Is this a bug or a feature?
Consider the following program:
# TestProgram.py
def Test():
# global x
x = 1
exec(compile('print([x+1,x+2])', 'MyTest', 'exec'))
exec(compile('print([x+i for i in range(1,3)])', 'MyTest', 'exec'))
Test()
In Python 2.7.15 the output is
[2, 3]
[2, 3]
In Python 3.6.5 the output is
[2, 3]
Traceback (most recent call last):
File "TestProgram.py", line 7, in <module>
Test()
File "TestProgram.py", line 6, in Test
exec(compile('print([x+i for i in range(1,3)])', 'MyTest', 'exec'))
File "MyTest", line 1, in <module>
File "MyTest", line 1, in <listcomp>
NameError: name 'x' is not defined
If the "global x" declaration is uncommented, this "fixes" the Python
3.6.5 behaviour,
i.e. no error occurs and the output is the same as for Python 2.7.15.
*In other words, it looks as if in Python 3.6.5, the compiled list
comprehension**
**can "see" a pre-existing global variable but not a local one.*
I have used dis to examine the code objects returned by compile()
(they are the same with or without the "global x"):
Python 2.7.15 first code object from 'print([x+1,x+2])':
1 0 LOAD_NAME 0 (x)
3 LOAD_CONST 0 (1)
6 BINARY_ADD
7 LOAD_NAME 0 (x)
10 LOAD_CONST 1 (2)
13 BINARY_ADD
14 BUILD_LIST 2
17 PRINT_ITEM
18 PRINT_NEWLINE
19 LOAD_CONST 2 (None)
22 RETURN_VALUE
Python 2.7.15 second code object from 'print([x+i for i in range(1,3)])':
1 0 BUILD_LIST 0
3 LOAD_NAME 0 (range)
6 LOAD_CONST 0 (1)
9 LOAD_CONST 1 (3)
12 CALL_FUNCTION 2
15 GET_ITER
>> 16 FOR_ITER 16 (to 35)
19 STORE_NAME 1 (i)
22 LOAD_NAME 2 (x)
25 LOAD_NAME 1 (i)
28 BINARY_ADD
29 LIST_APPEND 2
32 JUMP_ABSOLUTE 16
>> 35 PRINT_ITEM
36 PRINT_NEWLINE
37 LOAD_CONST 2 (None)
40 RETURN_VALUE
Python 3.6.5 first code object from 'print([x+1,x+2])':
1 0 LOAD_NAME 0 (print)
2 LOAD_NAME 1 (x)
4 LOAD_CONST 0 (1)
6 BINARY_ADD
8 LOAD_NAME 1 (x)
10 LOAD_CONST 1 (2)
12 BINARY_ADD
14 BUILD_LIST 2
16 CALL_FUNCTION 1
18 POP_TOP
20 LOAD_CONST 2 (None)
22 RETURN_VALUE
Python 3.6.5 second code object from 'print([x+i for i in range(1,3)])':
1 0 LOAD_NAME 0 (print)
2 LOAD_CONST 0 (<code object <listcomp> at
0x00000000029F79C0, file "MyTest", line 1>)
4 LOAD_CONST 1 ('<listcomp>')
6 MAKE_FUNCTION 0
8 LOAD_NAME 1 (range)
10 LOAD_CONST 2 (1)
12 LOAD_CONST 3 (3)
14 CALL_FUNCTION 2
16 GET_ITER
18 CALL_FUNCTION 1
20 CALL_FUNCTION 1
22 POP_TOP
24 LOAD_CONST 4 (None)
26 RETURN_VALUE
You will see that in Python 3.6.5 the dis output for the second code object
does not show the internals of the listcomp, and in particular whether,
and how, it refers to the variable 'x'. I don't know how to investigate
further.
Best wishes
Rob Cliffe
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