defer_doc.py

from collections import UserString


class ResolvedDocString(UserString):
    __class__ = str


class DeferDocString(UserString):
    def __init__(self, f):
        self._f = f

    def __getattribute__(self, attr):
        if attr == '__class__':
            return str
        elif attr == '_f':
            return super().__getattribute__(attr)

        val = self._f()
        del self._f
        self.__class__ = ResolvedDocString
        self.data = val
        return getattr(self, attr)

session

In [1]: from defer_doc import DeferDocString                                                   

In [2]: class C: 
   ...:     def m(self): 
   ...:         pass 
   ...:                                                                                        

In [3]: C.m.__doc__ = DeferDocString(lambda: 'test')                                           

In [4]: C.m?                                                                                   
Signature: C.m(self)
Docstring: test
File:      ~/projects/tmp/<ipython-input-2-871863a5a2d3>
Type:      function

In [5]: C.m?                                                                                   
Signature: C.m(self)
Docstring: test
File:      ~/projects/tmp/<ipython-input-2-871863a5a2d3>
Type:      function

In [6]: C.m.__doc__[0]                                                                         
Out[6]: 't'

In [7]: C.m.__doc__.upper()                                                                    
Out[7]: 'TEST'