Count the occurrences of a string in a file

count_a_string.py

def count(fileName, count_string):
"""
Count the occurrences of a string in a file
"""
    count = 0
    input = open(fileName, 'r')
    line = input.readline()
    while line <> '':
        if count_string in line:
            count = count + 1
    return count