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

Yury Selivanov yselivanov.ml at gmail.com
Mon Apr 30 12:53:32 EDT 2018

On Mon, Apr 30, 2018 at 11:32 AM Chris Angelico <rosuav at gmail.com> wrote:

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 inputdata if (y := f(x)) > 0]

Easy:

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

Longer, but way more readable and debuggable if you're into that. This has worked for us many years and only a handful of people complained about this.

OTOH, I see plenty of people complaining that nested list comprehensions are hard to read. In my own code reviews I ask people to avoid using complex comprehensions all the time.

Do that one with a lambda function.

Why would I? Is using lambda functions mandatory?

> 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.

That's a weird argument, Chris :-)

If f(x) has no meaningful name, then what is the result of the comprehension? Perhaps some meaningless data? ;)

> 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.

Hand-waving the question the way you do simply alienates more core devs to the PEP. And PEP 572 hand-waves a lot of questions and concerns. Asking people to dig for answers in 700+ emails about the PEP is a bit too much, don't you agree?

I think it's PEP's author responsibility to address questions right in their PEP.

> 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

Another hand-waving. Should we deprecate passing arguments by name if their corresponding parameters are not keyword-only?

Mark shows another potential confusion between '=' and ':=' that people will have, and it's an interesting one.

want a linter that recognizes unused variables, get a linter that recognizes unused variables.

Many want Python to be readable and writeable without linters.

Neither of these is the fault of the proposed syntax; you could just as easily write this:

dothings(plantflowers==True)

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

Strange. I see people who struggle to format their code properly or use the language properly every day ;)

Yury

More information about the Python-Dev mailing list