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

[Chris Angelico]

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?

ChrisA