文件IO操作可以对文件对象使用上下文管理,使用with..as
语法
with open('test') as f:
pass
仿照上例写一个自己的类,实现上下文管理
class Point:
pass
with Point() as p: # AttributeError: __exit__
pass
提示属性错误,没有__exit__
,看了需要这个属性
某些版本会显示没有__enter__
当一个对象同时实现了__enter()__
和__exit__()
方法,它就属于上席文管理的对象
方法 | 意义 |
---|---|
__enter__ |
进入与此对象相关的上下文。如果存在该方法,with语法会把该方法的返回值作为绑定到as子句中指定的变量上 |
__exit__ |
退出与此对象相关的上下文 |
import time
class Point():
def __init__(self):
print('init =========')
time.sleep(1)
print('init over =====')
def __enter__(self):
print('enter ========')
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit =========')
with Point() as p:
print('in with =========')
time.sleep(2)
print('with over ========')
print('======end======')
# 执行结果
init =========
init over =====
enter ========
in with =========
with over ========
exit =========
======end======
__enter__
方法,然后执行语句体,最后离开with语句块的时候,调用__exit__
方法import time
class Point():
def __init__(self):
print('init =========')
time.sleep(1)
print('init over =====')
def __enter__(self):
print('enter ========')
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit =========')
with Point() as p:
print('in with =========')
raise Exception('error')
time.sleep(2)
print('with over ========')
print('======end======')
执行结果
虽然有异常弹出,但是enter和exit照样执行,上下文管理是安全的
import time
class Point():
def __init__(self):
print('init =========')
time.sleep(1)
print('init over =====')
def __enter__(self):
print('enter ========')
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit =========')
with Point() as p:
print('in with =========')
import sys
sys.exit(1)
time.sleep(2)
print('with over ========')
print('======end======')
执行结果
init =========
init over =====
enter ========
in with =========
exit =========
Process finished with exit code 1
从执行结果来看,依然执行了__exit__
函数,哪怕是退出Python运行环境
说明上下文管理很安全
class Point():
def __init__(self):
print('init')
def __enter__(self):
print('enter')
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit')
f = open('t3.py')
with f as p:
print(f)
print(p)
print(1,f is p) # 打印什么
print(2,f == p) # 打印什么
p = Point()
with p as f:
print('in with-----')
print(3,p == f)
print('with over')
print('======end======')
执行结果
<_io.TextIOWrapper name='t3.py' mode='r' encoding='cp936'>
<_io.TextIOWrapper name='t3.py' mode='r' encoding='cp936'>
1 True
2 True
init
enter
in with-----
3 False
with over
exit
======end======
问题在于__enter__
方法上,它将自己的返回值赋给f。修改上例
class Point():
def __init__(self):
print('init')
def __enter__(self):
print('enter')
return self # 增加返回值
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit')
p = Point()
with p as f:
print('in with-----')
print(p == f)
print('with over')
print('======end======')
执行结果
init
enter
in with-----
True
with over
exit
======end======
with语句,会调用with后的对象的__enter__
方法,如果有as,则将该方法的返回值赋给as子句的变量
上例,可以等价为f = p.__enter__()
__enter__
方法 没有其他参数
__exit__
方法有3个参数
__exit__(self, exc_type, exc_value, traceback)
exc_type
,异常类型exc_value
,异常的值traceback
,异常的追踪信息__exit__
方法返回一个等效True的值,则压制异常;否则,继续抛出异常class Point():
def __init__(self):
print('init')
def __enter__(self):
print('enter')
return self # 增加返回值
def __exit__(self, exc_type, exc_val, exc_tb):
print(1, exc_type)
print(2, exc_val)
print(3, exc_tb)
print('exit')
return 'abc' # return 值恒等为True,压制报错
# return None # 0 # []
p = Point()
with p as f:
print('in with-----')
raise Exception('Error')
print('with over')
print('======end======')
执行结果
init
enter
in with-----
1 <class 'Exception'>
2 Error
3 <traceback object at 0x00000259B3BFDB88>
exit
======end======
为加法函数计时
方法1、使用装饰器显示该函数的执行市场
方法2、使用上下文管理方法来显示该函数的执行时长
import time
def add(x, y):
time.sleep(2)
return x + y
import time
import datetime
from functools import wraps
def timeit(fn):
@wraps(fn)
def wrapper(*args, **kwargs):
start = datetime.datetime.now()
ret = fn(*args, **kwargs)
delta = (datetime.datetime.now() - start).total_seconds()
print('{} took {}s'.format(fn.__name__,delta))
return ret
return wrapper
@timeit
def add(x, y):
time.sleep(2)
return x + y
print(add(4,5))
def add(x, y):
time.sleep(2)
return x + y
# 执行结果
add took 2.000913s
9
import time
import datetime
from functools import wraps
def timeit(fn):
@wraps(fn)
def wrapper(*args, **kwargs):
start = datetime.datetime.now()
ret = fn(*args, **kwargs)
delta = (datetime.datetime.now() - start).total_seconds()
print('{} took {}s'.format(fn.__name__,delta))
return ret
return wrapper
@timeit
def add(x, y):
time.sleep(2)
return x + y
class TimeIt:
def __init__(self, fn):
self.fn = fn
def __enter__(self):
self.start = datetime.datetime.now()
return self.fn
def __exit__(self, exc_type, exc_val, exc_tb):
delta = (datetime.datetime.now() - self.start).total_seconds()
print("{} took {}s".format(self.fn.__name__,delta))
with TimeIt(add) as fn:
print(add(4, 7))
# 执行结果
add took 2.000563s
11
add took 2.000563s
import time
import datetime
from functools import wraps
def timeit(fn):
@wraps(fn)
def wrapper(*args, **kwargs):
start = datetime.datetime.now()
ret = fn(*args, **kwargs)
delta = (datetime.datetime.now() - start).total_seconds()
print('{} took {}s'.format(fn.__name__,delta))
return ret
return wrapper
@timeit
def add(x, y):
time.sleep(2)
return x + y
class TimeIt:
def __init__(self, fn):
self.fn = fn
def __enter__(self):
self.start = datetime.datetime.now()
return self.fn
def __exit__(self, exc_type, exc_val, exc_tb):
delta = (datetime.datetime.now() - self.start).total_seconds()
print("{} took {}s".format(self.fn.__name__,delta))
def __call__(self, x, y):
print(x, y)
return self.fn(x, y)
with TimeIt(add) as timeitobj:
print(timeitobj(4, 7))
# 执行结果
add took 2.000302s
11
add took 2.000302s
将上面代码的类当装饰器使用
import time
import datetime
from functools import wraps
class TimeIt:
def __init__(self, fn):
self.fn = fn
def __enter__(self):
self.start = datetime.datetime.now()
return self.fn
def __exit__(self, exc_type, exc_val, exc_tb):
delta = (datetime.datetime.now() - self.start).total_seconds()
print("{} took {}s".format(self.fn.__name__,delta))
pass
def __call__(self, *args, **kwargs):
self.start = datetime.datetime.now()
ret = self.fn(*args, **kwargs)
self.delta = (datetime.datetime.now() - self.start).total_seconds()
print('{} took {}s'.format(self.fn.__name__, self.delta))
return ret
@TimeIt
def add(x, y):
"""This is add function."""
time.sleep(2)
return x + y
add(4, 5)
print(add.__doc__)
# print(add.__name__) # 异常,没有此属性
# 执行结果
add took 2.000581s
None
解决文档字符串的问题
__doc__
class TimeIt:
def __init__(self, fn=None):
self.fn = fn
# 把函数对象的文档字符串赋给类
self.__doc__ = fn.__foc__
functools.wraps
函数import time
import datetime
from functools import wraps
class TimeIt:
"""This is A Class"""
def __init__(self, fn):
self.fn = fn
# 把函数对象的文档字符串赋给类
# self.__doc__ = fn.__doc__
# update_wrapper(self, fn)
wraps(fn)(self)
def __enter__(self):
self.start = datetime.datetime.now()
return self.fn
def __exit__(self, exc_type, exc_val, exc_tb):
delta = (datetime.datetime.now() - self.start).total_seconds()
print("{} took {}s".format(self.fn.__name__,delta))
def __call__(self, *args, **kwargs):
self.start = datetime.datetime.now()
ret = self.fn(*args, **kwargs)
self.delta = (datetime.datetime.now() - self.start).total_seconds()
print('{} took {}s'.format(self.fn.__name__, self.delta))
return ret
@TimeIt
def add(x, y):
"""This is add function."""
time.sleep(2)
return x + y
print(add(4, 5))
print(add.__doc__)
print(TimeIt(add).__doc__)
# 执行结果
add took 2.000641s
9
This is add function.
This is add function.
上面的类即可以用在上下文管理,又可以用作装饰器
__enter__
中处理__enter__
和__exit__
方法import contextlib
@contextlib.contextmanager
def foo():
print('enter') # 相当于__enter__()
yield # yield 5, yield的值只能有一个,作为__enter__方法的返回值
print('exit') # 相当于 __exit__()
with foo() as f:
# raise Exception()
print(f)
# 执行结果
enter
None
exit
as后的f接受yield语句的返回值
raise Exception()
语句,则print('exit')
不会执行try finally
import contextlib
@contextlib.contextmanager
def foo():
print('enter') # 相当于__enter__()
try:
yield # yield 5, yield的值只能有一个,作为__enter__方法的返回值
finally:
print('exit') # 相当于 __exit__()
with foo() as f:
raise Exception()
print(f)
执行结果
上例当yield发生处为生成器函数增加了上下文管理。这就是为函数增加上下文机制的方式
__enter__
方法执行__exit__
方法执行__enter__
的返回值import contextlib
import datetime
import time
@contextlib.contextmanager
def foo(x, y): # 为生成器函数增加了上下文管理
start = datetime.datetime.now()
try:
time.sleep(2)
yield x + y #yield的值只能有一个,__enter__方法的返回值
finally:
delta = (datetime.datetime.now() - start).total_seconds()
print(delta) # 相当于 __exit__()
with foo(4, 5) as f:
# raise Exception()
print(f)
# 执行结果
9
2.000907
如果业务逻辑加单可以使用函数加contextlib.contextmanager
装饰器方式,如果业务复杂,用类的方式加__enter__
和__exit__
方法方便