[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20 (original) (raw)
Wes Turner wes.turner at gmail.com
Fri Apr 27 16:06:23 EDT 2018
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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(computerscience) : > >> 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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