There's a problem with genexp's that I think really needs to get fixed. See http://python.org/sf/1167751 the details are below. This code:

...

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foo(a = i for i in range(10))

generates "NameError: name 'i' is not defined" when run because: 2 0 LOAD_GLOBAL 0 (foo) 3 LOAD_CONST 1 ('a') 6 LOAD_GLOBAL 1 (i) 9 CALL_FUNCTION 256 12 POP_TOP 13 LOAD_CONST 0 (None) 16 RETURN_VALUE If you add parens around the code: foo(a = i for i in range(10)) You get something quite different: 2 0 LOAD_GLOBAL 0 (foo) 3 LOAD_CONST 1 ('a') 6 LOAD_CONST 2 (<code object <generator expression> at 0x2a960baae8, file "<stdin>", line 2>) 9 MAKE_FUNCTION 0 12 LOAD_GLOBAL 1 (range) 15 LOAD_CONST 3 (10) 18 CALL_FUNCTION 1 21 GET_ITER 22 CALL_FUNCTION 1 25 CALL_FUNCTION 256 28 POP_TOP 29 LOAD_CONST 0 (None) 32 RETURN_VALUE I agree with the bug report that the code should either raise a SyntaxError or do the right thing. n