Difference between __getattr__ vs __getattribute__ Difference between __getattr__ vs __getattribute__ python python

Difference between __getattr__ vs __getattribute__


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__.


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=mymaxobj1 = 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 0obj1 = 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__ raisesAttributeError 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 objectobj1 = Count(1,10)print(obj1.mymin)print(obj1.mymax)print(obj1.current)


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 valuef = Foo()print f.x #output >>> looking up x 42f.x = 3print f.x #output >>> 3print ('__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


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