On Fri, Nov 4, 2011 at 9:24 AM, Joshua Landau
<joshua.landau...@gmail.com> wrote:
> Non-lambdas work as expected, but a lambda inside a non-lambda still
> exhibits this behaviour.
> Conclusion:
> When setting defaults to keyword-only arguments in lambdas which are inside
> non-global scopes, cPython is unable to access the global scope and raises
> SystemError.
Fascinating! I did some introspection with this:
>>> def foo():
a=1 # to make sure local variables do exist
return (lambda *,keyword_only=global_variable: None)()
>>> dis.dis(foo)
2 0 LOAD_CONST 1 (1)
3 STORE_FAST 0 (a)
3 6 LOAD_CONST 2 ('keyword_only')
9 LOAD_NAME 0 (global_variable)
12 LOAD_CONST 3 (<code object <lambda> at
0x00FC7340, file "<pyshell#18>", line 3>)
15 MAKE_FUNCTION 256
18 CALL_FUNCTION 0
21 RETURN_VALUE
>>> def foo():
a=1
return (lambda argument=global_variable: None)()
>>> dis.dis(foo)
2 0 LOAD_CONST 1 (1)
3 STORE_FAST 0 (a)
3 6 LOAD_GLOBAL 0 (global_variable)
9 LOAD_CONST 2 (<code object <lambda> at
0x00F95E30, file "<pyshell#23>", line 3>)
12 MAKE_FUNCTION 1
15 CALL_FUNCTION 0
18 RETURN_VALUE
Variations on the theme show that during compilation of foo, the name
is normally cemented as either a global or a local - but if it's
keyword-only, then a LOAD_NAME opcode is emitted. It's the code for
LOAD_NAME that looks for locals, which presumably a lambda doesn't
have.
ChrisA
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