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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