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I definitely agree this is a big problem.

Here's what the code above generates:
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 I put parens around the genexp, I get:
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