in python 2.6 how do i change a global variable within a for loop

Question

i want to change the value of a variable within a for loop, but then have the new preserved variable changed when the loop finishes

my attempt (kinda simplified from what i'm actually doing)

SNP is a list and compar_1 is a list of lists

line_1 = empty_row
for SNP1 in compar_1:
    global line_1
    if SNP[3] == SNP1[3] 
        compare_line_1 = SNP1 
print = line_1
output_file.write(to_print)

if it finds a match within the loop i want it to change the variable to that so that's what's printed, if it doesn't find a match in the for loop i want it to print the "empty_row" (a string defined earlier)

Answer 1

for loops do not affect variable scope in Python (like they could in C)

If you were using variables inside a function, you can use global [var] to declare that when you refer to [var] that you mean the global one.

This is a decent overview of scope in Python

Answer 2

If you had tried to just change the variable, you would have seen that it just works. Python does not have separate scope for loops.

Answer 3

for SNP1 in compar_1:
    if SNP[3] == SNP1[3]:
        line_1 = SNP1 
        break
else:
    line_1 = empty_row
to_print = line_1
output_file.write(to_print)