[Python-Dev] assignment expressions: an alternative proposal (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Tue Apr 24 11:03:35 EDT 2018

On Tue, Apr 24, 2018 at 10:56 AM, Chris Angelico <rosuav at gmail.com> wrote: [..]

A lot of other questions arise though. PEP 572 proposes:

a = 1 # assignment a := 1 # also assignment (a := 1) # also assignment (a = 1) # error, why? Your third example is just the same as the second, with parentheses around it. In most of Python, parentheses (if legal) have no effect other than grouping; "a + b * c" is the same thing as "(a + b) * c", just done in the other order. The last one is a clear demonstration that "=" is a statement, not an expression. Are people confused by this sort of thing: if x > 1: print("x is more than 1") (if x > 1:) print("SyntaxError")

This is a very far-fetched example :)

My point was that when you see lots of '=' and ':=' used at the statement level, one might try to write "if x = 1" instead of "if x := 1" -- boom, we have an unexpected SyntaxError for some users.

In my opinion adding any assignment expression syntax to Python will create this sort of issues. PEP 572 isn't free of them, my proposal isn't free of them. My proposal doesn't add a new ':=' operator at the cost of slightly complicating rules around '='. PEP 572 avoids complicating '=', but adds an entirely new form of assignment.

Yury

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