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

Mike Miller python-dev at mgmiller.net
Fri Apr 20 12:50:29 EDT 2018

On 2018-04-19 23:52, Chris Angelico wrote:

And are limited to conditions that check the truthiness/falsiness of the value you care about. So that works for re.match, but not for anything that might return -1 (a lot of C APIs do that, so if you're working with a thin wrapper, that might be all you get), and it'll encourage people to use this form when "is not None" would be more appropriate (setting up for a failure if ever the API returned a

From the previously discussed code, it might look like this:

 while (file.get_next_token() as token) != -1:
     doc += token

Shouldn't be needed often, but I find it readable enough.

More generally, I've been -0 on this idea because I've come to appreciate Python's less-clever i.e. "dumb" loop syntax, and ":=" combined with assignment-expressions doesn't feel like Python at all but rather Pascal and C had a love-child, haha.

I could mildly support the "as" syntax however, since it is so darn readable and has analogues in other places.

That leaves what to do with "with". Guess I missed the part in the discussion where we couldn't fit the syntax into it. Would requiring parens here not work?

 with (expr() as name) as conman:
     pass

This should rarely be necessary or useful, correct? Perhaps disallow for now.

On assignment to names/subscripts, just names sounds simpler for the first round.

Also the current "while" itself could be a bit simpler by making the expression optional and slightly less verbose:

 while:
     points = learner.get(static_hint)
     if not points:
        break

Thanks for the hard work, -Mike

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