Question

I’m creating a class in Python, which then has some nested classes added using setattr().

class T( object ):
    def __init__( self ):
        cDict = {}
        cDict['__doc__'] = 'Inner class doc string'
        setattr( self, 'C', type('C', (), cDict ) )

However, calling help( T ) does not then include and information about C. Constructing a T, and then a C within it, works fine.

Doing this the traditional way works fine:

class T2( object ): 
    class C2( object ):
        __doc__ = 'Inner class doc string'

Calling help( T2 ) displays information about C2.

Can somebody shed some light on what’s happening here? Thanks.

Answer 1

The two snippets you showed are not equivalent.

This:

class T( object ):
    def __init__( self ):
        cDict = {}
        cDict['__doc__'] = 'Inner class doc string'
        setattr( self, 'C', type('C', (), cDict ) )

will set a C attribute on each T instance:

>>> T.C
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'T' has no attribute 'C'
>>> t = T()
>>> t.C
<class '__main__.C'>

This is because you placed the setattr inside __init__.

While this:

class T2( object ): 
    class C2( object ):
        __doc__ = 'Inner class doc string'

will add an attribute on T2 itself:

>>> T2.C2
<class '__main__.C2'>