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我们前面文章介绍了迭代器和可迭代对象，这次介绍python的上下文管理。在python中实现了__enter__和__exit__方法，即支持上下文管理器协议。上下文管理器就是支持上下文管理器协议的对象，它是为了with而生。当with语句在开始运行时，会在上下文管理器对象上调用 __enter__ 方法。with语句运行结束后，会在上下文管理器对象上调用 __exit__ 方法
with的语法:
with expr as var:
block
这是上面语法的伪代码:
我要评论mgr = (expr)
exit = type(mgr).__exit__ # not calling it yet
value = type(mgr).__enter__(mgr)
exc = true
try:
try:
var = value # only if "as var" is present
block
except:
# the exceptional case is handled here
exc = false
if not exit(mgr, *sys.exc_info()):
raise
# the exception is swallowed if exit() returns true
finally:
# the normal and non-local-goto cases are handled here
if exc:
exit(mgr, none, none, none)
1、生成上下文管理器mgr
2、如果没有发现__exit__, __enter__两个方法，解释器会抛出attributeerror异常
3、调用上下文管理器的 __enter__() 方法
4、如果语法里的as var没有写，那么 伪代码里的 var= 这部分也会同样被忽略
5、如果block中的代码正常结束，或者是通过break, continue ,return 来结束，__exit__()会使用三个none的参数来返回
6、如果执行过程中出现异常，则使用 sys.exc_info的异常信息为参数调用 __exit__(exc_type, exc_value, exc_traceback)
之前我们对文件的操作是这样的:
try:
f = open('filename')
except:
print("unexpected error:", sys.exc_info()[0])
else:
print(f.readlines())
f.close()
现在有了with语句可以使代码更加简洁，减少编码量,下面的语句会在执行完后自动关闭文件(即使出现异常也会)。:
with open('example.info', 'r') as f:
print(f.readlines())
一个例子:
class tmptest:
def __init__(self,filename):
self.filename=filename
def __enter__(self):
self.f = open(self.filename, 'r')
# return self.f
def __exit__(self, exc_type, exc_val, exc_tb):
self.f.close()
test=tmptest('file')
with test as t:
print ('test result: {}'.format(t))
返回:
test result: none
这个例子里面__enter__没有返回，所以with语句里的"as t"到的是none，修改一下上面的例子:
class tmptest:
def __init__(self,filename):
self.filename=filename
def __enter__(self):
self.f = open(self.filename, 'r')
return self.f
def __exit__(self, exc_type, exc_val, exc_tb):
self.f.close()
test=tmptest('file')
with test as t:
print ('test result: {}'.format(t))
返回:
test result: <_io.textiowrapper name='file' mode='r' encoding='cp936'>
如果在__init__或者__enter__中抛出异常，则不会进入到__exit__中:
class tmptest:
def __init__(self,filename):
self.filename=filename
print("__init__")
raise importerror
def __enter__(self):
self.f = open(self.filename, 'r')
print("__enter__")
return self.f
def __exit__(self, exc_type, exc_val, exc_tb):
print("__exit__")
self.f.close()
test=tmptest('file')
with test as t:
print ('test result: {}'.format(t))
返回:
__init__
traceback (most recent call last):
file "d:/pythonscript/leetcode/leetcode.py", line 14, in <module>
test=tmptest('file')
file "d:/pythonscript/leetcode/leetcode.py", line 5, in __init__
raise importerror
importerror
如果在__exit__中返回true,则不会产生异常:
class tmptest:
def __init__(self,filename):
self.filename=filename
print("__init__")
def __enter__(self):
self.f = open(self.filename, 'r')
print("__enter__")
return self.f
def __exit__(self, exc_type, exc_val, exc_tb):
print("__exit__ {} ".format(exc_type))
self.f.close()
return true
test=tmptest('file')
with test as t:
print ('test result: {}'.format(t))
raise importerror
print("no error")
返回:
__init__ __enter__ test result: <_io.textiowrapper name='file' mode='r' encoding='cp936'> __exit__ <class 'importerror'> no error
参考: https://python3-cookbook.readthedocs.io/zh_cn/latest/c08/p03_make_objects_support_context_management_protocol.html?highlight=with
https://docs.python.org/3/library/stdtypes.html#typecontextmanager
https://www.python.org/dev/peps/pep-0343/
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