python3 no.6 字符串的查找与替换

#!/usr/bin/env python3
'''
题目：
创建三个函数：
函数1， 从指定字符串里从前面查找指定单个字符
函数2， 从指定字符串里从后面开始查找指定单个字符
函数3， 输入两个字符串（一个要替换的， 二个被替换的）， 从指定字符串里面进行替换

（不能用string.*find()或string.*index函数和方法
'''

def showmenu():
welcome = '''
(s)tart find
(e)nd find
(r)ep find
(q)uit
'''
print(welcome)

def findstart():
while True:
findstring = input('Input a string:')
if len(findstring) < 1:
print('Error input.)')
else:
break
print('stringdata: ', stringdata)
findstring = findstring[0]
print('findstart:', findstring)
if findstring in stringdata:
index_string = 1
for check in stringdata:
if findstring == check:
break
else:
index_string += 1
print('return:', index_string)
else:
print('return: None' )

def findend():
while True:
findstring = input('Input a string:')
if len(findstring) < 1:
print('Error input.)')
else:
break
print('stringdata: ', stringdata)
findstring = findstring[0]
print('findend:', findstring)
if findstring in stringdata:
index_string = -1
for i in range(1, len(stringdata)+1):
if findstring == stringdata[-i]:
break
else:
index_string -= 1
print('return:', index_string)
else:
print('return: None' )

def repfind():
#全部替换
global stringdata
#引入全局变量， 进行必要修改
print('database_string: ', stringdata)
repstring = input('Input two string(A rep to B):').strip()
repstring = repstring.split()
if len(repstring) == 2:
#先分割， 再连接split-->join, 用指定字符分割， 再用指定字符连接
newstringdata = stringdata.split(repstring[0])
stringdata = repstring[1].join(newstringdata)
print('now the new stringdata: ', stringdata)
else:
print('Error, you must input two string.')

#if __name__ == '__main__':
#    showmenu()

cmd = {'s':findstart, 'e':findend, 'r':repfind}

stringdata = input('Input stringdatabase:')
while True:
showmenu()
try:
choose = (input('Input you choose:')).strip()[0].lower()
except:
print('Error, continue.')
continue
if choose in 'ser':
cmd[choose]()
else:
if choose == 'q':
print('Bye')
break
else:
print('You input error.')
input()