99uu备用APP99uu备用APP


优优官方导航

python中的__enter__ __exit__

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:

mgr = (EXPR) exit = type(mgr).__exit__ # Not calling it yetvalue = 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 truefinally: # 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mgr2、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 ImportErrorImportError

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 Truetest=TmpTest("file")with test as t: print ("test result: {}".format(t)) raise ImportErrorprint("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/

, 1, 0, 9);

欢迎阅读本文章: 陶先生

优优官方APP

优优官方导航