[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 Sat, Apr 28, 2018 at 6:06 AM, Wes Turner <wes.turner at gmail.com> wrote:
> >
> >
> > 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.
>
> I'm not sure I follow. In what situation would some code appear to be
> covered when it isn't, due to an assignment expression?


When an assignment expression is in the else clause of a ternary
expression, but the else clause does not execute because the condition is
true, the assignment expression does not execute and so isn't covered.

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

Is this a new opportunity for misunderstanding?

Assignment expressions, though they are noticeable :=, may not actually
define the variable in cases where that part of the line doesn't run but
reads as covered.




>
> ChrisA
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