python中的__enter__ __exit__，,我们前面文章介绍了迭

python中的__enter__ __exit__，,我们前面文章介绍了迭

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 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/

python中的__enter__ __exit__