidden away in a non-standard header. > What's so bad about mailing lists that you don't want your messages to > be seen on them? He's Stefan Ram :-). hp -- _ | Peter J. Holzer| Story must make more sense than reality. |_|_) || | | | h...@hjp.at |-- Charles Stross, "Creative writing __/ | http://www.hjp.at/ | challenge!" signature.asc Description: PGP signature -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On 1/28/2023 2:50 PM, Johannes Bauer wrote:
Am 28.01.23 um 02:51 schrieb Thomas Passin:
This is literally the version I described myself, except using triple
quotes. It only modifies the underlying problem, but doesn't solve it.
Ok, so now we are in the territory of "Tell us what you are trying to
accomplish". And part of that is why you cannot put some constraints
on what your string fragments are. The example I gave, copied out of
your earlier message, worked and now you are springing triple quotes
on us.
It works in this particular case, yes. Just like the example I gave in
my original case:
eval("f'" + s + "'")
"works" if there are no apostrophes used. And just like
eval("f\"" + s + "\"")
"works" if there are no quotation marks used.
I don't want to have to care about what quotation is used inside the
string, as long as it could successfully evaluate using the f-string
grammar.
Stop with the rock management already and explain (briefly if
possible) what you are up to.
I have a string. I want to evaluate it as if it were an f-string. I.e.,
there *are* obviously restrictions that apply (namely, the syntax and
semantics of f-strings), but that's it.
Well, yes, we do see that. What we don't see is what you want to
accomplish by doing it, and why you don't seem willing to accept some
restrictions on the string fragments so that they will evaluate correctly.
IOW, perhaps there is a more practical way to accomplish what you want.
Except that we don't know what that is.
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Re: Evaluation of variable as f-string
On Sun, 29 Jan 2023 at 14:36, Stefan Ram wrote: > > Johannes Bauer writes: > >I have a string. I want to evaluate it as if it were an f-string. I.e., > >there *are* obviously restrictions that apply (namely, the syntax and > >semantics of f-strings), but that's it. > > (This message was written for Usenet. If you read it in a > mailing list or the Web, it has been stolen from Usenet.) > I'm curious as to the copyright protections available to you, but if you're going to threaten python-list's owners with legal action for daring to rebroadcast a public post, I would have to recommend that you get promptly banned from the list in order to reduce liability. What's so bad about mailing lists that you don't want your messages to be seen on them? ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On Sun, 29 Jan 2023 at 11:56, Johannes Bauer wrote: > > Am 28.01.23 um 00:41 schrieb Chris Angelico: > > On Sat, 28 Jan 2023 at 10:08, Rob Cliffe via Python-list > > wrote: > >> > >> Whoa! Whoa! Whoa! > >> I appreciate the points you are making, Chris, but I am a bit taken > >> aback by such forceful language. > > > > The exact same points have already been made, but not listened to. > > Sometimes, forceful language is required in order to get people to > > listen. > > An arrogant bully's rationale. Personally, I'm fine with it. I've been > to Usenet for a long time, in which this way of "educating" people was > considered normal. But I do think it creates a deterring, toxic > environment and reflects back to you as a person negatively. Arrogant bully? Or someone who has tried *multiple times* to explain to you that what you're asking for is IMPOSSIBLE, and you need to ask a better question if you want a better answer? If that's "bullying", then fine, ban me for bullying, and go find somewhere else where you'll be coddled and told that your question is fine, it's awesome, and yes, wouldn't it be nice if magic were a thing. > Exactly. This is precisely what I want to avoid. Essentially, proper > quotation of such a string requires to write a fully fledged f-string > parser, in which case the whole problem solves itself. > > >>> Don't ask how to use X to do Y. Ask how to do Y. > >> Good advice. > > > > Exactly. As I have shown, asking how to use f-strings to achieve this > > is simply not suitable, and there's no useful way to discuss other > > than to argue semantics. If we had a GOAL to discuss, we could find > > much better options. > > I was not asking how to use f-strings. I was asking to evaluate a string > *as if it were* an f-string. Those are two completely different things > which you entirely ignored. They're not different things, because what you asked for is NOT POSSIBLE without the caveats that I gave. It is *fundamentally not possible* to "evaluate a string as if it were an f-string", other than by wrapping it in an f-string and evaluating it - with the consequences of that. > In other words, if there were a magic function: > > evalfstring(s, x = x) > > That would have been the ideal answer. There does not seem to be one, > however. So I'm back to silly workarounds. > Right. Exactly. Now if you'd asked for what you REALLY need, maybe there'd be a solution involving format_map, but no, you're so utterly intransigent that you cannot adjust your question to fit reality. If that makes me a bad guy, then fine. I'll be the bad guy. But you're not going to change the laws of physics. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On Sun, 29 Jan 2023 at 11:53, Johannes Bauer wrote: > I don't want to have to care about what quotation is used inside the > string, as long as it could successfully evaluate using the f-string > grammar. > Not possible. An f-string can contain other f-strings, and it is entirely possible to use EVERY quote type. So you can never add quotes around the outside of a string and then evaluate it as an f-string, without making sure that it doesn't already contain that string. (That MAY be changing in a future version of Python, but it's currently true.) ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
Am 27.01.23 um 23:10 schrieb Christian Gollwitzer:
Am 27.01.23 um 21:43 schrieb Johannes Bauer:
I don't understand why you fully ignore literally the FIRST example I
gave in my original post and angrily claim that you solution works
when it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
It's because "you're holding it wrong!". Notice the error message; it
says that the key 'y' does not exist.
Ah, that is neat! I didn't know that. Thanks for the info.
In my case, I do also however want to have some functionality that
actually does math or even calls functions. That would be possible with
templates or f-strings, but not format:
x = { "t": 12345 }
s = "{x['t'] // 60:02d}:{x['t'] % 60:02d}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'t'"
and
s = "{x[t] // 60:02d}:{x[t] % 60:02d}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
ValueError: Only '.' or '[' may follow ']' in format field specifier
but of course:
print(f"{x['t'] // 60:02d}:{x['t'] % 60:02d}")
205:45
Best,
Johannes
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Re: Evaluation of variable as f-string
Am 28.01.23 um 00:41 schrieb Chris Angelico:
On Sat, 28 Jan 2023 at 10:08, Rob Cliffe via Python-list
wrote:
Whoa! Whoa! Whoa!
I appreciate the points you are making, Chris, but I am a bit taken
aback by such forceful language.
The exact same points have already been made, but not listened to.
Sometimes, forceful language is required in order to get people to
listen.
An arrogant bully's rationale. Personally, I'm fine with it. I've been
to Usenet for a long time, in which this way of "educating" people was
considered normal. But I do think it creates a deterring, toxic
environment and reflects back to you as a person negatively.
Addressing your points specifically:
1) I believe the quote character limitation could be overcome. It
would need a fair amount of work, for which I haven't (yet) the time or
inclination.
No problem. Here, solve it for this string:
eval_me = ' f"""{f\'\'\'{f"{f\'{1+2}\'}"}\'\'\'}""" '
F-strings can be nested, remember.
Exactly. This is precisely what I want to avoid. Essentially, proper
quotation of such a string requires to write a fully fledged f-string
parser, in which case the whole problem solves itself.
Don't ask how to use X to do Y. Ask how to do Y.
Good advice.
Exactly. As I have shown, asking how to use f-strings to achieve this
is simply not suitable, and there's no useful way to discuss other
than to argue semantics. If we had a GOAL to discuss, we could find
much better options.
I was not asking how to use f-strings. I was asking to evaluate a string
*as if it were* an f-string. Those are two completely different things
which you entirely ignored.
In other words, if there were a magic function:
evalfstring(s, x = x)
That would have been the ideal answer. There does not seem to be one,
however. So I'm back to silly workarounds.
Cheers,
Johannes
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Re: Evaluation of variable as f-string
Am 28.01.23 um 02:51 schrieb Thomas Passin:
This is literally the version I described myself, except using triple
quotes. It only modifies the underlying problem, but doesn't solve it.
Ok, so now we are in the territory of "Tell us what you are trying to
accomplish". And part of that is why you cannot put some constraints on
what your string fragments are. The example I gave, copied out of your
earlier message, worked and now you are springing triple quotes on us.
It works in this particular case, yes. Just like the example I gave in
my original case:
eval("f'" + s + "'")
"works" if there are no apostrophes used. And just like
eval("f\"" + s + "\"")
"works" if there are no quotation marks used.
I don't want to have to care about what quotation is used inside the
string, as long as it could successfully evaluate using the f-string
grammar.
Stop with the rock management already and explain (briefly if possible)
what you are up to.
I have a string. I want to evaluate it as if it were an f-string. I.e.,
there *are* obviously restrictions that apply (namely, the syntax and
semantics of f-strings), but that's it.
Best,
Johannes
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Re: Evaluation of variable as f-string
On 2023-01-27 21:31:05 +0100, Johannes Bauer wrote:
> > But if you really REALLY know what you're doing, just use eval()
> > directly.
>
> I do, actually, but I hate it. Not because of the security issue, not
> because of namespaces, but because it does not reliably work:
>
> >>> s = "{\"x\" * 4}"
> >>> eval("f'" + s + "'")
> ''
That's exactly the result I expected. What did you expect?
hp
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Re: Evaluation of variable as f-string
On 2023-01-27 21:43:09 +0100, Johannes Bauer wrote:
> x = { "y": "z" }
> s = "-> {x['y']}"
> print(s.format(x = x))
> Traceback (most recent call last):
> File "", line 1, in
> KeyError: "'y'"
>
> This. Does. Not. Work.
>
> I want to pass a single variable as a dictionary and access its members
> inside the expression.
You can do that (see other responses), but you can't have arbitrary
Python expressions inside a format string.
It works with f-strings because f-strings are a compiler construct. The
f-string never exists at run-time. Instead the compiler transforms
f"-> {x['y']}"
into the equivalent of
"-> " + format_value(x["y"]) + ""
So either you need to pass it to the Python compiler (via eval), or you
need to implement enough of a Python parser/interpreter to cover the
cases you are interested in. The latter might be an interesting
exercise, but I would suggest looking at existing template engines like
Jinja2 for production purposes.
hp
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Re: Evaluation of variable as f-string
On 2023-01-27 20:56:49 -0500, Thomas Passin wrote:
> On 1/27/2023 5:10 PM, Christian Gollwitzer wrote:
> > Am 27.01.23 um 21:43 schrieb Johannes Bauer:
> > > x = { "y": "z" }
> > > s = "-> {x['y']}"
> > > print(s.format(x = x))
> > > Traceback (most recent call last):
> > > File "", line 1, in
> > > KeyError: "'y'"
> > >
> > > This. Does. Not. Work.
> >
> > It's because "you're holding it wrong!". Notice the error message; it
^^^
> > says that the key 'y' does not exist.
[...]
> > In [1]: x = { "y": "z" }
> > In [2]: s = "-> {x[y]}"
> > In [3]: print(s.format(x = x))
> > -> z
> > In [4]:
>
> Oops, that's not quite what he wrote.
>
> You: s = "-> {x[y]}"# Works
> Him: s = "-> {x['y']}" # Fails
That was the point.
hp
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RE: Evaluation of variable as f-string
May I point out that some dynamic situations can in a sense be normalized?
The example below posits a dynamically allocated dictionary during run time.
But why can't you have a placeholder variable name in place and make your
placeholder a link to the dictionary (or other item) before invoking the
existing f-string with the placeholder built-in, rather than trying to
evaluate an F♯ ???
Of course many situations may not have as much of a possible work-around.
But as so many have noted, we never got a really good explanation of what
the OP really wants to do. There have been replies that may be suitable
solutions and some clearly have potential to be security holes if you let
the users dynamically create strings to be evaluated.
In some languages, many of the facets of the language can be passed along as
a function with some name to be used in functional programming techniques
and this can be very useful. The "operator" module can be used for quite a
few things like operator.add or operator.__add__ or operator.concat and many
more. If the logic used to evaluate an f-string (and for that matter the
other string variants like b'..' and r'...') could be encapsulated in a
function like that, it would be potentially usable as in passing something
like dangerous_operator.f_string and a list of strings and having that
return a list of evaluated strings.
The fact that something like this is not known to the people here may hint
that it is not something considered safe to use by amateurs. But then again,
anyone who wants to can use eval() as Chris points out.
Of course, there may be other reasons too. An f-string is evaluated in a
context that may be different if a string is passed along and then looked at
in another context.
-Original Message-
From: Python-list On
Behalf Of Stefan Ram
Sent: Friday, January 27, 2023 4:31 PM
To: python-list@python.org
Subject: Re: Evaluation of variable as f-string
Johannes Bauer writes:
>>Johannes Bauer writes:
>>>x = { "y": "z" }
>>>s = "-> {x['y']}"
>>>print(s.format(x = x))
>>x = { "y": "z" }
>>def s( x ): return '-> ' + x[ 'y' ]
>>print( s( x = x ))
>Except this is not at all what I asked for. The string "s" in my
>example is just that, an example. I want to render *arbitrary* strings "s"
>together with arbitrary dictionaries "x".
I take this to mean that you want to process a dictionary
name, a string and a dictionary that is only specified as
late as at run time.
import string
name = input( 'name of the dictionary? ' ) string_ = input( 'string? ' ) #
"-> {x['y']}"
dictionary = eval( input( 'dictionary? ' )) print( eval( 'f"""' + string_ +
'"""', {name:dictionary} ))
name of the dictionary? x
string? -> {x['y']}
dictionary? { 'y': 'z' }
-> z
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Re: Evaluation of variable as f-string
On 1/27/2023 5:10 PM, Christian Gollwitzer wrote:
Am 27.01.23 um 21:43 schrieb Johannes Bauer:
I don't understand why you fully ignore literally the FIRST example I
gave in my original post and angrily claim that you solution works
when it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
It's because "you're holding it wrong!". Notice the error message; it
says that the key 'y' does not exist.
(base) Apfelkiste:Abschlussmeeting chris$ ipython
Python 3.8.8 (default, Apr 13 2021, 12:59:45)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: x = { "y": "z" }
In [2]: s = "-> {x[y]}"
In [3]: print(s.format(x = x))
-> z
In [4]:
Christian
Oops, that's not quite what he wrote.
You: s = "-> {x[y]}"# Works
Him: s = "-> {x['y']}" # Fails
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Re: Evaluation of variable as f-string
On 1/27/2023 3:33 PM, Johannes Bauer wrote:
Am 25.01.23 um 20:38 schrieb Thomas Passin:
x = { "y": "z" }
s = "-> {target}"
print(s.format(target = x['y']))
Stack overflow to the rescue:
No.
Search phrase: "python evaluate string as fstring"
https://stackoverflow.com/questions/47339121/how-do-i-convert-a-string-into-an-f-string
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
print(effify(s)) # prints as expected: "-> z"
Great.
s = '"""'
> def effify(non_f_str: str):
> return eval(f'f"""{non_f_str}"""')
>
> print(effify(s)) # prints as expected: "-> z"
>>> print(effify(s))
Traceback (most recent call last):
File "", line 1, in
File "", line 2, in effify
File "", line 1
f"
^
SyntaxError: unterminated triple-quoted string literal (detected at line 1)
This is literally the version I described myself, except using triple
quotes. It only modifies the underlying problem, but doesn't solve it.
Ok, so now we are in the territory of "Tell us what you are trying to
accomplish". And part of that is why you cannot put some constraints on
what your string fragments are. The example I gave, copied out of your
earlier message, worked and now you are springing triple quotes on us.
Stop with the rock management already and explain (briefly if possible)
what you are up to.
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Re: Evaluation of variable as f-string
Am 27.01.23 um 21:43 schrieb Johannes Bauer:
I don't understand why you fully ignore literally the FIRST example I
gave in my original post and angrily claim that you solution works when
it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
It's because "you're holding it wrong!". Notice the error message; it
says that the key 'y' does not exist.
(base) Apfelkiste:Abschlussmeeting chris$ ipython
Python 3.8.8 (default, Apr 13 2021, 12:59:45)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: x = { "y": "z" }
In [2]: s = "-> {x[y]}"
In [3]: print(s.format(x = x))
-> z
In [4]:
Christian
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Re: Evaluation of variable as f-string
Am 27.01.23 um 20:18 schrieb Chris Angelico:
All you tell us is
what you're attempting to do, which there is *no good way to achieve*.
Fair enough, that is the answer. It's not possible.
Perhaps someone will be inspired to write a function to do it.
See, we don't know what "it" is, so it's hard to write a function
that's any better than the ones we've seen. Using eval() to construct
an f-string and then parse it is TERRIBLE because:
1) It still doesn't work in general, and thus has caveats like "you
can't use this type of quote character"
Exactly my observation as well, which is why I was thinking there's
something else I missed.
2) You would have to pass it a dictionary of variables, which also
can't be done with full generality
Nonsense. I only am passing a SINGLE variable to eval, called "x". That
is fully general.
3) These are the exact same problems, but backwards, that led to
f-strings in the first place
I don't know what you mean by that.
4) eval is extremely slow and horrifically inefficient.
Let me worry about it.
For some reason, str.format() isn't suitable,
I don't understand why you fully ignore literally the FIRST example I
gave in my original post and angrily claim that you solution works when
it does not:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
This. Does. Not. Work.
I want to pass a single variable as a dictionary and access its members
inside the expression.
but *you haven't said
why*,
Yes I have, see above.
Well, yes. If you asked "how can I do X", hoping the answer would be
"with a runtime-evaluated f-string", then you're quite right - the
answer might not be what you were hoping for. But since you asked "how
can I evaluate a variable as if it were an f-string", the only
possible answer is "you can't, and that's a horrible idea".
"You can't" would have been sufficient. Pity. Your judgement is
unnecessary and, frankly, uncalled for as well. Multiple instances you
claim that you have no idea what I am doing so how would you even begin
to judge a solution as fit or unfit?
Don't ask how to use X to do Y. Ask how to do Y.
You don't have to be angry that my question does not have a solution. I
will manage and so might you.
Cheers,
Johannes
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Re: Evaluation of variable as f-string
Am 23.01.23 um 17:43 schrieb Stefan Ram:
Johannes Bauer writes:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
x = { "y": "z" }
def s( x ): return '-> ' + x[ 'y' ]
print( s( x = x ))
Except this is not at all what I asked for. The string "s" in my example
is just that, an example. I want to render *arbitrary* strings "s"
together with arbitrary dictionaries "x".
Cheers,
Johannes
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Re: Evaluation of variable as f-string
Am 25.01.23 um 20:38 schrieb Thomas Passin:
x = { "y": "z" }
s = "-> {target}"
print(s.format(target = x['y']))
Stack overflow to the rescue:
No.
Search phrase: "python evaluate string as fstring"
https://stackoverflow.com/questions/47339121/how-do-i-convert-a-string-into-an-f-string
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
print(effify(s)) # prints as expected: "-> z"
Great.
s = '"""'
> def effify(non_f_str: str):
> return eval(f'f"""{non_f_str}"""')
>
> print(effify(s)) # prints as expected: "-> z"
>>> print(effify(s))
Traceback (most recent call last):
File "", line 1, in
File "", line 2, in effify
File "", line 1
f"
^
SyntaxError: unterminated triple-quoted string literal (detected at line 1)
This is literally the version I described myself, except using triple
quotes. It only modifies the underlying problem, but doesn't solve it.
Cheers,
Johannes
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Re: Evaluation of variable as f-string
Am 23.01.23 um 19:02 schrieb Chris Angelico:
This is supposedly for security reasons. However, when trying to emulate
this behavior that I wanted (and know the security implications of), my
solutions will tend to be less secure. Here is what I have been thinking
about:
If you really want the full power of an f-string, then you're asking
for the full power of eval(),
Exactly.
and that means all the security
implications thereof,
Precisely, as I had stated myself.
not to mention the difficulties of namespacing.
Not an issue in my case.
Have you considered using the vanilla format() method instead?
Yes. It does not provide the functionality I want. Not even the utterly
trivial example that I gave. To quote myself again, let's say I have an
arbitrary dictionary x (with many nested data structures), I want an
expression to be evaluated that can access any members in there.
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
I also want to be able to say things like {'x' * 100}, which .format()
also does not do.
In other words: I want the evaluation of a variable as an f-string.
But if you really REALLY know what you're doing, just use eval()
directly.
I do, actually, but I hate it. Not because of the security issue, not
because of namespaces, but because it does not reliably work:
>>> s = "{\"x\" * 4}"
>>> eval("f'" + s + "'")
''
As I mentioned, it depends on the exact quoting. Triple quotes only
shift the problem. Actually replacing/escaping the relevant quotation
marks is also not trivial.
I don't really see what you'd gain from an f-string.
The full power of eval.
At very
least, work with a well-defined namespace and eval whatever you need
in that context.
That's what I'm doing.
Maybe, rather than asking for a way to treat a string as code, ask for
what you ACTUALLY need, and we can help?
I want to render data from a template using an easily understandable
syntax (like an f-string), ideally using native Python. I want the
template to make use of Python code constructs AND formatting (e.g.
{x['time']['runtime']['seconds'] // 60:02d}).
Cheers,
Johannes
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Re: Evaluation of variable as f-string
On 1/27/2023 5:54 PM, Rob Cliffe via Python-list wrote:
Whoa! Whoa! Whoa!
I appreciate the points you are making, Chris, but I am a bit taken
aback by such forceful language.
I generally agree with asking for what the intent is. In this case it
seems pretty clear that the OP wants to use these string fragments as
templates, and he needs to be able to insert variables into them at
runtime, not compile time.
So I think a good response would have been roughly
"It looks like you want to use these strings as templates, is that
right? If not, please tell us what you are trying to do, because it's
hard to help without knowing that. If it's right, here's a way you
could go about it."
Short and amiable.
On 27/01/2023 19:18, Chris Angelico wrote:
On Sat, 28 Jan 2023 at 05:31, Rob Cliffe via Python-list
wrote:
On 23/01/2023 18:02, Chris Angelico wrote:
Maybe, rather than asking for a way to treat a string as code, ask for
what you ACTUALLY need, and we can help?
ChrisA
Fair enough, Chris, but still ISTM that it is reasonable to ask (perhaps
for a different use-case) whether there is a way of evaluating a string
at runtime as if it were an f-string. We encourage people to ask
questions on this list, even though the answer will not always be what
they're hoping for.
No, it's not, because that's the "how do I use X to do Y" problem.
Instead, just ask how to do *what you actually need*. If the best way
to do that is to eval an f-string, then someone will suggest that.
But, much much much more likely, the best way to do it would be
something completely different. What, exactly? That's hard to say,
because *we don't know what you actually need*. All you tell us is
what you're attempting to do, which there is *no good way to achieve*.
If the above is addressed to the OP, I can't answer for him.
If it's addressed to me: How about if I wanted a program (a learning
tool) to allow the user to play with f-strings?
I.e. to type in a string, and then see what the result would be if it
had been an f-string?
I suspect there are other use cases, but I confess I can't think of one
right now.
I appreciate that the answer may be "No, because it would be a lot of
work - and increase the maintenance burden - to support a relatively
rare requirement".
What about: "No, because it's a terrible TERRIBLE idea, requires that
you do things horribly backwards, and we still don't even know what
you're trying to do"?
Perhaps someone will be inspired to write a function to do it.
See, we don't know what "it" is, so it's hard to write a function
that's any better than the ones we've seen.
Again, if this is addressed to the OP: I'm not his keeper.
If it's addressed to me: "it" means a function that will take a string
and evaluate it at runtime as if it were an f-string. Sure, with
caveats and limitations. And indeed Thomas Passim found this partial
solution on Stack Overflow:
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
Using eval() to construct
an f-string and then parse it is TERRIBLE because:
1) It still doesn't work in general, and thus has caveats like "you
can't use this type of quote character"
2) You would have to pass it a dictionary of variables, which also
can't be done with full generality
3) These are the exact same problems, but backwards, that led to
f-strings in the first place
4) eval is extremely slow and horrifically inefficient.
I understand these limitations. Nonetheless I can conceive that there
may be scenarios where it is an acceptable solution (perhaps the
learning tool program I suggested above).
Addressing your points specifically:
1) I believe the quote character limitation could be overcome. It
would need a fair amount of work, for which I haven't (yet) the time or
inclination.
2) Yes in general you would have to pass it one dictionary, maybe
two. I don't see this as an insuperable obstacle. I am not sure what
you mean by "can't be done with full generality" and perhaps that's not
important.
3) Not sure I understand this.
4) On the fairly rare occasions that I have used eval(), I can't
remember speed ever being a consideration.
For some reason, str.format() isn't suitable, but *you haven't said
why*, so we have to avoid that in our solutions. So, to come back to
your concern:
We encourage people to ask
questions on this list, even though the answer will not always be what
they're hoping for.
Well, yes. If you asked "how can I do X", hoping the answer would be
"with a runtime-evaluated f-string", then you're quite right - the
answer might not be what you were hoping for. But since you asked "how
can I evaluate a variable as if it were an f-string", the only
possible answer is "you can't, and that's a horrible idea".
I hope that I have shown that this is a somewhat dogmatic response.
Don't ask how to use X to do Y. Ask how to do Y.
Good advice.
Best wishes
Rob Cliffe
ChrisA
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Re: Evaluation of variable as f-string
On Sat, 28 Jan 2023 at 10:08, Rob Cliffe via Python-list
wrote:
>
> Whoa! Whoa! Whoa!
> I appreciate the points you are making, Chris, but I am a bit taken
> aback by such forceful language.
The exact same points have already been made, but not listened to.
Sometimes, forceful language is required in order to get people to
listen.
> If it's addressed to me: How about if I wanted a program (a learning
> tool) to allow the user to play with f-strings?
> I.e. to type in a string, and then see what the result would be if it
> had been an f-string?
> I suspect there are other use cases, but I confess I can't think of one
> right now.
Use the REPL, which will happily evaluate f-strings in their original
context, just like any other code would. You're already eval'ing, so
it's exactly what you'd expect. This is not the same thing as "typing
in a string", though - it's typing in code and seeing what the result
would be. (Except to the extent that source code can be considered a
string.)
> If it's addressed to me: "it" means a function that will take a string
> and evaluate it at runtime as if it were an f-string. Sure, with
> caveats and limitations.
And that's what I am saying is a terrible terrible idea. It will
evaluate things in the wrong context, it has all the normal problems
of eval, and then it introduces its own unique problems with quote
characters.
> And indeed Thomas Passim found this partial
> solution on Stack Overflow:
> def effify(non_f_str: str):
> return eval(f'f"""{non_f_str}"""')
You can find anything on Stack Overflow. Just because you found it
there doesn't mean it's any good - even if it's been massively
upvoted.
> Addressing your points specifically:
> 1) I believe the quote character limitation could be overcome. It
> would need a fair amount of work, for which I haven't (yet) the time or
> inclination.
No problem. Here, solve it for this string:
eval_me = ' f"""{f\'\'\'{f"{f\'{1+2}\'}"}\'\'\'}""" '
F-strings can be nested, remember.
> 2) Yes in general you would have to pass it one dictionary, maybe
> two. I don't see this as an insuperable obstacle. I am not sure what
> you mean by "can't be done with full generality" and perhaps that's not
> important.
>>> def func():
... x = 1
... class cls:
... y = 2
... print(f"{x=} {y=}")
... print(locals())
...
>>> func()
x=1 y=2
{'__module__': '__main__', '__qualname__': 'func..cls', 'y': 2}
Maybe you don't care. Maybe you do. But locals() is not the same as
"all names currently available in this scope". And, this example is
definitely not something I would recommend, but good luck making this
work with eval:
>>> def func():
... x = 1
... print(f"{(x:=2)}")
... print(x)
...
>>> func()
2
2
... x = 1
... print(eval("(x:=2)", globals(), locals()))
... print(x)
...
>>> func()
2
1
> 3) Not sure I understand this.
Before f-strings existed, one of the big problems with "just use
str.format_map" was that you can't just pass it locals() to get all
the available names. You also can't eval arbitrary code and expect to
get the same results, even if you pass it globals and locals. And
various other considerations here - the exact issues seen here, but
flipped on their heads. So the obvious question is: why not just use
str.format_map?
> > Well, yes. If you asked "how can I do X", hoping the answer would be
> > "with a runtime-evaluated f-string", then you're quite right - the
> > answer might not be what you were hoping for. But since you asked "how
> > can I evaluate a variable as if it were an f-string", the only
> > possible answer is "you can't, and that's a horrible idea".
> I hope that I have shown that this is a somewhat dogmatic response.
And I hope that I have shown that it is fully justified.
> > Don't ask how to use X to do Y. Ask how to do Y.
> Good advice.
Exactly. As I have shown, asking how to use f-strings to achieve this
is simply not suitable, and there's no useful way to discuss other
than to argue semantics. If we had a GOAL to discuss, we could find
much better options.
ChrisA
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Re: Evaluation of variable as f-string
Whoa! Whoa! Whoa!
I appreciate the points you are making, Chris, but I am a bit taken
aback by such forceful language.
On 27/01/2023 19:18, Chris Angelico wrote:
On Sat, 28 Jan 2023 at 05:31, Rob Cliffe via Python-list
wrote:
On 23/01/2023 18:02, Chris Angelico wrote:
Maybe, rather than asking for a way to treat a string as code, ask for
what you ACTUALLY need, and we can help?
ChrisA
Fair enough, Chris, but still ISTM that it is reasonable to ask (perhaps
for a different use-case) whether there is a way of evaluating a string
at runtime as if it were an f-string. We encourage people to ask
questions on this list, even though the answer will not always be what
they're hoping for.
No, it's not, because that's the "how do I use X to do Y" problem.
Instead, just ask how to do *what you actually need*. If the best way
to do that is to eval an f-string, then someone will suggest that.
But, much much much more likely, the best way to do it would be
something completely different. What, exactly? That's hard to say,
because *we don't know what you actually need*. All you tell us is
what you're attempting to do, which there is *no good way to achieve*.
If the above is addressed to the OP, I can't answer for him.
If it's addressed to me: How about if I wanted a program (a learning
tool) to allow the user to play with f-strings?
I.e. to type in a string, and then see what the result would be if it
had been an f-string?
I suspect there are other use cases, but I confess I can't think of one
right now.
I appreciate that the answer may be "No, because it would be a lot of
work - and increase the maintenance burden - to support a relatively
rare requirement".
What about: "No, because it's a terrible TERRIBLE idea, requires that
you do things horribly backwards, and we still don't even know what
you're trying to do"?
Perhaps someone will be inspired to write a function to do it.
See, we don't know what "it" is, so it's hard to write a function
that's any better than the ones we've seen.
Again, if this is addressed to the OP: I'm not his keeper.
If it's addressed to me: "it" means a function that will take a string
and evaluate it at runtime as if it were an f-string. Sure, with
caveats and limitations. And indeed Thomas Passim found this partial
solution on Stack Overflow:
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
Using eval() to construct
an f-string and then parse it is TERRIBLE because:
1) It still doesn't work in general, and thus has caveats like "you
can't use this type of quote character"
2) You would have to pass it a dictionary of variables, which also
can't be done with full generality
3) These are the exact same problems, but backwards, that led to
f-strings in the first place
4) eval is extremely slow and horrifically inefficient.
I understand these limitations. Nonetheless I can conceive that there
may be scenarios where it is an acceptable solution (perhaps the
learning tool program I suggested above).
Addressing your points specifically:
1) I believe the quote character limitation could be overcome. It
would need a fair amount of work, for which I haven't (yet) the time or
inclination.
2) Yes in general you would have to pass it one dictionary, maybe
two. I don't see this as an insuperable obstacle. I am not sure what
you mean by "can't be done with full generality" and perhaps that's not
important.
3) Not sure I understand this.
4) On the fairly rare occasions that I have used eval(), I can't
remember speed ever being a consideration.
For some reason, str.format() isn't suitable, but *you haven't said
why*, so we have to avoid that in our solutions. So, to come back to
your concern:
We encourage people to ask
questions on this list, even though the answer will not always be what
they're hoping for.
Well, yes. If you asked "how can I do X", hoping the answer would be
"with a runtime-evaluated f-string", then you're quite right - the
answer might not be what you were hoping for. But since you asked "how
can I evaluate a variable as if it were an f-string", the only
possible answer is "you can't, and that's a horrible idea".
I hope that I have shown that this is a somewhat dogmatic response.
Don't ask how to use X to do Y. Ask how to do Y.
Good advice.
Best wishes
Rob Cliffe
ChrisA
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Re: Evaluation of variable as f-string
On Sat, 28 Jan 2023 at 05:31, Rob Cliffe via Python-list wrote: > On 23/01/2023 18:02, Chris Angelico wrote: > > Maybe, rather than asking for a way to treat a string as code, ask for > > what you ACTUALLY need, and we can help? > > > > ChrisA > Fair enough, Chris, but still ISTM that it is reasonable to ask (perhaps > for a different use-case) whether there is a way of evaluating a string > at runtime as if it were an f-string. We encourage people to ask > questions on this list, even though the answer will not always be what > they're hoping for. No, it's not, because that's the "how do I use X to do Y" problem. Instead, just ask how to do *what you actually need*. If the best way to do that is to eval an f-string, then someone will suggest that. But, much much much more likely, the best way to do it would be something completely different. What, exactly? That's hard to say, because *we don't know what you actually need*. All you tell us is what you're attempting to do, which there is *no good way to achieve*. > I appreciate that the answer may be "No, because it would be a lot of > work - and increase the maintenance burden - to support a relatively > rare requirement". What about: "No, because it's a terrible TERRIBLE idea, requires that you do things horribly backwards, and we still don't even know what you're trying to do"? > Perhaps someone will be inspired to write a function to do it. See, we don't know what "it" is, so it's hard to write a function that's any better than the ones we've seen. Using eval() to construct an f-string and then parse it is TERRIBLE because: 1) It still doesn't work in general, and thus has caveats like "you can't use this type of quote character" 2) You would have to pass it a dictionary of variables, which also can't be done with full generality 3) These are the exact same problems, but backwards, that led to f-strings in the first place 4) eval is extremely slow and horrifically inefficient. For some reason, str.format() isn't suitable, but *you haven't said why*, so we have to avoid that in our solutions. So, to come back to your concern: > We encourage people to ask > questions on this list, even though the answer will not always be what > they're hoping for. Well, yes. If you asked "how can I do X", hoping the answer would be "with a runtime-evaluated f-string", then you're quite right - the answer might not be what you were hoping for. But since you asked "how can I evaluate a variable as if it were an f-string", the only possible answer is "you can't, and that's a horrible idea". Don't ask how to use X to do Y. Ask how to do Y. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On 23/01/2023 18:02, Chris Angelico wrote: On Tue, 24 Jan 2023 at 04:56, Johannes Bauer wrote: Hi there, is there an easy way to evaluate a string stored in a variable as if it were an f-string at runtime? ... This is supposedly for security reasons. However, when trying to emulate this behavior that I wanted (and know the security implications of), my solutions will tend to be less secure. Here is what I have been thinking about: If you really want the full power of an f-string, then you're asking for the full power of eval(), and that means all the security implications thereof, not to mention the difficulties of namespacing. Have you considered using the vanilla format() method instead? But if you really REALLY know what you're doing, just use eval() directly. I don't really see what you'd gain from an f-string. At very least, work with a well-defined namespace and eval whatever you need in that context. Maybe, rather than asking for a way to treat a string as code, ask for what you ACTUALLY need, and we can help? ChrisA Fair enough, Chris, but still ISTM that it is reasonable to ask (perhaps for a different use-case) whether there is a way of evaluating a string at runtime as if it were an f-string. We encourage people to ask questions on this list, even though the answer will not always be what they're hoping for. I appreciate that the answer may be "No, because it would be a lot of work - and increase the maintenance burden - to support a relatively rare requirement". Perhaps someone will be inspired to write a function to do it. Best wishes Rob Cliffe -- https://mail.python.org/mailman/listinfo/python-list
Re: Evaluation of variable as f-string
On 25/01/2023 19:38, Thomas Passin wrote:
Stack overflow to the rescue:
Search phrase: "python evaluate string as fstring"
https://stackoverflow.com/questions/47339121/how-do-i-convert-a-string-into-an-f-string
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
print(effify(s)) # prints as expected: "-> z"
Duh! Am I the only one who feels stupid not thinking of this?
Although of course it won't work if the string already contains triple
double quotes.
I believe this could be got round with some ingenuity (having the effify
function parse the string and replace genuine (not embedded in
single-quotes) triple double quotes with triple single quotes, though
there are some complications).
And the effify function can't be put in its own module unless it can be
passed the globals and/or locals dictionaries as needed for eval to
use. Something like this:
def effify(non_f_str, glob=None, loc=None):
return eval(f'f"""{non_f_str}"""',
glob if glob is not None else globals(),
loc if loc is not None else locals())
Best wishes
Rob Cliffe
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Re: Evaluation of variable as f-string
On 1/25/2023 1:26 PM, Antoon Pardon wrote:
Op 23/01/2023 om 17:24 schreef Johannes Bauer:
Hi there,
is there an easy way to evaluate a string stored in a variable as if
it were an f-string at runtime?
I.e., what I want is to be able to do this:
x = { "y": "z" }
print(f"-> {x['y']}")
This prints "-> z", as expected. But consider:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
Even though
s = "-> {x}"
print(s.format(x = x))
Prints the expected "-> {'y': 'z'}".
I am probably missing something but is there a reason why the following
wouldn't do what you want:
x = { "y": "z" }
s = "-> {target}"
print(s.format(target = x['y']))
Stack overflow to the rescue:
Search phrase: "python evaluate string as fstring"
https://stackoverflow.com/questions/47339121/how-do-i-convert-a-string-into-an-f-string
def effify(non_f_str: str):
return eval(f'f"""{non_f_str}"""')
print(effify(s)) # prints as expected: "-> z"
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Re: Evaluation of variable as f-string
Op 23/01/2023 om 17:24 schreef Johannes Bauer:
Hi there,
is there an easy way to evaluate a string stored in a variable as if
it were an f-string at runtime?
I.e., what I want is to be able to do this:
x = { "y": "z" }
print(f"-> {x['y']}")
This prints "-> z", as expected. But consider:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
Even though
s = "-> {x}"
print(s.format(x = x))
Prints the expected "-> {'y': 'z'}".
I am probably missing something but is there a reason why the following
wouldn't do what you want:
x = { "y": "z" }
s = "-> {target}"
print(s.format(target = x['y']))
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Re: Evaluation of variable as f-string
On Tue, 24 Jan 2023 at 04:56, Johannes Bauer wrote: > > Hi there, > > is there an easy way to evaluate a string stored in a variable as if it > were an f-string at runtime? > > ... > > This is supposedly for security reasons. However, when trying to emulate > this behavior that I wanted (and know the security implications of), my > solutions will tend to be less secure. Here is what I have been thinking > about: If you really want the full power of an f-string, then you're asking for the full power of eval(), and that means all the security implications thereof, not to mention the difficulties of namespacing. Have you considered using the vanilla format() method instead? But if you really REALLY know what you're doing, just use eval() directly. I don't really see what you'd gain from an f-string. At very least, work with a well-defined namespace and eval whatever you need in that context. Maybe, rather than asking for a way to treat a string as code, ask for what you ACTUALLY need, and we can help? ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Evaluation of variable as f-string
Hi there,
is there an easy way to evaluate a string stored in a variable as if it
were an f-string at runtime?
I.e., what I want is to be able to do this:
x = { "y": "z" }
print(f"-> {x['y']}")
This prints "-> z", as expected. But consider:
x = { "y": "z" }
s = "-> {x['y']}"
print(s.format(x = x))
Traceback (most recent call last):
File "", line 1, in
KeyError: "'y'"
Even though
s = "-> {x}"
print(s.format(x = x))
Prints the expected "-> {'y': 'z'}".
This is supposedly for security reasons. However, when trying to emulate
this behavior that I wanted (and know the security implications of), my
solutions will tend to be less secure. Here is what I have been thinking
about:
1. Somehow wrap "s" into an f-string, then eval. E.g.:
eval("f'" + s + "'")
This is a pain in the ass because you have to know what kind of
quotation signs are used inside the expression. In the given case, this
wouldn't work (but 'f"' prefix and '"' suffix would).
2. Parse the expression (regex?), then eval() the individual arguments,
then run through format(). Pain in the ass to get the exact same
behavior as f-strings. Probably by regex alone not even guaranteed to be
parsable (especially corner cases with escaped '{' signs or ':' or '{'
included inside the expression as a literal).
3. Somehow compile the bytecode representing an actual f-string
expression, then execute it. Sounds like a royal pain in the butt, have
not tried it.
All solutions are extremely undesirable and come with heavy drawbacks.
Is there any standard solution (Py3.10+) that does what I would?
Anything I'm missing?
Thanks,
Johannes
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