dir()
is basically a convenience method, it is not supposed to return everything, what it basically does is that it recursively returns everything found in the dictionary of a class and its bases.
PyPy's implementation of dir()
is quite easy to understand:
def dir(*args):
...
elif isinstance(obj, (types.TypeType, types.ClassType)):
# Don't look at __class__, as metaclass methods would be confusing.
return sorted(_classdir(obj))
...
def _classdir(klass):
"""Return a set of the accessible attributes of class/type klass.
This includes all attributes of klass and all of the base classes
recursively.
"""
names = set()
ns = getattr(klass, '__dict__', None)
if ns is not None:
names.update(ns)
bases = getattr(klass, '__bases__', None)
if bases is not None:
# Note that since we are only interested in the keys, the order
# we merge classes is unimportant
for base in bases:
names.update(_classdir(base))
return names
As each class basically inherits from object
you will see some dunder methods included because they are actually part of object
's dictionary:
>>> class A(object):
pass
...
>>> set(dir(A)) == set(list(object.__dict__) + list(A.__dict__))
True
Now what about __bases__
and other missing items?
First of all object
itself is an instance of something, well it's bit of a mess actually:
>>> isinstance(type, object)
True
>>> isinstance(object, type)
True
>>> issubclass(type, object)
True
>>> issubclass(object, type)
False
>>> type.mro(object)
[<type 'object'>]
>>> type.mro(type)
[<type 'type'>, <type 'object'>]
So, all of the attributes like __bases__
, __ge__
etc are actually part of type
:
>>> list(type.__dict__)
['__module__', '__abstractmethods__', '__getattribute__', '__weakrefoffset__', '__dict__', '__lt__', '__init__', '__setattr__', '__subclasses__', '__new__', '__base__', '__mro__', 'mro', '__dictoffset__', '__call__', '__itemsize__', '__ne__', '__instancecheck__', '__subclasscheck__', '__gt__', '__name__', '__eq__', '__basicsize__', '__bases__', '__flags__', '__doc__', '__delattr__', '__le__', '__repr__', '__hash__', '__ge__']
Hence when we do A.__bases__
we are actually looking up a descriptor on type of A
, i.e type
:
>>> A.__bases__
(<type 'object'>,)
>>> type(A).__dict__['__bases__'].__get__(A, type)
(<type 'object'>,)
So, as A
is an instance of type
these methods are not part of its own dictionary but its type's dictionary.
>> class A(object):
... spam = 'eggs'
...
>>> a = A()
>>> a.foo = 100
>>> a.bar = 200
>>> a.__dict__
{'foo': 100, 'bar': 200}
>>> A.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'A' objects>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'A' objects>, '__doc__': None, 'spam': 'eggs'})
As, type
is a subclass of object
, the dir()
call on type
will contain some items from object
:
>>> set(dir(type)) - set(type.__dict__)
set(['__reduce_ex__', '__str__', '__format__', '__reduce__', '__class__', '__subclasshook__', '__sizeof__'])