Misleading SyntaxError: cannot use assignment expressions with lambda

[wyattscarpenter]

Bug report

Bug description:

Python 3.12.8 (tags/v3.12.8:2dc476b, Dec  3 2024, 19:30:04) [MSC v.1942 64 bit (AMD64)] on win32
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>>> lambda x: foo := 2
  File "<stdin>", line 1
    lambda x: foo := 2
    ^^^^^^^^^^^^^
SyntaxError: cannot use assignment expressions with lambda
>>> lambda x: (foo := 2)
<function <lambda> at 0x00000229CC40E0C0>

IIRC, until today I believed that you could never use an assignment expression in a lambda. I was disabused of this notion only by reading PEP 572 itself. I believe the SyntaxError should ideally read something along the lines of "cannot use unparenthesized assignment expressions with lambda".

I wasn't sure to what category to file this under exactly, sorry if I chose wrong.

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[terryjreedy]

I agree that the error message should include 'unparenthesized'.

[terryjreedy]

Valid back to at least 3.8, and using the binding, even somewhat sensibly, is possible.

>>> (lambda x: ((foo := 2*x), foo**2))(3)
(6, 36)
>>> (lambda x: foo if (foo := 2*x) > 5 else foo**2)(3)
6

[serhiy-storchaka]

It is parsed as (lambda x: foo) := 2. So, including 'unparenthesized' will not help -- assignment to parenthesized lambda does not work either.

Other ambiguous examples, such as a = b := c or a, b := c, raise just "invalid syntax".

[wyattscarpenter]

@serhiy-storchaka It's the assignment expression that should be described as unparenthesized, not the lambda expression :)