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Python Decorator Problem with Docstrings

Answer 1

+1 A:

I found a solution, but don't know if it's really nice:

def decorator(f):
    def _decorator():
        print 'decorator active'
        f()
    _decorator.__name__=f.__name__
    _decorator.__doc__=f.__doc__
    return _decorator

The part with _decorator.__name__=f.__name__ seems a little bit hideous... What do you think?

Günther Jehle 2009-11-23 12:31:32

In fact this is (almost ?) exactly what functools.wraps do :)

thomas 2009-11-23 15:08:50

It doesn't look hideous to me. It says exactly what you want it to say. "I want the name of this function to be 'myfunction' instead of '_decorator'."

jcdyer 2009-11-23 16:20:19

Answer 2

+10 A:

Use functools.wraps() to update the attributes of the decorator:

from functools import wraps

def decorator(f):
    @wraps(f)
    def _decorator():
        print 'decorator active'
        f()
    return _decorator

@decorator
def foo():
    '''the magic foo function'''
    print 'this is function foo'

help(foo)

Also see the Standard Library documentation for functools.

Pär Wieslander 2009-11-23 12:33:04

This doesn't work if `foo` takes any arguments - they get replaced by whatever the `_decorator` uses. This is a problem especially when you want your decorator to take `*args, **kwds`. I've never been able to find a way to get the docstring correct using `functools.wraps`.

Scott Griffiths 2009-11-23 12:53:46

@Scott Griffiths: The docstring will still be correct even if `foo` takes arguments. However, `help(foo)` will display the parameter list of the `_decorator`, since it actually replaces the `foo` function. There's no good way around this if you're writing decorators that take arbitrary arguments using `*args, **kwargs`, but for me the important point is that the docstring is kept intact. Parameter details could always be specified in the docstring for clarity.

Pär Wieslander 2009-11-23 13:05:50

Thanks for the extra information. I've recently been failing to get the help description correct for decorated functions - it seems a pretty poor state of affairs, but I understand the difficulty as the decorated function could have a completely different signature. Still, there must be a way... :)

Scott Griffiths 2009-11-23 14:27:29

There is a way to do it. The decorator module http://pypi.python.org/pypi/decorator does it by using a trick. The trick is to rebuild the decorator signature and run exec on it. You can find the trick in line 118 in decorator.py. I think however that this approach is extreme.

Nadia Alramli 2009-11-23 15:11:08

Answer 3

+1 A:

Take a look at functools.wraps: http://docs.python.org/library/functools.html

Mark Byers 2009-11-23 12:34:52

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