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PEP 572: Python assignment expressions has been accepted

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317 points est | 2 comments | 03 Jul 18 08:30 UTC | HN request time: 0.478s | source
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gbfowler ◴[03 Jul 18 09:04 UTC] No.17448582[source]▶
It is not syntactic sugar, "x := 10" is an assignment expression in contrast with "x = 10", which is a statement.

Hence the former can be used in contexts like "if x := 10: pass", which is the whole point of the PEP.

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Aardwolf ◴[03 Jul 18 09:14 UTC] No.17448624[source]▶
Why is it not syntactic sugar? It looks like convenience. You could argue everything above machine language is syntactic sugar.
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akvadrako ◴[03 Jul 18 09:30 UTC] No.17448721[source]▶
Not so. Many things change the semantics, otherwise high level languages would just be glorified macros.

For example, import mod is NOT defined as

  mod = eval(open("mod.py").read())
but involves abstract load module operation, which is dependant on the environment.

That's why := is just syntactic sugar; there are no new semantics.

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junke ◴[03 Jul 18 09:45 UTC] No.17448810[source]▶
Honestly curious, how do you expand a "x := y" expression into an expression that returns y and affects x as a side-effect?
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1. akvadrako ◴[03 Jul 18 11:23 UTC] No.17449244[source]▶
I'm not sure exactly, but my point is that a program transformation can do it. However, no transformation can change an import to other operations while perserving semantics.

Just for fun, this seems to work:

  (locals().pop('x', None), locals().setdefault('x', y))[1]
replies(1): >>17449346 #
2. yorwba ◴[03 Jul 18 11:47 UTC] No.17449346[source]▶
That only works at top-level scope, where locals() == globals(). Inside a function, changing the dict returned by locals() doesn't actually change the variable.