Functools.update_wrapper() doesn't work properly

Question

I use Functools.update_wrapper() in my decorator, but It seems like update_wrapper rewrites only function attributes (such as __doc__, __name__), but does not affect on help() function.

I aware of these answers, but they don't work with decorator-class.

Here is my function.

import functools

class memoized(object):

    def __init__(self, func):
        self.func = func
        functools.update_wrapper(self, func)

    def __call__(self, *args):
        self.func(*args)

@memoized 
def printer(arg):
    "This is my function"
    print arg

Here is the output

>>> printer.__doc__
This is my function

>>> help(printer)
Help on memoized in module __main__ object:

printer = class memoized(__builtin__.object)
 |  Methods defined here:
 |  
 |  __call__(self, *args)
 |  
 |  __init__(self, func)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)

It looks like a bug, but how can I fix it?

Answer 1

functools.update_wrapper() sets the attribute on the instance, but in Python versions up to 3.8, help() looks at the information on the type.

So printer.__doc__ gives you the instance attribute, help() prints information about type(printer), e.g. the memoized class, which does not have a __doc__ attribute.

This was not considered a bug, this was all by design; help() will always look at the class when you pass in an instance. Don't use a class as decorator if you want help() to work for the decorated function, or upgrade your Python version.

This was eventually changed in Python 3.9, see the bug report discussion.