Python newb here. I m trying to count the number of letter "a"s in a given string. Code is below. It keeps returning 1 instead 3 in string "banana". Any input appreciated.

def count_letters(word, char):
    count = 0
    while count <= len(word):
        for char in word:
            if char == word[count]:
                count += 1
            return count



print count_letters('banana','a')