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 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/