[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Paul G paul at ganssle.io
Mon Apr 23 18:36:18 EDT 2018

On 04/23/2018 06:04 PM, Tim Peters wrote:

However, against "as" is that its current use in "with" statements does something quite different:

with f() as name: does not bind the result of f() to name, but the result of f()._enter_(). Whether that "should be" fatal, I don't know, but it's at least annoying ;-)

This could be read a different way, though, since with f() calls f().__enter__(), you could read it as (with f()) as name:, in which case it does the same thing as an as-based binding expression would. Viewing it that way also helps alleviate the cognitive problem that with f() as name and with (f() as name) do two different things - the parentheses there are changing the precedence in the same way that 2 + 4 * 3 and (2 + 4) * 3 do two different things.

This sorta also works for except, if you read it as (except SomeException) as e:, but I think this fiction falls apart when you look at import, since import foo already binds "foo" to a name, and import foo as bar not only binds foo to bar, but also doesn't bind foo to foo.

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