Missing Smallest positive integer - Codility lesson4

def solution(A):
    # write your code in Python 3.6
    if max(A) <1:
        return 1
    k=max(A)+1
    B=set(range(1,k))
    #print(B)
    C=B.difference(set(A))
    #print(C)
    if C:
        if max(C)<=0:
            return 1
        else:
            D=[]
            for i in C:
                if i>0:
                    D.append(i)
            return min(D)
    else:
        return k
    pass