def url_to_mid(url):
'''
>>> url_to_mid('z0JH2lOMb')
3501756485200075L
>>> url_to_mid('z0Ijpwgk7')
3501703397689247L
>>> url_to_mid('z0IgABdSn')
3501701648871479L
>>> url_to_mid('z08AUBmUe')
3500330408906190L
>>> url_to_mid('z06qL6b28')
3500247231472384L
>>> url_to_mid('yCtxn8IXR')
3491700092079471L
>>> url_to_mid('yAt1n2xRa')
3486913690606804L
'''
url = str(url)[::-1]
size = len(url) / 4 if len(url) % 4 == 0 else len(url) / 4 + 1
result = []
for i in range(size):
s = url[i * 4: (i + 1) * 4][::-1]
s = str(base62_decode(str(s)))
s_len = len(s)
if i < size - 1 and s_len < 7:
s = (7 - s_len) * '0' + s
result.append(s)
result.reverse()
return int(''.join(result))
mid转为url也就很简单了，对于一个mid，我们从后向前每7位一组，用base62编码来encode，拼起来即可。同样要注意的是，每7个一组的数字，除了开头一组，如果得到的62进制数字不足4位，需要补足0。
def mid_to_url(midint):
'''
>>> mid_to_url(3501756485200075)
'z0JH2lOMb'
>>> mid_to_url(3501703397689247)
'z0Ijpwgk7'
>>> mid_to_url(3501701648871479)
'z0IgABdSn'
>>> mid_to_url(3500330408906190)
'z08AUBmUe'
>>> mid_to_url(3500247231472384)
'z06qL6b28'
>>> mid_to_url(3491700092079471)
'yCtxn8IXR'
>>> mid_to_url(3486913690606804)
'yAt1n2xRa'
'''
midint = str(midint)[::-1]
size = len(midint) / 7 if len(midint) % 7 == 0 else len(midint) / 7 + 1
result = []
for i in range(size):
s = midint[i * 7: (i + 1) * 7][::-1]
