(original) (raw)

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
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
File "MyTest", line 1, in
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 ( at 0x00000000029F79C0, file "MyTest", line 1>)
4 LOAD\_CONST 1 ('')
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