[Tutor] Namespace Problem

[Carroll, Barry]

I have a function that makes use of several global variables:

##########
Include struct

ABC = 1
DEF = 2

xyz = 0
# Other variables 

def do_stuff(in_str):
    hdr = struct.pack('@2BH',ABC|DEF,xyz,len(in_str))
    newstr = hdr+in_str

    # rest of code snipped
##########

When I run the program containing this code I get this error:

>>>>>>>>>
Traceback (most recent call last):
  File "sample.py", line 43, in ?
    ret_data = do_stuff(data)
  File "sample.py", line 17, in do_stuff
    hdr = struct.pack('@2BH', ABC|DEF,xyz,len(in_str))
UnboundLocalError: local variable 'xyz' referenced before assignment
>>>>>>>>>

The error goes away if I include a 'global' statement at the top of the
function:

##########
def do_stuff(in_str):
    global xyz
    hdr = struct.pack('@2BH',ABC|DEF,xyz,len(in_str))
##########

Why does the interpreter choke on xyz and not on ABC or DEF?

Barry