[Python-Dev] Assignment expression and coding style: the while True case (original) (raw)

Victor Stinner vstinner at redhat.com
Thu Jul 5 08:13:48 EDT 2018

2018-07-05 9:10 GMT+02:00 Tim Peters <tim.peters at gmail.com>:

I'm all in favor of what Victor is doing: looking at how this stuff will work in actual code. That's a great antidote to the spread of theoretical fears.

FYI I'm trying to use assignment expressions on the stdlib because all examples of the PEP 572 look artificial to me.

Like "group = re.match(data).group(1) if re.match(data) else None" which is followed by "(TODO: Include Guido's evidence, and do a more systematic search.)" I cannot find such inefficient code in the stdlib. I'm not saying that nobody writes code like that, just that developers with a good Python expertise would avoid to write such code.

"filtered_data = [y for x in data if (y := f(x)) is not None]" also seems artificial. In the 711,617 lines of Python code of the stdlib, I only found one example:

labels = [label.strip() for label
                        in self._file.readline()[1:].split(b',')
                        if label.strip()]

=> https://github.com/python/cpython/pull/8098/files

And I also only found a single for loop which can be converted to a list comprehension thanks to assignement expression.

lines = []
for raw_line in raw_lines:
    match = line_pat.search(raw_line.strip())
    if match:
        lines.append(match.group(1, 2))

Wholesale changes to the std lib are unlikely to happen regardless. Broad patches just to spell things differently without need are discouraged.

So PEP 572 is purely syntax sugar? It doesn't bring anything to the stdlib for example?

My current 3 pull requests showing how assignment expressions can be used in the stdlib:

while True: https://github.com/python/cpython/pull/8095/files match/group: https://github.com/python/cpython/pull/8097/files list comp: https://github.com/python/cpython/pull/8098/files

Right now, I'm still not really excited by the new code.

If you spotted other parts of the stdlib where assignment expressions would be appropriate, please tell me and I will try to write more pull requests :-)

Victor

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