Why is getattr() so much slower than self.__dict__.get()?

Question

The example below is from a REST database driver on Python 2.7.

In the __setattr__ method below, if I use the commented out getattr() line, it reduces the object instantiation performance from 600 rps to 230.

Why is getattr() so much slower than self.__dict__.get() in this case?

class Element(object):

    def __init__(self, client):
        self._client = client
        self._data = {}
        self._initialized = True

    def __setattr__(self, key, value):
        #_initialized = getattr(self, "_initialized", False)
        _initialized = self.__dict__.get("_initialized", False)
        if key in self.__dict__ or _initialized is False:
            # set the attribute normally
            object.__setattr__(self, key, value)
        else:
            # set the attribute as a data property
            self._data[key] = value

Rumanian

On a side note, if you are having such a performance difference, do cache self.__dict__ in a local variable - even for only two access to it. (dict_ = self.__dict__ at the start of __setattr__) – Coquille

Answer 1

In short: because getattr(foo,bar) does the same thing as foo.bar, which is not the same thing as just accessing the __dict__ property (for a start, getattr has to select the right __dict__, but there's a whole lot more going on).

An example for illustration:

>>> class A:
...   a = 1
...
>>> class B(A):
...   b = 2
...
>>> dir(B)
['__doc__', '__module__', 'a', 'b']
>>> B.a
1
>>> B.__dict__
{'__module__': '__main__', 'b': 2, '__doc__': None}
>>> B.__dict__['a']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'a'
>>> B.__dict__.get('a')
>>>

Details contained in, or linked to here: http://docs.python.org/reference/datamodel.html (search for "getattr").

Ratel

@bereal: you need to re-read things. The necessary operations for foo.__dict__.get() are a proper subset of getattr(). – Georgie

@bereal Bear in mind that python bytecodes do not correspond to primitive operations. You can have a very small amount of bytecode to call a very complex function, while one simple expression can generate a lot more bytecode. Also, what bukzor said is correct and relevant. – Ratel

@bereal In no way does this disprove that foo.__dict__ does the same as getattr(foo, '__dict__'), because it is possible for calling getattr(foo,'__dict__') to be more efficient than dereferencing an arbitrary attribute. – Ratel