python - Asymmetric behavior for getattr, newstyle vs oldstyle classes

Question

this is the first time I write here, sorry if the message is unfocuessed or too long.

I was interested in understanding more about how objects'attributes are fetched when needed. So I read the Python 2.7 documentation titled "Data Model" here, I met __getattr__ and, in order to check whether I understood or not its behavior, I wrote these simple (and incomplete) string wrappers.

class OldStr:
  def __init__(self,val):
    self.field=val

  def __getattr__(self,name):
    print "method __getattr__, attribute requested "+name

class NewStr(object):
  def __init__(self,val):  
    self.field=val

  def __getattr__(self,name):
    print "method __getattr__, attribute requested "+name

As you can see they are the same except for being an oldstyle vs newstyle classes. Since the quoted text says __getattr__ is "Called when an attribute lookup has not found the attribute in the usual places", I wanted to try a + operation on two instances of those classes to see what happened, expecting an identical behavior.

But the results I got puzzled me a little bit:

>>> x=OldStr("test")
>>> x+x
method __getattr__, attribute requested __coerce__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable

Good! I did not define a method for __coerce__ (although I was expecting a request for __add__, nevermind :), so __getattr__ got involved and returned a useless thing. But then

>>> y=NewStr("test")
>>> y+y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NewStr' and 'NewStr'

Why this asymmetric behavior between __getattr__ in oldstyle classes and newstyle ones when using a built-in operator like +? Could someone please help me to understand what is going on, and what was my mistake in reading the documentation?

Thanks a lot, your help is strongly appreciated!

Please don't use old style classes for anything. They're deprecated for obvious reasons -- like this one. Since they're deprecated, knowing how they used to work isn't helpful. Discard them and move on, please. — S.Lott
Comparing the behavior of old and new classes helps illustrate how the new-style classes work, especially if you are familiar with old-style classes. It is indeed unsurprising they work in different ways, and it's comforting to know the new classes work as expected. Still, there is value in trying to understand why code is interpreted the way it is no matter the version. — stw_dev

lunaryorn lunaryorn · Accepted Answer · 2010-08-05T15:00:21

See Special method lookup for new-style classes.

Special methods are directly looked up in the class object, the instance objects and consequently __getattr__() and __getattribute__() are bypassed. For precisely the same reason, instance.__add__ = foobar doesn't work.

This is done to speed up attribute access. Special methods are called very frequently, and making them subject to the standard, rather complex attribute lookup would significantly slow down the interpreter.

Attribute lookup for old-style classes is much less complex (most notably old style classes do not support descriptors), so there is no reason to treat special methods differently in old style classes.

python - Asymmetric behavior for __getattr__, newstyle vs oldstyle classes

3 Answers

python - Asymmetric behavior for getattr, newstyle vs oldstyle classes