Using getattr in python

Question

The getattr function is defined as follows:

getattr(object, name[, default])

Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object's attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar . If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

Which method does getattr() call? For example, does it call:

• __getattr__
• __get__
• __getattribute__
• something else?

Answer 1

getattr() goes to __getattribute__() first, same as the dot operator:

>>> class A:
...     def __getattr__(self, obj):
...         print("Called __getattr__")
...         return None
...     def __getattribute__(self, obj):
...         print("Called __getattribute__")
...         return None
...     def __get__(self, obj):
...         print("Called __get__")
...         return None
... 
>>> a = A()
>>> a.foobar
Called __getattribute__
>>> getattr(a, 'foobar')
Called __getattribute__

Convention is to use getattr() only when you don't know at compile-time what the attribute name is supposed to be. If you do, then use the dot operator ("explicit is better than implicit"...).

As @Klaus D. mentioned in a comment, the python Data Model documentation goes into more detail about how .__getattribute__() and .__getattr__() interact. Suffice it to say that, at a high level, the latter is a fall-back option of sorts for if the former fails. Note that .__getattr__() and the built-in getattr() are not directly related - IIRC this is a quirk of naming that originated in earlier versions of python and was granfathered into python 3.