[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Rob Cliffe via Python-ideas]

The fact that it is not allowed is nothing to do with the 'return'. You 
*can* write

    return (x := x+1)
What you can't (at present) do is use the walrus operator with an attribute:
    x = (self.a := self.a + 1) # SyntaxError
I too have found times when this would be convenient.
As Stephen says, Python often adds new features conservatively, then 
extends them later if/when it seems desirable.  This happened with the 
'@' decorater: originally it came with all manner of restrictions, but 
eventually they were removed.  IMO this was a good thing because it made 
the language more uniform, meaning you didn't have to remember what was 
allowed and what wasn't; so I hope the same will happen with the walrus 
operator.

Best wishes
Rob Cliffe

On 09/10/2023 02:17, Dom Grigonis wrote:

Is there a reason why this is not allowed?
return  (self.mode :=  self.mode_valid(mode))

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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Stephen J. Turnbull]

Dom Grigonis writes:

 > Why would this not be a good option? 1 extra line compared to
 > walrus, but no DRY issue.
 >   with open(“fn") as f:
 > while True:
 > line = f.readline()
 > if line and check(line):
 > process(line)
 > else:
 > break

Works for me.  I suspect you can construct situations where it would
be a lot uglier.  But I'm not a fan of the walrus in the first place,
so I'm not going to take this any further.

Steve
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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Dom Grigonis]

I just want to note that this is just a gentle suggestion/observation, in case 
this wasn’t done yet.

I am not pushing this.

——

All good points, thank you.

——

Why would this not be a good option? 1 extra line compared to walrus, but no 
DRY issue.
with open(“fn") as f:
while True:
line = f.readline()
if line and check(line):
process(line)
else:
break
DG

> On 9 Oct 2023, at 15:52, Stephen J. Turnbull 
>  wrote:
> 
> Dom Grigonis writes:
> 
>> Mistake, late night. I just meant to portray initialisation via
>> walrus inside statement, nothing more. It doesn’t work with while
>> loop.
> 
> It *does* work with a while loop, just not that one.  See below.  The
> problem is that you're writing new code full of trash you don't need
> to explain the concept.  Keep these examples as simple as possible.
> For example, my "pass" example is too simple if you want to show how
> the walrus can save typing because it doesn't save any lines of code;
> you need something a little more complicated to demonstrate that
> saving.
> 
> In the progress of any proposal to acceptance, at some point someone
> is going to say, "I see that it works and solves a theoretical
> problem, but who would ever use it?"  At that point you go into a
> large body of code such as the Python standard library or numpy to
> find realistic examples.  But that's old code, and only needed to
> demonstrate actual utility.
> 
>> Generally, this would be useful for all examples of
>> https://peps.python.org/pep-0572/ 
>> , where “initial” value is an attribute or dictionary item.
> 
> That's not the question though.  The question is are those examples
> themselves frequently useful.  And that's when you go find them in the
> stdlib.
> 
>> I see. Could you give an example of this? I vaguely remember this
>> sometimes being the case, but can not find anything now on this in
>> relation to walrus operator. Is there no way to bring that
>> execution within the body without walrus operator?
> 
> The problem is code like this:
> 
>with open("a_file") as f:
>while (line := f.readline())
>if check(line):
>process(line)
>else:
>break
> 
> There is no way to do that without an assignment, and without the
> walrus you need an assignment before the while, and the same
> assignment within the loop body.  Is that a big deal?  No.  In fact, I
> have never used the walrus operator in anger, nor wanted to.  I'm just
> as happy to write
> 
>with open("a_file") as f:
>line = f.readline()
>while (line)
>if check(line):
>process(line)
>else:
>break
>line = f.readline()
> 
> But some people find loop-and-a-half extremely ugly, and while I
> differ on "extremely", I can't deny that it's ugly.
> 
> You *could* do this for small files with
> 
>with open("a_file") as f:
>lines = f.readlines()
>for (line in lines):
>if check(line):
>process(line)
>else:
>break
> 
> but that's not possible with an infinite iterable, undesireable for
> most non-file streams, etc.
> 
> Re "simple examples", see why I used the "if check()" stuff?  If I
> wasn't going to talk about infinite iterables and pausing external
> streams, that would just be (potentially wrong!) complexity that
> doesn't help explain anything.
> 
> Steve
> 
> 

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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Stephen J. Turnbull]

Dom Grigonis writes:

 > Mistake, late night. I just meant to portray initialisation via
 > walrus inside statement, nothing more. It doesn’t work with while
 > loop.

It *does* work with a while loop, just not that one.  See below.  The
problem is that you're writing new code full of trash you don't need
to explain the concept.  Keep these examples as simple as possible.
For example, my "pass" example is too simple if you want to show how
the walrus can save typing because it doesn't save any lines of code;
you need something a little more complicated to demonstrate that
saving.

In the progress of any proposal to acceptance, at some point someone
is going to say, "I see that it works and solves a theoretical
problem, but who would ever use it?"  At that point you go into a
large body of code such as the Python standard library or numpy to
find realistic examples.  But that's old code, and only needed to
demonstrate actual utility.

 > Generally, this would be useful for all examples of
 > https://peps.python.org/pep-0572/ 
 > , where “initial” value is an attribute or dictionary item.

That's not the question though.  The question is are those examples
themselves frequently useful.  And that's when you go find them in the
stdlib.

 > I see. Could you give an example of this? I vaguely remember this
 > sometimes being the case, but can not find anything now on this in
 > relation to walrus operator. Is there no way to bring that
 > execution within the body without walrus operator?

The problem is code like this:

with open("a_file") as f:
while (line := f.readline())
if check(line):
process(line)
else:
break

There is no way to do that without an assignment, and without the
walrus you need an assignment before the while, and the same
assignment within the loop body.  Is that a big deal?  No.  In fact, I
have never used the walrus operator in anger, nor wanted to.  I'm just
as happy to write

with open("a_file") as f:
line = f.readline()
while (line)
if check(line):
process(line)
else:
break
line = f.readline()

But some people find loop-and-a-half extremely ugly, and while I
differ on "extremely", I can't deny that it's ugly.

You *could* do this for small files with

with open("a_file") as f:
lines = f.readlines()
for (line in lines):
if check(line):
process(line)
else:
break

but that's not possible with an infinite iterable, undesireable for
most non-file streams, etc.

Re "simple examples", see why I used the "if check()" stuff?  If I
wasn't going to talk about infinite iterables and pausing external
streams, that would just be (potentially wrong!) complexity that
doesn't help explain anything.

Steve


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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Dom Grigonis]

On 9 Oct 2023, at 08:58, Stephen J. Turnbull 
 wrote:
> ...
> Not sure what you're getting at here, that's an infloop.  Did you mean
> something like this:
> 
>class A:
>def func(self):
>while (self.a := self.a += 1) < 5:
>pass
>return self.a
Mistake, late night. I just meant to portray initialisation via walrus inside 
statement, nothing more. It doesn’t work with while loop. The intention was 
this, which is also principal use-case from previous reply:
class A:
def func(self, string):
if self.a := re.match('.*', string):
print(self.a)
return self.a.group()
Generally, this would be useful for all examples of
https://peps.python.org/pep-0572/ 
, where “initial” value is an attribute or dictionary item.

>> Same argument as for “walrus” operator itself - convenient
>> feature. Or is there more to it?
> 
> There's more to it.  The loop and a half construct, where you execute
> the body of the loop once outside the loop for some reason is widely
> considered a really big wart (DRY failure).  The walrus allows us to
> eliminate most of those.
> 
> On the other hand, an assignment and a return statement are two
> different things; it's not a DRY viotion to have the same identifier
> in both.
I see. Could you give an example of this? I vaguely remember this sometimes 
being the case, but can not find anything now on this in relation to walrus 
operator. Is there no way to bring that execution within the body without 
walrus operator?

>> I was actually surprised that this didn’t work - I thought that
>> this operator is a literal composite of assignment and “retriever",
>> rather than having it’s own restrictive logic.
> 
> This is a common practice in Python development.  Try a little bit of
> a new idea, avoid engineering, and expand later if that seems useful
> too.
Fair.

>> If it doesn’t break anything, doesn’t have any undesirable side
>> effects and community likes it, then could be a good addition.
> 
> "Community likes it" is a null predicate, very hard to verify in edge
> cases.  When the support in the community is strong enough that
> "community likes it" is common knowledge, it's always the case that
> there are objective reasons why it's a good thing.
> 
> All additions have a 0 * infinity cost: a negligible cost of learning
> (for one user) times *all* the users.
Ok, thanks.

DG



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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Stephen J. Turnbull]

Dom Grigonis writes:

 > This nuance can also be encountered in “principal use-case”. E.g.:
 > class A:
 > def func(self):
 > while (self.a := 1) < 5:
 > self.a += 1
 > return self.a

Not sure what you're getting at here, that's an infloop.  Did you mean
something like this:

class A:
def func(self):
while (self.a := self.a += 1) < 5:
pass
return self.a

I'm pretty sure that exact idiom, modeled on the C "copy string" idiom

void strcpy(char *s, char *t) {
while (*t++ = *s++)

would generally be considered unpythonic, but I suppose some people
might like it if instead of pass you had a real suite there.  I think
it's a real "meh" use case, since in most cases it can be rewritten

class A:
def func(self):
while self.a < (5 - 1):  # expression to make the
 # transformation clear
self.a += 1
return self.a

and you save a couple characters and at most one line (none in the
case you present).

 > Same argument as for “walrus” operator itself - convenient
 > feature. Or is there more to it?

There's more to it.  The loop and a half construct, where you execute
the body of the loop once outside the loop for some reason is widely
considered a really big wart (DRY failure).  The walrus allows us to
eliminate most of those.

On the other hand, an assignment and a return statement are two
different things; it's not a DRY viotion to have the same identifier
in both.

 > I was actually surprised that this didn’t work - I thought that
 > this operator is a literal composite of assignment and “retriever",
 > rather than having it’s own restrictive logic.

This is a common practice in Python development.  Try a little bit of
a new idea, avoid engineering, and expand later if that seems useful
too.

 > If it doesn’t break anything, doesn’t have any undesirable side
 > effects and community likes it, then could be a good addition.

"Community likes it" is a null predicate, very hard to verify in edge
cases.  When the support in the community is strong enough that
"community likes it" is common knowledge, it's always the case that
there are objective reasons why it's a good thing.

All additions have a 0 * infinity cost: a negligible cost of learning
(for one user) times *all* the users.

Other projects feel differently about it, but Python tends to be quite
conservative about additions.

Steve
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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[Dom Grigonis]

> Having:
> 
>self.mode = self.mode_valid(mode)
>return self.mode
> 
> isn't too bad.

No, not bad at all. But to me, being able to use walrus would be convenient.

———

This nuance can also be encountered in “principal use-case”. E.g.:
class A:
def func(self):
while (self.a := 1) < 5:
self.a += 1
return self.a

A().func()
Could be useful in iterator protocol implementations, where/or_where state is 
managed with property setters managing objects that implement  custom 
`__iadd__`, etc or just simply convenience as in my former example.

Same argument as for “walrus” operator itself - convenient feature. Or is there 
more to it?

———

I was actually surprised that this didn’t work - I thought that this operator 
is a literal composite of assignment and “retriever", rather than having it’s 
own restrictive logic.

Could be a reason for it, maybe to avoid some undesirable mis-usage. But I 
can’t see it. Any issues arising from such construct would be the same issues 
as for:
expr = 1
expr
This could be an actual assignment + retrieval of left-hand-side, then it would 
work generically for all constructs, such as:
a()[‘a'] := 1
a().attr := 1
If walrus currently has its own specific logic, this would integrate it nicely 
into assignment operator. Then it would benefit automatically from any language 
extension that makes use of it.

Or is it already it, just with added restrictions?

All in all, I think would be a generalisation of a convenience feature, which 
doesn’t invent anything new, but extends its convenience to more general 
application.

These are my thoughts, whether it’s worthwhile and sensible - I have no idea.

If it doesn’t break anything, doesn’t have any undesirable side effects and 
community likes it, then could be a good addition.

Regards,
DG

P.S. Deferred evaluation will be able to handle this one too___
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[Python-ideas] Re: SyntaxError: cannot use assignment expressions with attribute

[MRAB]

On 2023-10-09 02:17, Dom Grigonis wrote:

Is there a reason why this is not allowed?


return  (self.mode :=  self.mode_valid(mode))


The principal use-case for the operator is in conditions, for example:
if m := re.match(pattern_1, string):
...
elif m := re.match(pattern_2, string):
...
else:
...

or:

while line := file.readline():
...

Do you have a convincing argument that it should be expanded from a 
simple name?


Having:

self.mode = self.mode_valid(mode)
return self.mode

isn't too bad.
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