python - functools.wraps 不允许我用 Python 3 中的类包装函数

Question

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

Answer 1

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()