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

Nick Coghlan ncoghlan at gmail.com
Sun Apr 22 01:48:05 EDT 2018

On 22 April 2018 at 03:41, Steven D'Aprano <steve at pearwood.info> wrote:

We already have the rule that the outermost iterable is special, except it isn't a rule precisely, since (as far as I know) it isn't documented anywhere, nor was it ever planned as a feature.

It's a deliberate feature of generator expressions: https://www.python.org/dev/peps/pep-0289/#the-details

Covered in the language reference here: https://docs.python.org/3/reference/expressions.html#generator-expressions

It's then inherited by comprehensions through the intentional semantic equivalences between:

[x for x in iterable] <-> list(x for x in iterable)
{x for x in iterable} <-> set(x for x in iterable)
{k(x):v(x) for x in iterable} <-> set((k(x), v(x) for x in iterable))

The consequences of those equivalences weren't historically spelled out in the language reference, but we fixed that omission when deprecating the use of "yield" & "yield from" in comprehensions for 3.7: https://github.com/python/cpython/commit/73a7e9b10b2ec9636e3c6396cf7b3695f8ed1856 (we had to in order to explain why you can still use "yield" and "yield from" in the outermost iterable - it's because the restriction only applies to the implicitly nested scope).

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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