Metaclass conundrum - binding value from an outer scope

Peter Otten __peter__ at
Thu Apr 20 16:19:45 EDT 2017

Skip Montanaro wrote:

> For various reasons, I have a class which delegates much functionality to
> a singleton instance of another class (exposed by pybind11) instead of
> inheriting from that class. So, the construction looks like this (this is
> in Python 2.7):
> from someothermodule import SomeOtherClass as _SomeOtherClass
> class SomeClass(object):
>     _instance = None
>     def __init__(self):
>         if self.__class__._instance is None:
>             self._instance = _SomeOtherClass.instance()
>     def __getattr__(self, key):
>         return getattr(self._instance, key)
>     ... and so on ...
> If someone tries help(SomeClass) or dir(SomeClass) today, none of the
> attributes or docstrings defined in SomeOtherClass are shown.

If being helpful really is the only purpose of the metaclass you can 
implement a SomeClass.__dir__() method instead:

     def __dir__(self):
         names = dir(self._instance)
         # <snip whatever post-processing you want>
         return names         

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