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/