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