Python solution to determining a function

isFunction

def isFunction(A, B, f):
    x = []
    for i in range(len(f)):
        x.append(f[i][0])
        #print(x)
    if len(x) != len(A):
        return False
    else:
        for s in A:
            #print(s)
            if not s in x:
                return False
        else:
            return True

A = [1, 2, 3]
B = [4, 5]
f = [[1, 4], [2, 5], [3, 4]]
print(isFunction(A, B, f))