unbounded variable

[deanhystad]

Variables are "created" when Python code is parsed, not when it is executed. Before main() was ever called, the code for main "defined" a local vairable named "take_sum". You can see that if you look at the generated bytecode. Below I simplify the function to focus on the variable.

import dis


def main():
    if "take_sum" not in globals():
        take_sum = 1
    print(take_sum)


print(dis.dis(main))

Output:
             16 PRECALL                  0
             20 CALL                     0
             30 CONTAINS_OP              1
             32 POP_JUMP_FORWARD_IF_FALSE     2 (to 38)

  6          34 LOAD_CONST               2 (1)
             36 STORE_FAST               0 (take_sum)

  7     >>   38 LOAD_GLOBAL              3 (NULL + print)
             50 LOAD_FAST                0 (take_sum)
             52 PRECALL                  1
             56 CALL                     1
             66 POP_TOP
             68 LOAD_CONST               0 (None)
             70 RETURN_VALUE

Even if the comparison in line 5 (of the python code) causes execution to skip the assignment in line 6, main() still has a variable named (take_sum). It is part of the function's code.

main() creates (take_sum) because line 6 does an assignment to the variable. (take_sum) is a local variable because main() does not declare it as global.
Notice how the code changes if we declare take_sum as global.

import dis


def main():
    global take_sum
    if "take_sum" not in globals():
        take_sum = 1
    print(take_sum)


print(dis.dis(main))

Output:
  4           0 RESUME                   0

  6           2 LOAD_CONST               1 ('take_sum')
              4 LOAD_GLOBAL              1 (NULL + globals)
             16 PRECALL                  0
             20 CALL                     0
             30 CONTAINS_OP              1
             32 POP_JUMP_FORWARD_IF_FALSE     2 (to 38)

  7          34 LOAD_CONST               2 (1)
             36 STORE_GLOBAL             1 (take_sum)

  8     >>   38 LOAD_GLOBAL              5 (NULL + print)
             50 LOAD_GLOBAL              2 (take_sum)
             62 PRECALL                  1
             66 CALL                     1
             76 POP_TOP
             78 LOAD_CONST               0 (None)
             80 RETURN_VALUE

Now main() uses STORE_GLOBAL and LOAD_GLOBAL when referencing (take_sum).

To fix your code, you could modify main() to always assign something to take_sum:

def take_sum(a, b, c):
    return a+b+c
 
def main():
    if "take_sum" in globals():
        take_sum = globals()["take_sum"]
    else:
        take_sum = lambda x,y,z: x+y+z
    print(take_sum)

main()

Or you could declare take_sum as global

def take_sum(a, b, c):
    return a+b+c


def main():
    global take_sum
    if "take_sum" not in globals():
        take_sum = lambda x,y,z: x+y+z
    print(take_sum)
     

main()