Is Python dict an Object?

Question

I have a dict like this:

>>> my_dict = {u'2008': 6.57, u'2009': 4.89, u'2011': 7.74,
...            u'2010': 7.44, u'2012': 7.44}

Output with has_key:

>>> my_dict.has_key(unicode(2012))
True

Output with hasattr:

>>> hasattr(my_dict, unicode(2012))
False

I couldn't understand why this behaves differently. I googled and found out that it is because dict and objects are different.

But, still I couldn't understand the difference properly.

(BTW : I am using python 2.7)

Answer 1

dict instances are objects too. But their keys are just not exposed as as attributes.

Exposing the keys as attributes (too or instead of item access) would lead to namespace pollution; you'd never be able to use a has_key key, for example. has_key is already an attribute on dictionaries:

>>> hasattr({}, 'has_key')
True
>>> {}.has_key
<built-in method has_key of dict object at 0x7fa2a8461940>

Attributes of objects and the contents of dictionaries are two separate things, and the separation is deliberate.

You can always subclass dict to add attribute access using the __getattr__() hook method:

class AttributeDict(dict):
    def __getattr__(self, name):
        if name in self:
            return self[name]
        raise AttributeError(name)

Demo:

>>> demo = AttributeDict({'foo': 'bar'})
>>> demo.keys()
['foo']
>>> demo.foo
'bar'

Existing attributes on the dict class take priority:

>>> demo['has_key'] = 'monty'
>>> demo.has_key
<built-in method has_key of AttributeDict object at 0x7fa2a8464130>

Answer 2

has_key checks for the existence of a key in the dictionary. (One your code defines while creating a dictionary) hasattr checks if the object has an attribute.

Dictionaries are objects, and they have certain attributes. hasattr checks for those.

>>> hasattr(dict, 'has_key')
True
>>> hasattr(dict, 'items')
True
>>> newDict = {'a': 1, 'b':2}
>>> newDict.has_key('a')
True

You can use dir() which lists out the valid attributes for an object.

>>> dir(dict)
['__class__', '__cmp__', '__contains__', '__delattr__', '__delitem__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'has_key', 'items', 'iteritems', 'iterkeys', 'itervalues', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values', 'viewitems', 'viewkeys', 'viewvalues']