Confusing error message: lambda walruses

Peter J. Holzer hjp-python at
Sun Oct 3 07:44:20 EDT 2021

On 2021-10-03 10:39:38 +1100, Chris Angelico wrote:
> Using assignment expressions in lambda functions sometimes works, but
> sometimes doesn't.

> ...     return lambda x: (x, x := 2, x)
syntax ok

>     return lambda: n := 1
syntax error

> ...     return lambda: (n := 1)
syntax ok

>     return (lambda: n := 1)
syntax error

> ...     return (lambda: (n := 1)), (lambda: n)
syntax ok, but doesn't assign

> What's going on here?

I think it's the parentheses:

The right side of the lambda is defined as an «expression», not an an
«assignment_expression», and there is no direct derivation from one to
the other in the grammar. So you can't put an «assignment_expression» there
directly. But if you put it in parentheses, it becomes a «starred_item»
which is a «starred_expression» which is then part of the «parenth_form»
which is an «enclosure» which is an «atom» which is (through several
more steps) an «expression» which can be used on the right side of a


   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at         |    -- Charles Stross, "Creative writing
__/   | |       challenge!"
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