在字符串中找出与给定字符串相同的字符串（find a substring from a string）

import sys

def is_substring(actual_str,pattern_str):
    if actual_str is None or pattern_str is None:
        print 'empty string'
        return
    if len(pattern_str)>len(actual_str):
        print 'substring pattern is longer than catual string'
        return
    indexes=[]
    for i in range(len(actual_str) - len(pattern_str) + 1):
        if pattern_str[0]==actual_str[i] and\
        pattern_str[len(pattern_str)-1]==actual_str[i+len(pattern_str)-1]:
            indexes.append(i)
    if len(indexes)==0:
        print 'substring could not be found'
        return
    mismatch=False
    for j in range(len(indexes)):
        index=indexes[j]
        for k in range(len(pattern_str)):
            if(actual_str[index]!=pattern_str[k]):
                print "Mismatch : %s - %s"%(actual_str[index], pattern_str[k])
                mismatch=True
                break
            index+=1
        if mismatch==True:
            mismatch=False
            continue
        print 'substring found at indexes:',indexes[j]

if __name__=='__main__':
    if len(sys.argv)!=3:
        is_substring('122234542223212527','222')
    else:
        is_substring(str(sys.argv(1),str(sys.argv[2])))

substring found at indexes: 1
substring found at indexes: 8
Mismatch : 3 - 2
Mismatch : 1 - 2
Mismatch : 5 - 2
>>>