Hi, I have a question about the use of CALL_FUNCTION_EX in https://github.com/python/cpython/blob/master/Python/compile.c#L3624. Looking at the code it appears that the argument will be either 1 or 0 depending on whether or not the function is taking keywords arguments (which means that CALL_FUNCTION_EX cannot be used on function taking no argument). Executing that opcode will remove from the stack the function code, the positional arguments (packed in a tuple) and the potential keyword arguments packed in a dict and push the return value. So the stack effect will be either -1 or -2 (could be 0 if the possibility to pass 0 arguments existed). Looking at the stack effect computation (https://github.com/python/cpython/blob/master/Python/compile.c#L1047), it appears that the stack effect will be 0 if the argument is 0, -1 for either 1 or 2, and -2 for 3. Which means that the code generated at https://github.com/python/cpython/blob/master/Python/compile.c#L3624 can never allow to compute the right stack effect using the stack_effect function (as it will return either 0 or -1 instead of -1 and -2) I would say that this is a bug and that the oparg should be 1 + 2 if keywords arguments are present at line 3624. I am not sure what consequence this can have on CPython but it means the bytecode becomes weird as a the stack looks like it can grow during a list comprehension (calling a function f using * syntax) : BUILD_LIST 0 1 LOAD_FAST .0 1 FOR_ITER 22 1 ---> after each jump it looks like the stack is higher by one STORE_FAST i -1 LOAD_GLOBAL f 1 LOAD_DEREF a 1 LOAD_FAST i 1 BINARY_SUBSCR None -1 CALL_FUNCTION_EX 0 0 LIST_APPEND 2 -1 JUMP_ABSOLUTE 4 0 RETURN_VALUE None -1 What do you think ? Should I open an issue on https://bugs.python.org/ ? Best regards Matthieu