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

Łukasz Langa lukasz at langa.pl
Wed Apr 25 16:21:19 EDT 2018

PEP 572 caused a strong emotional reaction in me. I wanted to first understand
my intuitive objection to the idea before posting anything.

I feel that (name := expression) doesn't fit the narrative of PEP 20. It
doesn't remove complexity, it only moves it. What was its own assignment before
now is part of the logic test. This saves on vertical whitespace but makes
parsing and understanding logic tests harder. This is a bad bargain: logic
tests already contain a lot of complexity that human readers have to cope with.

Proponents of := argue it makes several patterns flatter (= better than nested)
to express. Serial regular expression matching is a popular example. However,
(name := expression) itself is making logic tests more nested, not flatter. It
makes information in the logic test denser (= worse than sparse). Since it also
requires an additional pair of parentheses, it forces the reader to decompose
the expression in their head.

:= also goes against having one obvious way to do it. Since it's an expression,
it can also be placed on its own line or in otherwise weird places like
function call arguments. I anticipate PEP 8 would have to be extended to
explicitly discourage such abuse. Linters would grow rules against it. This is

I'm -1 on PEP 572, I think it's very similar in spirit to the rejected PEP 463.

-- Ł

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