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

Chris Angelico rosuav at gmail.com
Fri Apr 20 22:30:36 EDT 2018

On Sat, Apr 21, 2018 at 11:38 AM, Anthony Flury via Python-Dev <python-dev at python.org> wrote:

I am entirely new to this list, but if I can I would like share my comments :

* I do think this proposal := has merit in my opinion; it does make some code more readable. * I think readability is only improved if :

There's that word "readability" again. Sometimes I wish the Zen of Python didn't use it, because everyone seems to think that "readable" means "code I like".

* The current expectations of how comprehensions work should also be honored; I don't claim to have fully followed all of the discussions around this, but it seems to me that comprehensions work in a particular way because of a concerted effect (especially in Python 3) to make them that way. They are self contained and don't leak values in their containing scope. Similarly I think that setting variables within a comprehension is just for the benefit of readable code within the comprehension - i.e. :

stuff = [[y, x/y] for x in range(5) for y in [f(x)]] can become : stuff = [[y := f(x), x/y] for x in range(5)] So - overall from me a conditional +1 - conditions as above; if they are not possible then -1 from me.

Perfectly self-contained. They do everything in their own scope. Except ... except that they don't.

$ python3.7 Python 3.7.0a4+ (heads/master:95e4d58913, Jan 27 2018, 06:21:05) [GCC 6.3.0 20170516] on linux Type "help", "copyright", "credits" or "license" for more information.

class X: ... x = ["spam", "ham"] ... print(["1: "+x for x in x]) ... print(["2: "+x for _ in range(1) for x in x]) ... ['1: spam', '1: ham'] Traceback (most recent call last): File "", line 1, in File "", line 4, in X File "", line 4, in UnboundLocalError: local variable 'x' referenced before assignment class Y: ... prefix = "3: " ... print([prefix + x for x in ["spam", "ham"]]) ... Traceback (most recent call last): File "", line 1, in File "", line 3, in Y File "", line 3, in NameError: name 'prefix' is not defined

Introducing expression assignments will make these oddities even more obvious. You'd be able to demonstrate things like this at function scope, not just with a class. But the oddities are there, and they are inherent to the current definition of a comprehension. That's why the changes are being recommended. They will simplify the peculiarities of comprehensions, make them far closer to a naive transformation into longhand, and extremely close to a non-naive-but-still-simple transformation into longhand.

ChrisA

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