[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Wes Turner]

On Friday, April 27, 2018, Chris Angelico <rosuav at gmail.com> wrote:

> On Fri, Apr 27, 2018 at 8:18 PM, Wes Turner <wes.turner at gmail.com> wrote:
> > IDK, I could just be resistant to change, but this seems like something
> that
> > will decrease readability -- and slow down code review -- without any
> real
> > performance gain. So, while this would be useful for golfed-down (!)
> > one-liners with pyline,
> > I'm -1 on PEP 572.
>
> PEP 572 has never promised a performance gain, so "without any real
> performance gain" is hardly a criticism.
>
> > How do I step through this simple example with a debugger?
> >
> >     if re.search(pat, text) as match:
> >         print("Found:", match.group(0))
>
> Step the 'if' statement. It will call re.search() and stash the result
> in 'match'. Then the cursor would be put either on the 'print' (if the
> RE matched) or on the next executable line (if it didn't).


Right. Pdb doesn't step through the AST branches of a line, so ternary
expressions and list comprehensions and defining variables at the end of
the line are 'debugged' after they're done.

Similarly, code coverage is line-based; so those expressions may appear to
be covered but are not.


>
> > From https://en.wikipedia.org/wiki/Assignment_(computer_science) :
> >
> >> In some languages the symbol used is regarded as an operator (meaning
> that
> >> the assignment has a value) while others define the assignment as a
> >> statement (meaning that it cannot be used in an expression).
> >
> >
> > PEP 572 -- Assignment Expressions
> > PEP 572 -- Assignment Operator (:=) and Assignment Expressions
>
> Huh? I don't get your point.


Q: What is ':='? (How do I searchengine for it?)
A: That's the assignment operator which only works in Python 3.8+.

Q: When are variables defined -- or mutable names bound -- at the end of
the expression accessible to the left of where they're defined?
Q: What about tuple unpacking? Is there an ECMAscript-like destructuring
PEP yet?
A: Ternary expressions; list, dict, generator comprehensions;
(@DOCS PLEASE HELP EXPLAIN THIS)

Q: do these examples of the assignment expression operator all work?

"""
- debuggers have no idea what to do with all of this on one line
- left-to-right doesn't apply to comprehensions

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

- left-to-right doesn't apply to ternary expressions

  if (y := func(x)) if (x := 3) else 0:
  while (y := func(x)) if (x := 3) else 0:

- left-to-right does apply to everything else?

- *these* are discouraged:

  if (x := 3) or (y := func(x)):
  if (3) or (func(3)):

  if ((x := 3) if 1 else (y := func(x))):
"""


> ChrisA
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