[Python-Dev] Python3 compiled listcomp can't see local var (original) (raw)

Rob Cliffe rob.cliffe at btinternet.com
Wed Jun 6 01:31:35 EDT 2018

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 (<code object 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

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