[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Tim Peters tim.peters at gmail.com
Mon Apr 23 11:57:13 EDT 2018

[Tim]

Which this alternative expresses directly:

if (diff := x - xbase) and (g := gcd(diff, n)) > 1: return g That's so Pythonic I could cry ;-)

[Antoine]

It looks like C to me. That won't make me cry (I write C++ code daily these days), but it's certainly not the same language as Python.

The second part, especially, where you use the result of an assignment expression as a comparison operand, looks definitely un-Pythonic.

You snipped the part explaining what's "Pythonic" about it:

It's _really_ an "and" test:  if the diff isn't 0 and gcd(diff, n) >
1, return the gcd.  That's how I _thought_ of it from the start.

"Expresses directly" is the Pythonic part; the syntax is minor to me. Seeing that the intent is an "and test" is a pattern-matching puzzle in the original spelling (which essentially turned me into a compiler, writing low-level code for the concepts I had in mind from the start):

diff = x - x_base if diff: g = gcd(diff, n) if g > 1: return g

But note that the part of the PEP I support is just the "binding expression" part: giving a simple name (binding an identifier) to the result of an expression. I don't want the full potential complexity of assignment statements in expressions. There's nothing "un-Pythonic" about merely giving a name to an expression result, apart from that there are few contexts that currently support that in a sanely usable way.

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