On Mon, Apr 30, 2018 at 11:32 AM Chris Angelico <rosuav@gmail.com> wrote:
On Tue, May 1, 2018 at 12:30 AM, Mark Shannon <mark@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]
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 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.
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 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
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:
do_things(plant_flowers==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