Python实现计算两个时间之间相差天数的方法

#-*- encoding:UTF-8 -*-
from datetime import date
import time
nowtime = date.today()
def convertstringtodate(stringtime):
  "把字符串类型转换为date类型"
  if stringtime[0:2] == "20":
    year=stringtime[0:4]
    month=stringtime[4:6]
    day=stringtime[6:8]
    begintime=date(int(year),int(month),int(day))
    return begintime
  else :
    year="20"+stringtime[0:2]
    month=stringtime[2:4]
    day=stringtime[4:6]
    begintime=date(int(year),int(month),int(day))
    return begintime
def comparetime(nowtime,stringtime):
  "比较两个时间,并返回两个日期之间相差的天数"
  if isinstance(nowtime,date):
    pass
  else:
    nowtime=convertstringtodate(nowtime)
  if isinstance(stringtime,date):
    pass
  else:
    stringtime=convertstringtodate(stringtime)
  result=nowtime-stringtime
  return result.days
"""
  if stringtime[0:2] == "20":
    year=stringtime[0:4]
    month=stringtime[4:6]
    day=stringtime[6:8]
    begintime=date(int(year),int(month),int(day))
    endtime=nowtime
    result=endtime-begintime
    return result.days
  else :
    year="20"+stringtime[0:2]
    month=stringtime[2:4]
    day=stringtime[4:6]
    begintime=date(int(year),int(month),int(day))
    endtime=nowtime
    result=endtime-begintime
    return result.days
"""
print isinstance("20141012",date)
print comparetime(nowtime,"140619")