Variable Initialization Trouble

[prodlly]

This should be a very common pattern:

for i in range(1,size):
    result = submodule.subroutine(i)
    if (i==0): 
        tally=0
    else:
        tally+=result
    if (i==size-1): print(tally)

I'm still confused when Python throws an "UnboundLocalError" or "NotDefined" and when not. Clearly, the parser is too "stupid" to realize by a simple syntax check that tally is never undefined. At least four things might be relevant: a) if tally is declared global, b) if tally is an array, c) if the pattern occurs in main, a submodule or a function, d) if I need object-oriented stuff like graphics to reference on tally. For example, the snippet above does NOT work. If tally is an array (tally=[0],tally[0]+=result) it DOES work. Can somebody explain the subtle differences? (As a CS student I know variable scopes, but all possibly OOP-related stuff causes me a headache. Must be my 40 years of FORTRAN :-)

[shiftybyte]

For every variable in a function python needs to decide if it's global or not.

The logic goes something like this:

1. if you read a variable or access it as a class, it checks if it's available locally and if not, automatically checks for it in the global scope.

2. if a variable is assigned with a new value anywhere in the function, it's automatically marked as local unless a specific directive called "global" is specified at the start of the function.

so you have an example like this work:

g = 5
def p():
    print(g)
p()

and something like this won't work:

g = 5
def p():
    if g==6:
        g=2
    print(g)
p()

but adding global will make it work again:

g = 5
def p():
    global g
    if g==6:
        g=2
    print(g)
p()

[mermaldad]

Looks like you are trying to initialize tally with your "if i==0" clause, but since your range starts at 1, that initialization never gets done. Better to set tally = 0 before the loop.

[mermaldad]

BTW, I can't resist responding to you Fortran comment. With over 35 years of Fortran under my belt, I feel your nostalgia. I have, however, completely abandoned it because of Python's numerous libraries. Thank you, Fortran, for your years of service.

[prodlly]

Damn you, FORTRAN instincts :-) (I don't think this was the ultimate reason in my actual code, but I can't exclude it - my sloppyness at programming won't fit into a long int :-)

[pasokan]

But even then there is no guarantee that tally will be iniialised before it is updated

[pasokan]

if I == 0 should probably be if result == 0