How do I programmatically set the docstring?

Question

I have a wrapper function that returns a function. Is there a way to programmatically set the docstring of the returned function? If I could write to __doc__ I'd do the following:

def wrapper(a):
    def add_something(b):
       return a + b
    add_something.__doc__ = 'Adds ' + str(a) + ' to `b`'
    return add_something

Then I could do

>>> add_three = wrapper(3)
>>> add_three.__doc__
'Adds 3 to `b`

However, since __doc__ is read-only, I can't do that. What's the correct way?

---

Edit: Ok, I wanted to keep this simple, but of course this is not what I'm actually trying to do. Even though in general __doc__ is writeable in my case it isn't.

I am trying to create testcases for unittest automatically. I have a wrapper function that creates a class object that is a subclass of unittest.TestCase:

import unittest
def makeTestCase(filename, my_func):
    class ATest(unittest.TestCase):
        def testSomething(self):
            # Running test in here with data in filename and function my_func
            data  = loadmat(filename)
            result = my_func(data)
            self.assertTrue(result > 0)

    return ATest

If I create this class and try to set the docstring of testSomething I get an error:

>>> def my_func(): pass
>>> MyTest = makeTestCase('some_filename', my_func)
>>> MyTest.testSomething.__doc__ = 'This should be my docstring'
AttributeError: attribute '__doc__' of 'instancemethod' objects is not writable

Answer 1

An instancemethod gets its docstring from its __func__. Change the docstring of __func__ instead. (The __doc__ attribute of functions are are writeable.)

>>> class Foo(object):
...     def bar(self):
...         pass
...
>>> Foo.bar.__func__.__doc__ = "A super docstring"
>>> help(Foo.bar)
Help on method bar in module __main__:

bar(self) unbound __main__.Foo method
    A super docstring

>>> foo = Foo()
>>> help(foo.bar)
Help on method bar in module __main__:

bar(self) method of __main__.Foo instance
    A super docstring

From the 2.7 docs:

User-defined methods

A user-defined method object combines a class, a class instance (or None) and any callable object (normally a user-defined function).

Special read-only attributes: im_self is the class instance object, im_func is the function object; im_class is the class of im_self for bound methods or the class that asked for the method for unbound methods; __doc__ is the method’s documentation (same as im_func.__doc__); __name__ is the method name (same as im_func.__name__); __module__ is the name of the module the method was defined in, or None if unavailable.

Changed in version 2.2: im_self used to refer to the class that defined the method.

Changed in version 2.6: For 3.0 forward-compatibility, im_func is also available as __func__, and im_self as __self__.