When setting defaults to keyword-only arguments in lambdas which are inside non-global scopes, cPython doesn't push the name to it's closure's co_freevars.

EXAMPLE:
global_variable = None
(lambda: (lambda *, keyword_only=global_variable: None))()

Because the inner lambda hasn't told the outer lambda to put global_variable in co_freevars, it fails to create the default to keyword_only. This only happens if the inner function is a lambda and you are setting a keyword_only variable.

It does not cause a crash if global_variable is local to the outer lambda, as the opcode LOAD_FAST is still created properly (as opposed to LOAD_NAME).

It does not crash if global_variable is used elsewhere in the outer function as co_freevars will be updated with it, allowing LOAD_NAME to retrieve it.



I've never done a bug here before and I'm unsure what to say, so please be nice and correct me if I'm doing it wrong.

Here is a patch, with a minimal test.

same patch, without tabs.

Patch looks good to me.

New changeset 1e0e821d2626 by Amaury Forgeot d'Arc in branch '3.2':
Issue #13343: Fix a SystemError when a lambda expression uses a global
http://hg.python.org/cpython/rev/1e0e821d2626

New changeset bddb455439d0 by Amaury Forgeot d'Arc in branch 'default':
Issue #13343: Fix a SystemError when a lambda expression uses a global
http://hg.python.org/cpython/rev/bddb455439d0

It was a bug in Python compiler, thanks for the report!

Glad to help :)
It's made my day. I get to boast at school now!

> It's made my day. I get to boast at school now!
You could do more of that if you got a patch committed!  See http://docs.python.org/devguide and http://mail.python.org/mailman/listinfo/core-mentorship for more info if you’re interested.