Difference between __getattr__ and __getattribute__
Question:
I am trying to understand when to define __getattr__
or __getattribute__
. The python documentation mentions __getattribute__
applies to new-style classes. What are new-style classes?
Answers:
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
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 sets 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 do 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]
IMPORTANT
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)
In reading through Beazley & Jones PCB, I have stumbled on an explicit and practical use-case for __getattr__
that helps answer the “when” part of the OP’s question. From the book:
“The __getattr__()
method is kind of like a catch-all for attribute lookup. It’s a method that gets called if code tries to access an attribute that doesn’t exist.” We know this from the above answers, but in PCB recipe 8.15, this functionality is used to implement the delegation design pattern. If Object A has an attribute Object B that implements many methods that Object A wants to delegate to, rather than redefining all of Object B’s methods in Object A just to call Object B’s methods, define a __getattr__()
method as follows:
def __getattr__(self, name):
return getattr(self._b, name)
where _b is the name of Object A’s attribute that is an Object B. When a method defined on Object B is called on Object A, the __getattr__
method will be invoked at the end of the lookup chain. This would make code cleaner as well, since you do not have a list of methods defined just for delegating to another object.
- getattribute: Is used to retrieve an attribute from an instance. It captures every attempt to access an instance attribute by using dot notation or getattr() built-in function.
- getattr: Is executed as the last resource when attribute is not found in an object. You can choose to return a default value or to raise AttributeError.
Going back to the __getattribute__ function; if the default implementation was not overridden; the following checks are done when executing the method:
- Check if there is a descriptor with the same name (attribute name) defined in any class in the MRO chain (method object resolution)
- Then looks into the instance’s namespace
- Then looks into the class namespace
- Then into each base’s namespace and so on.
- Finally, if not found, the default implementation calls the fallback getattr() method of the instance and it raises an AttributeError exception as default implementation.
This is the actual implementation of the object.__getattribute__ method:
.. c:function:: PyObject* PyObject_GenericGetAttr(PyObject *o,
PyObject *name) Generic attribute getter function that is meant to
be put into a type object’s tp_getattro slot. It looks for a
descriptor in the dictionary of classes in the object’s MRO as well
as an attribute in the object’s :attr:~object.dict (if
present). As outlined in :ref:descriptors, data descriptors take
preference over instance attributes, while non-data descriptors
don’t. Otherwise, an :exc:AttributeError is raised.
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__
.
I am trying to understand when to define __getattr__
or __getattribute__
. The python documentation mentions __getattribute__
applies to new-style classes. What are new-style classes?
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
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 sets 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 do 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]
IMPORTANT
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)
In reading through Beazley & Jones PCB, I have stumbled on an explicit and practical use-case for __getattr__
that helps answer the “when” part of the OP’s question. From the book:
“The __getattr__()
method is kind of like a catch-all for attribute lookup. It’s a method that gets called if code tries to access an attribute that doesn’t exist.” We know this from the above answers, but in PCB recipe 8.15, this functionality is used to implement the delegation design pattern. If Object A has an attribute Object B that implements many methods that Object A wants to delegate to, rather than redefining all of Object B’s methods in Object A just to call Object B’s methods, define a __getattr__()
method as follows:
def __getattr__(self, name):
return getattr(self._b, name)
where _b is the name of Object A’s attribute that is an Object B. When a method defined on Object B is called on Object A, the __getattr__
method will be invoked at the end of the lookup chain. This would make code cleaner as well, since you do not have a list of methods defined just for delegating to another object.
- getattribute: Is used to retrieve an attribute from an instance. It captures every attempt to access an instance attribute by using dot notation or getattr() built-in function.
- getattr: Is executed as the last resource when attribute is not found in an object. You can choose to return a default value or to raise AttributeError.
Going back to the __getattribute__ function; if the default implementation was not overridden; the following checks are done when executing the method:
- Check if there is a descriptor with the same name (attribute name) defined in any class in the MRO chain (method object resolution)
- Then looks into the instance’s namespace
- Then looks into the class namespace
- Then into each base’s namespace and so on.
- Finally, if not found, the default implementation calls the fallback getattr() method of the instance and it raises an AttributeError exception as default implementation.
This is the actual implementation of the object.__getattribute__ method:
.. c:function:: PyObject* PyObject_GenericGetAttr(PyObject *o,
PyObject *name) Generic attribute getter function that is meant to
be put into a type object’s tp_getattro slot. It looks for a
descriptor in the dictionary of classes in the object’s MRO as well
as an attribute in the object’s :attr:~object.dict (if
present). As outlined in :ref:descriptors, data descriptors take
preference over instance attributes, while non-data descriptors
don’t. Otherwise, an :exc:AttributeError is raised.
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__
.