dict_from_class()

In this section we define a functions that gets a dictionary from a class. This dictionary contains all the information supplied in the body of a class statement, except for the doc-string.

The __dict__ of the empty class

Here’s our empty class again:

>>> class A(object):
...     pass

As seen in Constructing classes, even for the empty class its class dictionary has entries. Handling these always-there entries is a nuisance when deconstructing classes. Here, once again, is the list of entries.

>>> sorted(A.__dict__.keys())
['__dict__', '__doc__', '__module__', '__weakref__']

The __dict__ and __weakref__ entries are there purely for system purposes. This makes them easier to deal with.

The class docstring __doc__ is None unless the user supplies a value.

>>> A.__doc__ is None
True

>>> class A2(object):
...     'This is the docstring'

>>> A2.__doc__
'This is the docstring'

Ordinarily, __module__ is the name of the module in which the class is defined. However, because of the way Sphinx uses doctest, it gets the name of the module wrong. Please don’t worry about this. Despite what it says below, it’s the name of the module.

>>> A.__module__
'__builtin__'

Is the doc-string part of the body?

Soon we will define a function that copies the body, as a dictionary, out of a class. But first we must answer the question: Is the doc-string part of the body of a class?

There is no completely satisfactory answer to this question, as there are good arguments on both sides. We choose NO, because for example using the -OO command line option will remove doc-strings, and so they are not an essential part of the body of the class. (However, -OO does not remove doc-strings produced explicitly, by assigning to __doc__.)

The keys to be excluded are precisely the ones that the empty class (which has an empty body) has.

>>> _excluded_keys = set(A.__dict__.keys())

Definition of dict_from_class()

This function simply filters the class dictionary, copying only the items whose key is not excluded.

>>> def dict_from_class(cls):
...     return dict(
...         (key, value)
...         for (key, value) in cls.__dict__.items()
...         if key not in _excluded_keys
...         )

As expected, the empty class has an empty body.

>>> dict_from_class(A)
{}

Here’s a class whose body is not empty.

>>> class B(object):
...     'This docstring is not part of the body.'''
...     s = 'a string'
...     def f(self): pass

We get what we expect for the body. (See [somewhere] for why we need the __func__.)

>>> dict_from_class(B) == dict(s='a string', f=B.f.__func__)
True

Here’s another way of expressing the same truth.

>>> sorted(dict_from_class(B).items())
[('f', <function f at 0x...>), ('s', 'a string')]