Question

I want to write a decorator for some functions that take file as the first argument. The decorator has to implement the context manager protocol (i.e. turn the wrapped function into a context manager), so I figured I needed to wrap the function with a class.

I’m not really experienced with the decorator pattern and have never implemented a context manager before, but what I wrote works in Python 2.7 and it also works in Python 3.3 if I comment out the wraps line.

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

The error I get when uncommenting the wraps line occurs inside update_wrapper:

/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'

I know the docs don’t say that I even can use functools.wraps to wrap a function with a class like this, but then again, it just works in Python 2. Can someone please explain what exactly this traceback is telling me and what I should do to achieve the effects of wraps on both versions of Python?

---

EDIT: I was mistaken. The code above does not do what I want it to. I want to be able to use the function both with and without with, like the builtin open.

The code above turns the decorated function into a context manager. I want to be able to do:

reader = func('source.txt', arg)
for item in reader:
    pass

as well as

with func('source.txt', arg) as reader:
    for item in reader:
        pass

So my version of the code should probably look approximately as follows:

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

Feel free to comment about anything I have overlooked.

Note: the class version by J.F. Sebastian seems to work then:

I basically removed the wraps from the class and changed return CManager to:

@wraps(func)
def helper(*args, **kwargs):
    return CManager(*args, **kwargs)
return helper

Answer 1

functools.wraps() is for wrapper functions:

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

Example

@file_reader
def f(file):
    print(repr(file.read(10)))
    return file

with f('prog.py') as file:
    print(repr(file.read(10)))

If you want to use a class-based context manager then a workaround is:

def file_reader(func):
    @functools.wraps(func)
    def helper(*args, **kwds):
        return File(func, *args, **kwds)
    return helper

To make it behave identically whether the decorated function is used directly or as a context manager you should return self in __enter__():

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()