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Why doesn't the loop in the following code work?

#Finds out what numbers are prime.
primes = [2,3,5,7,11,13,17,19,23]
finish = int(raw_input("What number would you like to check to? "))
start = 24
while start >= finish:
    x = 0
    while start % primes[x] != 0:
        if IndexError:
            break
        else:
            x = x + 1
    else:
        primes.append[start]
    start = start + 1
print primes

When run, it just prints out the default set of primes after taking the number you want the program to check up to.
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IndexError is a class whose instance is raised as an exception if an invalid index is used on a sequence, therefore it cannot be false as a logical expression. Use try and except instead:

 
#Finds out what numbers are prime.
primes = [2,3,5,7,11,13,17,19,23]
finish = int(raw_input("What number would you like to check to? "))
start = 24
while start >= finish:
    x = 0
    try:
        while start % primes[x] != 0:
           x = x + 1
    except IndexError:
        primes.append(start)
    start = start + 1
print primes