Question

I have a container class (subclassed from list) where I have overridden the geattr method;

def __getattr__(self, name):
    def _multiplexed(*args, **kwargs):
        return [getattr(R, name)(*args, **kwargs) for R in self]
    return _multiplexed

The trouble is that now I can’t ask Python for help about my object. When I do ask for help I get (traceback from iPython) the error which is copied below.

So the question is: How can I override getattr without losing my ability to call for help?

Thanks,
Jeremy

help(Collect)
/home/jlconlin/CustomPython/trunk/Collect.py in <module>()
----> 1 
      2 
      3 
      4 
      5 

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.pyc in __call__(self, *args, **kwds)
    455     def __call__(self, *args, **kwds):
    456         import pydoc
--> 457         return pydoc.help(*args, **kwds)
    458 
    459 def sethelper():

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.pyc in __call__(self, request)
   1721     def __call__(self, request=None):
   1722         if request is not None:
-> 1723             self.help(request)
   1724         else:
   1725             self.intro()

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.pyc in help(self, request)
   1768             elif request: doc(request, 'Help on %s:')
   1769         elif isinstance(request, Helper): self()
-> 1770         else: doc(request, 'Help on %s:')
   1771         self.output.write('\n')
   1772 

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.pyc in doc(thing, title, forceload)
   1506     """Display text documentation, given an object or a path to an object."""
   1507     try:
-> 1508         pager(render_doc(thing, title, forceload))
   1509     except (ImportError, ErrorDuringImport), value:
   1510         print value

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.pyc in render_doc(thing, title, forceload)
   1483     desc = describe(object)
   1484     module = inspect.getmodule(object)
-> 1485     if name and '.' in name:
   1486         desc += ' in ' + name[:name.rfind('.')]
   1487     elif module and module is not object:

TypeError: argument of type 'function' is not iterable

Answer 1

Looking at the pydoc source code, the culprit is:

getattr(thing, '__name__', None)

That tries to look up thing.__name__, and if it fails, returns None instead. Except, in your case, it doesn’t fail; it gets a value back from __getattr__ instead.

If you’re overriding __getattr__, it’s a good idea to special case all special attributes. Something like:

if name.startswith("__"):
    raise AttributeError

This ensures that if your class doesn’t have a particular special attribute (in this case, __name__), lookups for it will fail, rather than producing an unexpected value.