Python count sort

gistfile1.py

def count_sort(A, K):
    """ A is positive integer array each element is in range [0, K]"""
    count = [0] * (K + 1)
    for a in A:
        count[a] += 1
    
    for i in range(1, K + 1):
        count[i] = count[i - 1] + count[i]
        
    B = [None] * len(A)
    for i in range(len(A) - 1, -1, -1):
        B[count[A[i]] - 1] = A[i]
        count[A[i]] -= 1
    return B