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

Steven D'Aprano steve at pearwood.info
Tue Apr 24 11:27:59 EDT 2018

On Tue, Apr 24, 2018 at 11:03:35AM -0400, Yury Selivanov wrote:

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.

That's a good thing. They will then learn not to write x = 1 as an expression.

Also, if I write lots of x := 1 binding-expressions as statements, my code is bad and deserves to fail code-review. But why would I write the extra colon (one character, two key-presses) to use

x := 1

as a statement, when x = 1 will work? That's a sure sign that I don't know what I'm doing. (Or that I desperately wish I was writing Pascal.)

I don't think we need worry about this. The sort of code that is filled with binding-expressions used as statements will almost certainly be so un-Pythonic and ugly in many other ways, that this won't make any difference.

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.

Indeed. That is true: either way, we introduce complexity into the language. (But that will allow us to reduce complexity in our code.) Given that increasing complexity is inevitable regardless of whether we choose PEP 572 or your suggestion, it is better to choose the option which keeps binding-expressions and assignment statements separate, since they are two different concepts.

-- Steve

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