[Python-Dev] PEP 572: Assignment Expressions

[Chris Angelico]

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 meaningful_name(x):
>     t = f(x)
>     return t, x/t
>
> [meaningful_name(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 do_things(fire_missiles=False, plant_flowers=False): ...
> do_things(plant_flowers:=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