Difference between getattr vs getattribute

Question

I am trying to understand when to use   getattr   or   getattribute     The documentation mentions   getattribute   applies to new-style classes  What are new-style classes

User · Answer

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

User · Answer

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

User · Answer

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

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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   -- gt  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   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

User · Answer

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  gt  gt  gt  looking up x 42  f x   3 print f x   output  gt  gt  gt  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

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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

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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

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In reading through Beazley  amp  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

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