I am trying to understand when to use __getattr__
or __getattribute__
. The documentation mentions __getattribute__
applies to new-sty
I find that no one mentions this difference:
__getattribute__
has a default implementation, but __getattr__
does not.
class A:
pass
a = A()
a.__getattr__ # error
a.__getattribute__ # return a method-wrapper
This has a clear meaning: since __getattribute__
has a default implementation, while __getattr__
not, clearly python encourages users to implement __getattr__
.
New-style classes inherit from object
, or from another new style class:
class SomeObject(object):
pass
class SubObject(SomeObject):
pass
Old-style classes don't:
class SomeObject:
pass
This only applies to Python 2 - in Python 3 all the above will create new-style classes.
See 9. Classes (Python tutorial), NewClassVsClassicClass and What is the difference between old style and new style classes in Python? for details.
A key difference between __getattr__
and __getattribute__
is that __getattr__
is only invoked if the attribute wasn't found the usual ways. It's good for implementing a fallback for missing attributes, and is probably the one of two you want.
__getattribute__
is invoked before looking at the actual attributes on the object, and so can be tricky to implement correctly. You can end up in infinite recursions very easily.
New-style classes derive from object
, old-style classes are those in Python 2.x with no explicit base class. But the distinction between old-style and new-style classes is not the important one when choosing between __getattr__
and __getattribute__
.
You almost certainly want __getattr__
.
This is just an example based on Ned Batchelder's explanation.
__getattr__
example:
class Foo(object):
def __getattr__(self, attr):
print "looking up", attr
value = 42
self.__dict__[attr] = value
return value
f = Foo()
print f.x
#output >>> looking up x 42
f.x = 3
print f.x
#output >>> 3
print ('__getattr__ sets a default value if undefeined OR __getattr__ to define how to handle attributes that are not found')
And if same example is used with __getattribute__
You would get >>> RuntimeError: maximum recursion depth exceeded while calling a Python object
Lets see some simple examples of both __getattr__
and __getattribute__
magic methods.
__getattr__
Python will call __getattr__
method whenever you request an attribute that hasn't already been defined. In the following example my class Count has no __getattr__
method. Now in main when I try to access both obj1.mymin
and obj1.mymax
attributes everything works fine. But when I try to access obj1.mycurrent
attribute -- Python gives me AttributeError: 'Count' object has no attribute 'mycurrent'
class Count():
def __init__(self,mymin,mymax):
self.mymin=mymin
self.mymax=mymax
obj1 = Count(1,10)
print(obj1.mymin)
print(obj1.mymax)
print(obj1.mycurrent) --> AttributeError: 'Count' object has no attribute 'mycurrent'
Now my class Count has __getattr__
method. Now when I try to access obj1.mycurrent
attribute -- python returns me whatever I have implemented in my __getattr__
method. In my example whenever I try to call an attribute which doesn't exist, python creates that attribute and set it to integer value 0.
class Count:
def __init__(self,mymin,mymax):
self.mymin=mymin
self.mymax=mymax
def __getattr__(self, item):
self.__dict__[item]=0
return 0
obj1 = Count(1,10)
print(obj1.mymin)
print(obj1.mymax)
print(obj1.mycurrent1)
__getattribute__
Now lets see the __getattribute__
method. If you have __getattribute__
method in your class, python invokes this method for every attribute regardless whether it exists or not. So why we need __getattribute__
method? One good reason is that you can prevent access to attributes and make them more secure as shown in the following example.
Whenever someone try to access my attributes that starts with substring 'cur' python raises AttributeError
exception. Otherwise it returns that attribute.
class Count:
def __init__(self,mymin,mymax):
self.mymin=mymin
self.mymax=mymax
self.current=None
def __getattribute__(self, item):
if item.startswith('cur'):
raise AttributeError
return object.__getattribute__(self,item)
# or you can use ---return super().__getattribute__(item)
obj1 = Count(1,10)
print(obj1.mymin)
print(obj1.mymax)
print(obj1.current)
Important: In order to avoid infinite recursion in __getattribute__
method, its implementation should always call the base class method with the same name to access any attributes it needs. For example: object.__getattribute__(self, name)
or super().__getattribute__(item)
and not self.__dict__[item]
If your class contain both getattr and getattribute magic methods then __getattribute__
is called first. But if __getattribute__
raises
AttributeError
exception then the exception will be ignored and __getattr__
method will be invoked. See the following example:
class Count(object):
def __init__(self,mymin,mymax):
self.mymin=mymin
self.mymax=mymax
self.current=None
def __getattr__(self, item):
self.__dict__[item]=0
return 0
def __getattribute__(self, item):
if item.startswith('cur'):
raise AttributeError
return object.__getattribute__(self,item)
# or you can use ---return super().__getattribute__(item)
# note this class subclass object
obj1 = Count(1,10)
print(obj1.mymin)
print(obj1.mymax)
print(obj1.current)
New-style classes are ones that subclass "object" (directly or indirectly). They have a __new__
class method in addition to __init__
and have somewhat more rational low-level behavior.
Usually, you'll want to override __getattr__
(if you're overriding either), otherwise you'll have a hard time supporting "self.foo" syntax within your methods.
Extra info: http://www.devx.com/opensource/Article/31482/0/page/4