python学习笔记：利用contextlib和@contextmanager实现with语句上下文实例

import hashlib

from contextlib import contextmanager

#contextlib
'''
任何对象，只要正确实现了上下文管理，就可以用于with语句。
实现上下文管理是通过__enter__和__exit__这两个方法实现的，
也可以通过@contextmanager和closing函数实现
'''
print('用contextlib实现在函数调用前后打印log的功能：')
print('（1）contextlib: with...as...语句')
class Call(object):
def __init__(self, func):
self.__func = func

def __enter__(self):
print('Call %s()' % self.__func.__name__)
return self

def __exit__(self, exc_type, exc_value, traceback):
if exc_type:
print('Error')
else:
print('%s() End' % self.__func.__name__)

def doProcess(self):
self.__func()

def testFunc():
print('I am testFunc, I am doing something...')

'''
执行顺序：
1，执行Call里面的__enter__函数
2，然后执行with语句里面的c.doProcess()
3，再执行Call里面的__exit__函数
结果：
Call testFunc()
I am testFunc, I am doing something...
testFunc() End
'''
with Call(testFunc) as c:
c.doProcess()

#用@contextmanager实现和上面一样的功能，在函数调用前后打印log
print('（2）contextlib: with...as...语句，用@contextmanager实现')
from contextlib import contextmanager
class Call2(object): #不用实现__enter__和__exit__函数了
def __init__(self, func):
self.__func = func

def doProcess(self):
self.__func()

@contextmanager
def callWraper(func):
print('Call %s()' % func.__name__)
c = Call2(func)
yield c
print('%s() End' % func.__name__)

'''
执行流程：
1, with语句先执行callWraper函数里面yield之前的语句
2，yield调用会执行with语句内部的所有语句c.doProcess()
3，最后执行yield之后的语句
'''
with callWraper(testFunc) as c:
c.doProcess()

#实现用with语句可以在用户执行操作前连接数据库，在执行完操作后关闭数据库的功能

#由字符串获取md5摘要信息串
def md5DigestGet(str):
str += '!@#$%^'  #为了使简单的字符串不被黑客破解，将字符串添油加醋
md5 = hashlib.md5()
md5.update(str.encode('utf-8'))
return md5.hexdigest()

class myDb(object):
def __init__(self, users):
self.__linkFlag = False #连接标志
self.__usersLoginDigests = {} #保存用户名和密码的摘要信息
for name, passward in users.items():
md5Digest = md5DigestGet(name + passward)
self.__usersLoginDigests[name] = md5Digest

def getDbInfo(self):
return self.__usersLoginDigests

def loginCheck(self, name, passward):
if md5DigestGet(name + passward) == self.getDbInfo()[name]:
print('%s Login success' % name)
else:
print('%s Login fail' % name)

def linkDb(self):
self.__linkFlag = True
return self.loginCheck

def unlinkDb(self):
self.__linkFlag = False

def linkDb(db): #连接到数据库
print('Trying to link to db...')
checkFunc = db.linkDb()
print('Link db success..')
return checkFunc

def unlinkDb(db):
print('Trying to unlink db...')
db.unlinkDb()
print('unLink db success..')

@contextmanager
def userLoginCheck(db, name, passward):
checkFunc = linkDb(db)
yield checkFunc
unlinkDb(db)

users = {'kite':'abc_12345', 'jim':'xyz@1345', 'tom':'hello_***'} #用户名和密码
db = myDb(users)

print('-----------------------------------------------------------')
userName = 'kite'
passWard = 'abc_12345'
with userLoginCheck(db, userName, passWard) as checkFunc:
checkFunc(userName, passWard)
print('-----------------------------------------------------------')
userName = 'jim'
passWard = 'xyz@1346'
with userLoginCheck(db, userName, passWard) as checkFunc:
checkFunc(userName, passWard)
'''
执行结果：
-----------------------------------------------------------
Trying to link to db...
Link db success..
kite Login success
Trying to unlink db...
unLink db success..
-----------------------------------------------------------
Trying to link to db...
Link db success..
jim Login fail
Trying to unlink db...
unLink db success..
'''