Inherit a parent class docstring as __doc__ attribute

Question

There is a question about Inherit docstrings in Python class inheritance, but the answers there deal with method docstrings.

My question is how to inherit a docstring of a parent class as the __doc__ attribute. The usecase is that Django rest framework generates nice documentation in the html version of your API based on your view classes' docstrings. But when inheriting a base class (with a docstring) in a class without a docstring, the API doesn't show the docstring.

It might very well be that sphinx and other tools do the right thing and handle the docstring inheritance for me, but django rest framework looks at the (empty) .__doc__ attribute.

class ParentWithDocstring(object):
    """Parent docstring"""
    pass


class SubClassWithoutDoctring(ParentWithDocstring):
    pass


parent = ParentWithDocstring()
print parent.__doc__  # Prints "Parent docstring".
subclass = SubClassWithoutDoctring()
print subclass.__doc__  # Prints "None"

I've tried something like super(SubClassWithoutDocstring, self).__doc__, but that also only got me a None.

Answer 1

Since you cannot assign a new __doc__ docstring to a class (in CPython at least), you'll have to use a metaclass:

import inspect

def inheritdocstring(name, bases, attrs):
    if not '__doc__' in attrs:
        # create a temporary 'parent' to (greatly) simplify the MRO search
        temp = type('temporaryclass', bases, {})
        for cls in inspect.getmro(temp):
            if cls.__doc__ is not None:
                attrs['__doc__'] = cls.__doc__
                break

    return type(name, bases, attrs)

Yes, we jump through an extra hoop or two, but the above metaclass will find the correct __doc__ however convoluted you make your inheritance graph.

Usage:

>>> class ParentWithDocstring(object):
...     """Parent docstring"""
... 
>>> class SubClassWithoutDocstring(ParentWithDocstring):
...     __metaclass__ = inheritdocstring
... 
>>> SubClassWithoutDocstring.__doc__
'Parent docstring'

The alternative is to set __doc__ in __init__, as an instance variable:

def __init__(self):
    try:
        self.__doc__ = next(cls.__doc__ for cls in inspect.getmro(type(self)) if cls.__doc__ is not None)
    except StopIteration:
        pass

Then at least your instances have a docstring:

>>> class SubClassWithoutDocstring(ParentWithDocstring):
...     def __init__(self):
...         try:
...             self.__doc__ = next(cls.__doc__ for cls in inspect.getmro(type(self)) if cls.__doc__ is not None)
...         except StopIteration:
...             pass
... 
>>> SubClassWithoutDocstring().__doc__
'Parent docstring'

As of Python 3.3 (which fixed issue 12773), you can finally just set the __doc__ attribute of custom classes, so then you can use a class decorator instead:

import inspect

def inheritdocstring(cls):
    for base in inspect.getmro(cls):
        if base.__doc__ is not None:
            cls.__doc__ = base.__doc__
            break
    return cls

which then can be applied thus:

>>> @inheritdocstring
... class SubClassWithoutDocstring(ParentWithDocstring):
...     pass
... 
>>> SubClassWithoutDocstring.__doc__
'Parent docstring'