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

Chris Angelico rosuav at gmail.com
Mon Apr 30 11:30:37 EDT 2018

On Tue, May 1, 2018 at 12:30 AM, Mark Shannon <mark at hotpy.org> wrote:

List comprehensions ------------------- The PEP uses the term "simplifying" when it really means "shortening". One example is stuff = [[y := f(x), x/y] for x in range(5)] as a simplification of stuff = [(lambda y: [y,x/y])(f(x)) for x in range(5)]

Now try to craft the equivalent that captures the condition in an if:

results = [(x, y, x/y) for x in input_data if (y := f(x)) > 0]

Do that one with a lambda function.

IMO, the "simplest" form of the above is the named helper function.

def meaningfulname(x): t = f(x) return t, x/t [meaningfulname(i) for i in range(5)] Is longer, but much simpler to understand.

Okay, but what if there is no meaningful name? It's easy to say "pick a meaningful name". It's much harder to come up with an actual name that is sufficiently meaningful that a reader need not go look at the definition of the function.

I am also concerned that the ability to put assignments anywhere allows weirdnesses like these:

try: ... except (x := Exception) as x: ... with (x: = open(...)) as x: ...

We've been over this argument plenty, and I'm not going to rehash it.

def dothings(firemissiles=False, plantflowers=False): ... dothings(plantflowers:=True) # whoops!

If you want your API to be keyword-only, make it keyword-only. If you want a linter that recognizes unused variables, get a linter that recognizes unused variables. Neither of these is the fault of the proposed syntax; you could just as easily write this:

do_things(plant_flowers==True)

but we don't see myriad reports of people typing too many characters and blaming the language.

ChrisA

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