我想为一些将文件作为第一个参数的函数编写一个装饰器。装饰器必须实现 context manager protocol (即将包装函数变成上下文管理器),所以我想我需要用类包装函数。
我对装饰器模式并没有真正的经验,之前从未实现过上下文管理器,但我写的东西在 Python 2.7 中有效,如果我注释掉 wraps
,它在 Python 3.3 中也有效行。
from functools import wraps
def _file_reader(func):
"""A decorator implementing the context manager protocol for functions
that read files."""
# @wraps(func)
class CManager:
def __init__(self, source, *args, **kwargs):
self.source = source
self.args = args
self.kwargs = kwargs
self.close = kwargs.get('close', True)
def __enter__(self):
# _file_obj is a little helper that opens the file for reading
self.fsource = _file_obj(self.source, 'r')
return func(self.fsource, *self.args, **self.kwargs)
def __exit__(self, exc_type, exc_value, traceback):
if self.close:
self.fsource.close()
return False
return CManager
取消注释 wraps
行时出现的错误发生在 update_wrapper
中:
/usr/lib/python3.3/functools.py in update_wrapper(wrapper, wrapped, assigned, updated)
54 setattr(wrapper, attr, value)
55 for attr in updated:
---> 56 getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
57 # Return the wrapper so this can be used as a decorator via partial()
58 return wrapper
AttributeError: 'mappingproxy' object has no attribute 'update'
我知道文档没有说我什至可以使用 functools.wraps
用这样的类包装一个函数,但话又说回来,它只适用于 Python 2。有人可以解释一下这个回溯到底告诉我什么以及我应该怎么做才能实现 wraps
的效果在两个版本的 Python 上?
编辑:我错了。上面的代码没有做我想做的事。我希望能够同时使用 with 和 without with
功能,就像内置的 open
一样。
上面的代码将装饰函数变成了上下文管理器。我希望能够做到:
reader = func('source.txt', arg)
for item in reader:
pass
还有
with func('source.txt', arg) as reader:
for item in reader:
pass
所以我的代码版本应该大概如下所示:
def _file_reader(func):
"""A decorator implementing the context manager protocol for functions
that read files."""
@wraps(func)
class CManager:
def __init__(self, source, *args, **kwargs):
self.close = kwargs.get('close', True)
self.fsource = _file_obj(source, 'r')
self.reader = func(self.fsource, *args, **kwargs)
def __enter__(self):
return self.reader
def __iter__(self):
return self.reader
def __next__(self):
return next(self.reader)
def __exit__(self, exc_type, exc_value, traceback):
if self.close and not self.fsource.closed:
self.fsource.close()
return False
return CManager
请随时评论我忽略的任何内容。
注意:J.F. Sebastian 的类版本似乎可以工作:
我基本上从类中删除了 wraps
并将 return CManager
更改为:
@wraps(func)
def helper(*args, **kwargs):
return CManager(*args, **kwargs)
return helper
最佳答案
functools.wraps()
用于包装函数:
import contextlib
import functools
def file_reader(func):
@functools.wraps(func)
@contextlib.contextmanager
def wrapper(file, *args, **kwargs):
close = kwargs.pop('close', True) # remove `close` argument if present
f = open(file)
try:
yield func(f, *args, **kwargs)
finally:
if close:
f.close()
return wrapper
例子
@file_reader
def f(file):
print(repr(file.read(10)))
return file
with f('prog.py') as file:
print(repr(file.read(10)))
如果您想使用基于类的上下文管理器,那么解决方法是:
def file_reader(func):
@functools.wraps(func)
def helper(*args, **kwds):
return File(func, *args, **kwds)
return helper
无论是直接使用装饰函数还是作为上下文管理器,要使其表现相同,您应该在 __enter__()
中返回 self
:
import sys
class File(object):
def __init__(self, file, func, *args, **kwargs):
self.close_file = kwargs.pop('close', True)
# accept either filename or file-like object
self.file = file if hasattr(file, 'read') else open(file)
try:
# func is responsible for self.file if it doesn't return it
self.file = func(self.file, *args, **kwargs)
except: # clean up on any error
self.__exit__(*sys.exc_info())
raise
# context manager support
def __enter__(self):
return self
def __exit__(self, *args, **kwargs):
if not self.close_file:
return # do nothing
# clean up
exit = getattr(self.file, '__exit__', None)
if exit is not None:
return exit(*args, **kwargs)
else:
exit = getattr(self.file, 'close', None)
if exit is not None:
exit()
# iterator support
def __iter__(self):
return self
def __next__(self):
return next(self.file)
next = __next__ # Python 2 support
# delegate everything else to file object
def __getattr__(self, attr):
return getattr(self.file, attr)
Example
file = f('prog.py') # use as ordinary function
print(repr(file.read(20)))
file.seek(0)
for line in file:
print(repr(line))
break
file.close()
关于python - functools.wraps 不允许我用 Python 3 中的类包装函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14095025/