Walrus operator is not allowed inside the iterable of a comprehension which is itself the iterable of an outer comprehension

[Adarsh-Roy]

Bug report

Bug description:

The following works:

return sum(map(prod,pairwise(c for r in b if (c:=r.count('1')))))

But the following does not work:

return sum(a*b for a,b in pairwise(c for r in b if (c:=r.count('1'))))

The error:

SyntaxError: assignment expression cannot be used in a comprehension iterable expression
                                                        ^^^^^^^^^^^^^^^
    return sum(a*b for a,b in pairwise(c for r in b if (c:=r.count('1'))))
Line 3  (Solution.py)

CPython versions tested on:

3.11

Operating systems tested on:

Linux

[skirpichev]

N.B., following works:

>>> from itertools import product, pairwise
>>> def f2(b):
...     xxx = pairwise(c for r in b if (c:=r.count('1')))
...     return sum(a*b for a,b in xxx)
...     
>>> f2("12311145")
3

[terryjreedy]

The doc is here. It says an assignment expression is allowed in comprehension if-expressions if surrounded in parentheses. However, the error message suggests an intentional exception within "comprehension iterable expressions" and pairwise(c for r in b if (c:=r.count('1'))) is an example of such. So this may be handled as a doc fix rather than a code fix.

[pochmann3]

It says an assignment expression is allowed in comprehension if-expressions if surrounded in parentheses.

Not really. It says "Assignment expressions must be surrounded by parentheses when used [in] comprehension-if expressions". If I state "To enter a locked house, you must use a key", does that allow you to enter my house if you find the key?

[lysnikolaou]

This is explicitly specified in PEP 572. If you wanna advocate for changing this, there should probably be a wider discussion about it.