[Python-Dev] String Interpolation Best Practices

[Guido van Rossum]

Sounds awfully premature. Style guides are typically updated in response to
the occurrence of bad practice in the wild, not in anticipation of such bad
practice. I would give the users of Python 3.6 some more credit.

On Tue, Sep 8, 2015 at 12:17 AM, Mike Miller <python-dev at mgmiller.net>
wrote:

> Hi,
>
> I'd like to collect thinking on best practices that we can use as a style
> guide for string interpolation.  Now that arbitrary expressions are very
> likely to be included, it is more important to set guidelines than it would
> otherwise be.
>
> Below is a recent post with some good ideas (though it hopes to restrict
> expressions, which is not what we're discussing here, but rather creation
> of a style-guide for code-review a la PEP8).
>
> Would anyone else like to contribute?
>
> -Mike
>
> Recent posts:
>
> - On PEPs to augment PEP8:
>
>
> https://mail.python.org/pipermail/python-dev/2015-September/141473.html
>
> - On things to avoid in f-strings:
>
>
> https://mail.python.org/pipermail/python-dev/2015-September/141451.html
>     (part included below)
>
>
> On 09/05/2015 02:10 AM, haypo s (Victor Stinner) wrote:
> > Would it be possible to specify a subset of the Python language
> > allowed in f-string? For example, __import__ or lambda should not be
> > used in a f-string. I'm not convinced that a loop or
> > list/dict/set-comprehension is a good idea neither.
> >
> > I would prefer to keep as much code as possible *outside* f-string
> because:
> > - text editor knows well how to color it
> > - static analyzers know how to analyze it
> > - it encourage developers to indent and comment their code correctly,
> > whereas f-string has more restrictons on indentation (is it possible
> > to indent and comment code inside a f-string?)
> >
> > For example, for me it's a common practice to write a complex
> > list-comprehension on two lines for readability:
> >
> > newlist = [very_complex_expression(item)
> >              for item in oldlist]
> > # sorry, it's hard to indent correctly in a mail client, especially Gmail
> >
> > Well, I'm not convinced that we need a larger subset than what is
> > allowed currently in str.format(), simple expressions like: obj.attr,
> > obj[index], etc.
> >
> > I recall horrible examples in the previous mail threads showing how
> > much complex code you can put inside f-string.
> >
> > Even the following example from the PEP seems too complex to me:
> > print(f"Usage: {sys.argv[0]} [{'|'.join('--'+opt for opt in
> > valid_opts)}]", file=sys.stderr)
> >
> > Oh, first I read [...] as a list-comprehension :-p But it's part of
> > the output string, not of the Python code...
> >
> > I prefer to build the second parameter outside the f-string:
> > opts = '|'.join('--'+opt for opt in valid_opts)
> > print(f"Usage: {sys.argv[0]} [{opts}]", file=sys.stderr)
> >
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-- 
--Guido van Rossum (python.org/~guido)
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