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Tue Sep 8 09:17:38 CEST 2015 · https://mail.python.org/pipermail/python-dev/2015-September/141507.html

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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[Python-Dev] String Interpolation Best Practices