[Python-ideas] Re: writelines2?

[Steven D'Aprano]

On Tue, Jul 13, 2021 at 09:54:27AM -0400, Eric V. Smith wrote:

> I'm skeptical that people who won't read the documentation for 
> writelines will either read the documentation for writelines2, or 
> intuitively understand how it's different from writelines, or would know 
> there's a kwarg parameter to writelines.

Of course they won't. They'll just go on Twitter to bitch about what a 
horrible language Python is, because it's not "intuitive", until 
somebody points them to a few dozen StackOverflow posts that point out 
how to use it correctly.

*wink*

In this case I agree that the current API is a little surprising, but it 
seems to me that adding a kw parameter or a new method is an easy way to 
solve that. No, people who don't read the docs won't intuit their 
existence, but so long as *one* person reads the docs, the knowledge 
will slowly spread via mailing lists, StackOverflow, code reviews, etc.


> It seems your proposal would only matter if the argument were a list?

writelines doesn't require a list, it takes any iterable that yields 
strings.

(The docs are misleading: it still says "list".)

The current behaviour would have to remain the same, so the items 
would be written directly, whether or not they end with a newline. With 
the parameter, writelines would write a newline after each item in the 
iterable.



-- 
Steve
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[Python-ideas] Re: writelines2?

[Chris Angelico]

On Wed, Jul 14, 2021 at 11:22 AM Steven D'Aprano  wrote:
>
> On Tue, Jul 13, 2021 at 10:27:55PM +0300, Serhiy Storchaka wrote:
>
> > The writelines method is the part of interface. Adding new parameter or
> > new method to interface is a major breaking change. It will make invalid
> > all existing user code which implements this interface.
>
> I think that statement is a bit strong. How will it break existing code?
> The code will still work exactly the same as before.
>
> We added the BufferedIOBase.readinto1 method in Python 3.5, before that
> we added 'x' mode and `opener=None` parameter to FileIO in 3.3.
>
> https://docs.python.org/3/library/io.html#io.BufferedIOBase.readinto1
>
> https://docs.python.org/3/library/io.html#io.FileIO
>
> In 3.7 we allowed the BytesIO.read1 size parameter to be optional:
>
> https://docs.python.org/3/library/io.html#io.BytesIO.read1
>
> There are other changes to the IO interfaces through to Python 3.7. If
> that didn't break code, I don't see why this change would.
>
> I think that adding a new method or a keyword-only arg to writelines in
> 3.11 would be fine.
>

Breakage has to be justified. The value of readinto1 is more
significant than a change to writelines(), which - I'll be honest - is
an API that I've never bothered with. It's usually easiest to just
write().

But OTOH, the breakage here wouldn't be very significant. In theory,
it means adding the functionality to every file-like object. In
practice, nobody knows what "file-like object" really means, other
than "has that one method of file objects that I'm going to be
calling", so if other objects don't have this method or arg, it won't
hurt.

I'm -0 on this.

ChrisA
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[Python-ideas] Re: writelines2?

[Steven D'Aprano]

On Tue, Jul 13, 2021 at 08:59:21AM -0700, Guido van Rossum wrote:
> Let's leave poor writelines alone. It's an old API, if you want something
> different, there are a zillion other ways (e.g. print(*x, sep="\n") ).
> 
> I wonder how much research the OP did before they claimed "writelines is a
> very big misnomer to many python developers".

I don't know about research, but the API certainly fools me. I don't 
consider the delimiter at the end of the line (the newline) to be part 
of the line, so whenever I used readlines and writelines I am invariably 
surprised by the fact that writelines requires the newlines to already 
be there.

I'm also surprised that there's a readline but no writeline :-)


-- 
Steve
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[Python-ideas] Re: writelines2?

[Steven D'Aprano]

On Tue, Jul 13, 2021 at 10:27:55PM +0300, Serhiy Storchaka wrote:

> The writelines method is the part of interface. Adding new parameter or
> new method to interface is a major breaking change. It will make invalid
> all existing user code which implements this interface.

I think that statement is a bit strong. How will it break existing code? 
The code will still work exactly the same as before.

We added the BufferedIOBase.readinto1 method in Python 3.5, before that 
we added 'x' mode and `opener=None` parameter to FileIO in 3.3.

https://docs.python.org/3/library/io.html#io.BufferedIOBase.readinto1

https://docs.python.org/3/library/io.html#io.FileIO

In 3.7 we allowed the BytesIO.read1 size parameter to be optional:

https://docs.python.org/3/library/io.html#io.BytesIO.read1

There are other changes to the IO interfaces through to Python 3.7. If 
that didn't break code, I don't see why this change would.

I think that adding a new method or a keyword-only arg to writelines in 
3.11 would be fine.



-- 
Steve
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[Python-ideas] Re: writelines2?

[Steven D'Aprano]

On Tue, Jul 13, 2021 at 09:05:54AM -0700, Christopher Barker wrote:

> indeed. so if this idea is to be done (and there's something to be said for
> it), I think a similar option should be added to readlines as well --
> striping the newline.

I don't think that's as useful as you may expect. When processing lines 
of text, we often want to strip *any* trailing whitespace, not just 
newlines. So even if you strip the newline, you probably will still need 
to strip trailing whitespace. So why bother with stripping the newline? 
I think it will be extremely niche to want one but not the other.


> A couple other notes:
> 
> This would highlight the whole "a string is an iterable of strings" problem
> :-( -- should strings be special cased? I think not, that's an issue that
> Python programmers have to learn at one point or another anyway.

Sorry, have I missed something? How is this relevant? We're talking 
about a list of strings.


> As for whether it always puts in a newline, or if you can specify what you
> want to put in , I vote for the newline -- newlines are not the same on all
> platforms (though TextIO does translate), but I think it would get a bit
> confusing if someone explicitly put in "\n| and got "\r\n".

A precedent is str.splitlines:

splitlines(self, /, keepends=False)
Return a list of the lines in the string, breaking at line 
boundaries.

Line breaks are not included in the resulting list unless 
keepends is given and true.

except writelines (and readlines) would presumably reverse the default.

* readlines default to keeping the newlines
* writelines default to not appending newlines


> And the name IS writeLINES -- so why would anyone expect to use it for
> anything else -- we still have str.join() after all.

Repurposing functions to do something different from what the designers 
imagined is very common.


-- 
Steve
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[Python-ideas] Re: writelines2?

[Serhiy Storchaka]

13.07.21 14:15, sandhoners...@gmail.com пише:
> My suggestion is to have either a writelines2 or a newline kwarg which does 
> put new lines automatically at the end of every line written

The writelines method is the part of interface. Adding new parameter or
new method to interface is a major breaking change. It will make invalid
all existing user code which implements this interface.

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[Python-ideas] Re: writelines2?

[David Mertz]

I've written something like this far too many times:

lines = (line.rstrip('\n') for line in open(frame).readlines())

It's not unworkable, but it's definitely common to want the lines without
the newlines. The corresponding .writelines() hits me less, but it's still
a concern.

I slightly shorter and more intuitive spelling would be convenient. On the
model of print(), it seems like a 'sep=' argument would make sense, while
being backwards compatible. Obviously the default would need to be current
behavior.

On Tue, Jul 13, 2021, 10:08 AM Christopher Barker 
wrote:

> On Tue, Jul 13, 2021 at 9:00 AM <2qdxy4rzwzuui...@potatochowder.com>
> wrote:
>
>> As it stands, writelines is consistent with readlines.  Both preserve
>> newlines.
>>
>
> indeed. so if this idea is to be done (and there's something to be said
> for it), I think a similar option should be added to readlines as well --
> striping the newline.
>
> A couple other notes:
>
> This would highlight the whole "a string is an iterable of strings"
> problem :-( -- should strings be special cased? I think not, that's an
> issue that Python programmers have to learn at one point or another anyway.
>
> As for whether it always puts in a newline, or if you can specify what you
> want to put in , I vote for the newline -- newlines are not the same on all
> platforms (though TextIO does translate), but I think it would get a bit
> confusing if someone explicitly put in "\n| and got "\r\n".
>
> And the name IS writeLINES -- so why would anyone expect to use it for
> anything else -- we still have str.join() after all.
>
> -CHB
> --
> Christopher Barker, PhD (Chris)
>
> Python Language Consulting
>   - Teaching
>   - Scientific Software Development
>   - Desktop GUI and Web Development
>   - wxPython, numpy, scipy, Cython
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[Python-ideas] Re: writelines2?

[Eric V. Smith]

Ignore me on this, I wasn't focusing on the function at hand.

Eric

On 7/13/2021 10:18 AM, Eric V. Smith wrote:

On 7/13/2021 9:52 AM, Thomas Güttler wrote:



Am Di., 13. Juli 2021 um 15:02 Uhr schrieb >:


Right now, writelines is a very big misnomer to many python
developers, especially beginners who would expect writelines to
put new lines at the end of every element in the list

My suggestion is to have either a writelines2 or a newline kwarg
which does put new lines automatically at the end of every line
written


I like the idea of having a kwarg for writelines 
.


Do you want a boolean like "append_newlines=True" or a string like 
"append_string='\n'"


But it only applies if the argument is iterable, right?

That is, it would have no effect on something like:

fl.writelines(3, append_newlines=True)

Assuming so, the parameter would need to have some more appropriate 
name, like append_newlines_if_iterable.


Personally, I don't think this has any chance of being accepted, but 
that's me. There are numerous ways to achieve this already, and we 
don't need another one.


Eric


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--
Eric V. Smith

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[Python-ideas] Re: writelines2?

[Guido van Rossum]

Let's leave poor writelines alone. It's an old API, if you want something
different, there are a zillion other ways (e.g. print(*x, sep="\n") ).

I wonder how much research the OP did before they claimed "writelines is a
very big misnomer to many python developers".

On Tue, Jul 13, 2021 at 8:46 AM MRAB  wrote:

> On 2021-07-13 15:11, sandhoners...@gmail.com wrote:
> > Maybe (to be consistent with other functions like print), end= since
> that would allow even custom line endings
> >
> I'm not sure about the name 'end'. The 'print' function has 'end', but
> it's printed only at the end(!); .writelines would write it after every
> line.
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-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*

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[Python-ideas] Re: writelines2?

[2QdxY4RzWzUUiLuE]

On 2021-07-13 at 14:11:15 -,
sandhoners...@gmail.com wrote:

> Maybe (to be consistent with other functions like print), end= since
> that would allow even custom line endings

As it stands, writelines is consistent with readlines.  Both preserve
newlines.

All else being equal, the following copies a file:

input_file = open(input_pathname, 'r')
output_file = open(output_pathname, 'w')
lines = input_file.readlines()
output_file.writelines(lines)
input_file.close()
output_file.close()

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[Python-ideas] Re: writelines2?

[MRAB]

On 2021-07-13 15:18, Eric V. Smith wrote:

On 7/13/2021 9:52 AM, Thomas Güttler wrote:



Am Di., 13. Juli 2021 um 15:02 Uhr schrieb >:


Right now, writelines is a very big misnomer to many python
developers, especially beginners who would expect writelines to
put new lines at the end of every element in the list

My suggestion is to have either a writelines2 or a newline kwarg
which does put new lines automatically at the end of every line
written


I like the idea of having a kwarg for writelines 
.


Do you want a boolean like "append_newlines=True" or a string like 
"append_string='\n'"


But it only applies if the argument is iterable, right?

That is, it would have no effect on something like:

fl.writelines(3, append_newlines=True)


That raises an exception, and I wouldn't expect that to change.

Assuming so, the parameter would need to have some more appropriate 
name, like append_newlines_if_iterable.


Personally, I don't think this has any chance of being accepted, but 
that's me. There are numerous ways to achieve this already, and we don't 
need another one.



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[Python-ideas] Re: writelines2?

[MRAB]

On 2021-07-13 15:11, sandhoners...@gmail.com wrote:

Maybe (to be consistent with other functions like print), end= since that would 
allow even custom line endings

I'm not sure about the name 'end'. The 'print' function has 'end', but 
it's printed only at the end(!); .writelines would write it after every 
line.

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[Python-ideas] Re: writelines2?

[Paul Moore]

On Tue, 13 Jul 2021 at 15:56, Jonathan Fine  wrote:
>
> The interactive help message for writelines gives no help. I've made an 
> enhancement request to b.p.o.
>
> help(open('/dev/zero').writelines) gives no help
> https://bugs.python.org/issue44623
>
> Please take a look.

Works for me on Python 3.9:

>>> help(open("nul").writelines)
Help on built-in function writelines:

writelines(lines, /) method of _io.TextIOWrapper instance
Write a list of lines to stream.

Line separators are not added, so it is usual for each of the
lines provided to have a line separator at the end.
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[Python-ideas] Re: writelines2?

[Jonathan Fine]

The interactive help message for writelines gives no help. I've made an
enhancement request to b.p.o.

help(open('/dev/zero').writelines) gives no help
https://bugs.python.org/issue44623

Please take a look.

-- 
Jonathan
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[Python-ideas] Re: writelines2?

[sandhoners123]

It would produce a\nb\nc\n. Essentially it would put \n after every value 
gotten from a iterator
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[Python-ideas] Re: writelines2?

[Eric V. Smith]

On 7/13/2021 7:15 AM, sandhoners...@gmail.com wrote:

Right now, writelines is a very big misnomer to many python developers, 
especially beginners who would expect writelines to put new lines at the end of 
every element in the list

My suggestion is to have either a writelines2 or a newline kwarg which does put 
new lines automatically at the end of every line written

I'm skeptical that people who won't read the documentation for 
writelines will either read the documentation for writelines2, or 
intuitively understand how it's different from writelines, or would know 
there's a kwarg parameter to writelines.


It seems your proposal would only matter if the argument were a list? Or 
does it include tuples and other iterables? If lists, I assume that:


fl.writelines2(['a', 'b', 'c'])

would produce 3 lines:

a
b
c

But what would:

fl.writelines2('abc')

produce? It's an iterable.

Eric

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[Python-ideas] Re: writelines2?

[Eric V. Smith]

On 7/13/2021 9:52 AM, Thomas Güttler wrote:



Am Di., 13. Juli 2021 um 15:02 Uhr schrieb >:


Right now, writelines is a very big misnomer to many python
developers, especially beginners who would expect writelines to
put new lines at the end of every element in the list

My suggestion is to have either a writelines2 or a newline kwarg
which does put new lines automatically at the end of every line
written


I like the idea of having a kwarg for writelines 
.


Do you want a boolean like "append_newlines=True" or a string like 
"append_string='\n'"


But it only applies if the argument is iterable, right?

That is, it would have no effect on something like:

fl.writelines(3, append_newlines=True)

Assuming so, the parameter would need to have some more appropriate 
name, like append_newlines_if_iterable.


Personally, I don't think this has any chance of being accepted, but 
that's me. There are numerous ways to achieve this already, and we don't 
need another one.


Eric

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[Python-ideas] Re: writelines2?

[sandhoners123]

Maybe (to be consistent with other functions like print), end= since that would 
allow even custom line endings
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[Python-ideas] Re: writelines2?

[Thomas Güttler]

Am Di., 13. Juli 2021 um 15:02 Uhr schrieb :

> Right now, writelines is a very big misnomer to many python developers,
> especially beginners who would expect writelines to put new lines at the
> end of every element in the list
>
> My suggestion is to have either a writelines2 or a newline kwarg which
> does put new lines automatically at the end of every line written
>
>
I like the idea of having a kwarg for writelines
.

Do you want a boolean like "append_newlines=True" or a string like
"append_string='\n'"

Regards,
  Thomas
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