[英]Function acting as both decorator and context manager in Python?
这可能会把事情推得太远,但主要是出于好奇。
是否有可能以具有一个可调用的对象(功能/类),其同时作为上下文管理器,并在同一时间装饰器:
def xxx(*args, **kw):
# or as a class
@xxx(foo, bar)
def im_decorated(a, b):
print('do the stuff')
with xxx(foo, bar):
print('do the stuff')
从Python 3.2开始,对此的支持甚至包含在标准库中。 从类contextlib.ContextDecorator
派生,可以轻松编写可用作装饰器或上下文管理器的类。 这个功能可以很容易地向后移植到Python 2.x - 这是一个基本的实现:
class ContextDecorator(object):
def __call__(self, f):
@functools.wraps(f)
def decorated(*args, **kwds):
with self:
return f(*args, **kwds)
return decorated
从这个类派生上下文管理和定义__enter__()
和__exit__()
方法,像往常一样。
在Python 3.2+中,您可以使用@contextlib.contextmanager
contextlib.contextmanager定义一个也是装饰器的上下文管理器。
来自文档:
contextmanager()
使用ContextDecorator
因此它创建的上下文管理器可以用作装饰器以及with
语句
用法示例:
>>>
>>>
...
...
...
...
...
...
...
>>>
...
...
Starting printing Hello World
Hello, World!
Done printing Hello World
>>>
>>>
...
...
...
>>>
Starting running my function
Inside my function
Done running my function
class Decontext(object):
"""
makes a context manager also act as decorator
"""
def __init__(self, context_manager):
self._cm = context_manager
def __enter__(self):
return self._cm.__enter__()
def __exit__(self, *args, **kwds):
return self._cm.__exit__(*args, **kwds)
def __call__(self, func):
def wrapper(*args, **kwds):
with self:
return func(*args, **kwds)
return wrapper
现在你可以这样做:
mydeco = Decontext(some_context_manager)
这两者都允许
@mydeco
def foo(...):
do_bar()
foo(...)
和
with mydeco:
do_bar()
这是一个例子:
class ContextDecorator(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
print("init", foo, bar)
def __call__(self, f):
print("call")
def wrapped_f():
print("about to call")
f()
print("done calling")
return wrapped_f
def __enter__(self):
print("enter")
def __exit__(self, exc_type, exc_val, exc_tb):
print("exit")
with ContextDecorator(1, 2):
print("with")
@ContextDecorator(3, 4)
def sample():
print("sample")
sample()
这打印:
init 1 2
enter
with
exit
init 3 4
call
about to call
sample
done calling
虽然我同意(和赞成)@jterrace在这里,但我添加了一个非常小的变体,它返回了装饰函数,并包含了装饰器和装饰函数的参数。
class Decon:
def __init__(self, a=None, b=None, c=True):
self.a = a
self.b = b
self.c = c
def __enter__(self):
# only need to return self
# if you want access to it
# inside the context
return self
def __exit__(self, exit_type, exit_value, exit_traceback):
# clean up anything you need to
# otherwise, nothing much more here
pass
def __call__(self, func):
def decorator(*args, **kwargs):
with self:
return func(*args, **kwargs)
return decorator
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