Variable scope - "global x" didn't work...

[ptrivino]

I am struggling to understand updating a 'global' dictionary (i.e., it's defined in the main module and I want to update it in a subroutine). I understand that I need to do something since dict_1 in the mainline isn't what would be updated in a function. So I tried this experiment:

dict_1 = {}
dict_2 = {}

def upd_dict1(dict_1_copy):
    print("sub dict_1_copy a is " + str(dict_1_copy["a"]))
    dict_1_copy["a"] = 100
    print("sub dict_1_copy a is " + str(dict_1_copy["a"]))
    return dict_1_copy

if __name__ == "__main__":
    dict_1["a"] = 1
    dict_1["b"] = 2
    dict_2["a"] = 0
    dict_2["b"] = 22
    print("dict_1 a is " + str(dict_1["a"]) + " b4 upd - sb 1")
    print("dict_1 b is " + str(dict_1["b"]) + " b4 upd - sb 2")
    print("dict_2 a is " + str(dict_2["a"]) + " b4 upd - sb 0")
    print("dict_2 b is " + str(dict_2["b"]) + " b4 upd - sb 22\n")
    dict_2 = upd_dict1(dict_1)
    print("dict_1 a is " + str(dict_1["a"]) + " aft upd - sb 1")
    print("dict_1 b is " + str(dict_1["b"]) + " aft upd - sb 2")
    print("dict_2 a is " + str(dict_2["a"]) + " aft upd - sb 100")
    print("dict_2 b is " + str(dict_2["b"]) + " aft upd - sb 2\n")
    dict_1 = dict_2
    print("dict_1 a is " + str(dict_1["a"]) + " aft copy - sb 100")
    print("dict_1 b is " + str(dict_1["b"]) + " aft copy - sb 2")
    print("dict_2 a is " + str(dict_2["a"]) + " aft copy - sb 100")
    print("dict_2 b is " + str(dict_2["b"]) + " aft copy - sb 2")

Which *should* give the results shown as "sb X" in the print statements. But what I get is

Output:
dict_1 a is 1 b4 upd - sb 1
dict_1 b is 2 b4 upd - sb 2
dict_2 a is 0 b4 upd - sb 0
dict_2 b is 22 b4 upd - sb 22
sub dict_1_copy a is 1
sub dict_1_copy a is 100
dict_1 a is 100 aft upd - sb 1     <-- THIS
dict_1 b is 2 aft upd - sb 2
dict_2 a is 100 aft upd - sb 100
dict_2 b is 2 aft upd - sb 2
dict_1 a is 100 aft copy - sb 100
dict_1 b is 2 aft copy - sb 2
dict_2 a is 100 aft copy - sb 100
dict_2 b is 2 aft copy - sb 2

I completely do not understand why the line marked THIS get what it gets.

Can anyone tell me what I'm missing here?

Paul