List comprehension - NameError: name '_[1]' is not defined ?

Terry Reedy tjreedy at udel.edu
Thu Jan 15 23:35:16 CET 2009


Peter Otten wrote:

> List comprehensions delete the helper variable after completion:

I do not believe they did in 2.4.  Not sure of 2.5.  There is certainly 
  a very different implementation in 3.0 and, I think, 2.6.  OP 
neglected to mention Python version he tested on. Code meant to run on 
2.4 to 3.0 cannot depend on subtle listcomp details.

>>>> def f(): [i for i in [1]]
> ...
>>>> dis.dis(f)
>   1           0 BUILD_LIST               0
>               3 DUP_TOP
>               4 STORE_FAST               0 (_[1])
>               7 LOAD_CONST               1 (1)
>              10 BUILD_LIST               1
>              13 GET_ITER
>         >>   14 FOR_ITER                13 (to 30)
>              17 STORE_FAST               1 (i)
>              20 LOAD_FAST                0 (_[1])
>              23 LOAD_FAST                1 (i)
>              26 LIST_APPEND
>              27 JUMP_ABSOLUTE           14
>         >>   30 DELETE_FAST              0 (_[1])
>              33 POP_TOP
>              34 LOAD_CONST               0 (None)
>              37 RETURN_VALUE
>

In 3.0
 >>> def f(): [i for i in [1]]

 >>> import dis
 >>> dis.dis(f)
   1           0 LOAD_CONST               1 (<code object <listcomp> at 
0x01349BF0, file "<pyshell#12>", line 1>)
               3 MAKE_FUNCTION            0
               6 LOAD_CONST               2 (1)
               9 BUILD_LIST               1
              12 GET_ITER
              13 CALL_FUNCTION            1
              16 POP_TOP
              17 LOAD_CONST               0 (None)
              20 RETURN_VALUE

Running OP code in 3.0 with print ()s added gives

pre 0.a 1.b 2.c 3.d  post

Traceback (most recent call last):
   File "C:\Programs\Python30\misc\temp7.py", line 32, in <module>
     """ % gie)
   File "C:\Programs\Python30\misc\temp7.py", line 8, in __getitem__
     return eval(expr, self.globals, self.locals)
   File "<string>", line 7, in <module>
   File "<string>", line 7, in <listcomp>
   File "C:\Programs\Python30\misc\temp7.py", line 12, in ts
     return ts % self
   File "C:\Programs\Python30\misc\temp7.py", line 8, in __getitem__
     return eval(expr, self.globals, self.locals)
   File "<string>", line 2, in <module>
   File "<string>", line 1, in <listcomp>
NameError: global name 'i' is not defined

> If you manage to run two nested listcomps in the same namespace you get a
> name clash and the inner helper variable overwrites/deletes the outer:
> 
>>>> def xeval(x): return eval(x, ns)
> ...
>>>> ns = dict(xeval=xeval)
>>>> xeval("[xeval('[k for k in ()]') for i in (1,)]")
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 1, in xeval
>   File "<string>", line 1, in <module>
> NameError: name '_[1]' is not defined

Which Python?  3.0 prints "[[]]"! But I think the nested listcomp *is* 
in a separate namespace here.  I will leave it to you or OP to disect 
how his and your code essentially differ from 3.0 (and maybe 2.6) 
implementation's viewpoint.

Terry Jan Reedy




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