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

MRAB python at mrabarnett.plus.com
Thu Jul 5 11:39:15 EDT 2018

On 2018-07-05 16:07, Victor Stinner wrote:

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.rootsection.formathelp() if help: help = self.longbreakmatcher.sub('\n\n', help) help = help.strip('\n') + '\n' return help Bad? if (help := self.rootsection.formathelp()): help = self.longbreakmatcher.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"). I think it's OK if it's:

 Do something, and if that succeeded, act on it.

but not if it's:

 Do something, but if that failed, do something else.

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