[Python-Dev] PEP 572: intended scope of assignment expression (original) (raw)

Victor Stinner vstinner at redhat.com
Thu Jul 5 11:07:58 EDT 2018

2018-07-05 15:14 GMT+02:00 Gustavo Carneiro <gjcarneiro at gmail.com>:

I don't know if you're trying to propose something clever here, like "if (x := func()):" would assign to 'x' only inside the "then" body of the if, but IMHO that would be a terrible idea:

I don't propose to change the PEP 572. I'm trying to explain to myself where "if (var := expr): ..." would add anything to readers compared to "var = expr; if var: ...". I know that var is still valid after the if.

I'm not asking to raise a syntax error or even emit a linter warning if var is used after the if. I'm just thinking loudly where the PEP 572 is appropriate.

IMHO the following code should not use assignement expression.

Good:

    help = self._root_section.format_help()
    if help:
        help = self._long_break_matcher.sub('\n\n', help)
        help = help.strip('\n') + '\n'
    return help

Bad?

    if (help := self._root_section.format_help()):
        help = self._long_break_matcher.sub('\n\n', help)
        help = help.strip('\n') + '\n'
    return help

IHMO using "help :=" here would send the wrong signal to the reader: as if help is not going to be used after the if, whereas it's the case ("return help").

Victor

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