Class method differences in Python bound unbound and static

Question

What is the difference between the following class methods   Is it that one is static and the other is not   class Test object     def method one self       print  Called method one     def method two        print  Called method two   a test   Test   a test method one   a test method two

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that is an error   first of all  first line should be like this  be careful of capitals   class Test object     Whenever you call a method of a class  it gets itself as the first argument  hence the name self  and method two gives this error    gt  gt  gt  a method two   Traceback  most recent call last   File   lt stdin gt    line 1  in  lt module gt  TypeError  method two   takes no arguments  1 given

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Bound method   instance method  Unbound method   static method

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Methods in Python are a very  very simple thing once you understood the basics of the descriptor system   Imagine the following class   class C object       def foo self           pass   Now let s have a look at that class in the shell    gt  gt  gt  C foo  lt unbound method C foo gt   gt  gt  gt  C   dict    foo    lt function foo at 0x17d05b0 gt    As you can see if you access the foo attribute on the class you get back an unbound method  however inside the class storage  the dict  there is a function   Why s that   The reason for this is that the class of your class implements a   getattribute   that resolves descriptors   Sounds complex  but is not   C foo is roughly equivalent to this code in that special case    gt  gt  gt  C   dict    foo     get   None  C   lt unbound method C foo gt    That s because functions have a   get   method which makes them descriptors   If you have an instance of a class it s nearly the same  just that None is the class instance    gt  gt  gt  c   C    gt  gt  gt  C   dict    foo     get   c  C   lt bound method C foo of  lt   main   C object at 0x17bd4d0 gt  gt    Now why does Python do that   Because the method object binds the first parameter of a function to the instance of the class   That s where self comes from   Now sometimes you don t want your class to make a function a method  that s where staticmethod comes into play    class C object      staticmethod   def foo       pass   The staticmethod decorator wraps your class and implements a dummy   get   that returns the wrapped function as function and not as a method    gt  gt  gt  C   dict    foo     get   None  C   lt function foo at 0x17d0c30 gt    Hope that explains it

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Unbound Methods  Unbound methods are methods that are not bound to any particular class instance yet   Bound Methods  Bound methods are the ones which are bound to a specific instance of a class    As its documented here  self can refer to different things depending on the function is bound  unbound or static    Take a look at the following example    class MyClass          def some method self           return self    For the sake of the example   gt  gt  gt  MyClass   some method    lt   main   MyClass object at 0x10e8e43a0 gt   This can also be written as  gt  gt  gt  obj   MyClass     gt  gt  gt  obj some method    lt   main   MyClass object at 0x10ea12bb0 gt     Bound method call   gt  gt  gt  obj some method 10  TypeError  some method   takes 1 positional argument but 2 were given    WHY IT DIDN T WORK    obj some method 10  bound call translated as   MyClass some method obj  10  unbound method and it takes 2    arguments now instead of 1     ----- USING THE UNBOUND METHOD ------  gt  gt  gt  MyClass some method 10  10   Since we did not use the class instance     obj     on the last call  we can kinda say it looks like a static method   If so  what is the difference between MyClass some method 10  call and a call to a static function decorated with a  staticmethod decorator   By using the decorator  we explicitly make it clear that the method will be used without creating an instance for it first  Normally one would not expect the class member methods to be used without the instance and accesing them can cause possible errors depending on the structure of the method   Also  by adding the  staticmethod decorator  we are making it possible to be reached through an object as well   class MyClass          def some method self           return self           staticmethod     def some static method number           return number   gt  gt  gt  MyClass some static method 10      without an instance 10  gt  gt  gt  MyClass   some static method 10      Calling through an instance 10   You can   t do the above example with the instance methods  You may survive the first one  as we did before  but the second one will be translated into an unbound call MyClass some method obj  10  which will raise a TypeError since the instance method takes one argument and you unintentionally tried to pass two   Then  you might say     if I can call static methods through both an instance and a class  MyClass some static method and MyClass   some static method should be the same methods     Yes

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Accurate explanation from Armin Ronacher above  expanding on his answers so that beginners like me understand it well   Difference in the methods defined in a class  whether static or instance method there is yet another type - class method - not discussed here so skipping it   lay in the fact whether they are somehow bound to the class instance or not  For example  say whether the method receives a reference to the class instance during runtime  class C      a           def foo self           pass  C   this is the class object C a   is a list object  class property object  C foo   is a function object  class property object  c   C    c   this is the class instance   The   dict   dictionary property of the class object holds the reference to all the properties and methods of a class object and thus    gt  gt  gt  C   dict    foo    lt function foo at 0x17d05b0 gt    the method foo is accessible as above  An important point to note here is that everything in python is an object and so references in the dictionary above are themselves pointing to other objects  Let me call them Class Property Objects - or as CPO within the scope of my answer for brevity   If a CPO is a descriptor  then python interpretor calls the   get     method of the CPO to access the value it contains   In order to determine if a CPO is a descriptor  python interpretor checks if it implements the descriptor protocol  To implement descriptor protocol is to implement 3 methods  def   get   self  instance  owner  def   set   self  instance  value  def   delete   self  instance    for e g     gt  gt  gt  C   dict    foo     get   c  C    where    self is the CPO  it could be an instance of list  str  function etc  and is supplied by the runtime instance is the instance of the class where this CPO is defined  the object  c  above  and needs to be explicity supplied by us owner is the class where this CPO is defined the class object  C  above  and needs to be supplied by us  However this is because we are calling it on the CPO  when we call it on the instance  we dont need to supply this since the runtime can supply the instance or its class polymorphism  value is the intended value for the CPO and needs to be supplied by us   Not all CPO are descriptors  For example    gt  gt  gt  C   dict    foo     get   None  C   lt function C foo at 0x10a72f510 gt    gt  gt  gt  C   dict    a     get   None  C  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   list  object has no attribute    get      This is because the list class doesnt implement the descriptor protocol   Thus the argument self in c foo self  is required because its method signature is actually this C   dict    foo     get   c  C   as explained above  C is not needed as it can be found out or polymorphed  And this is also why you get a TypeError if you dont pass that required instance argument   If you notice the method is still referenced via the class Object C and the binding with the class instance is achieved via passing a context in the form of the instance object into this function    This is pretty awesome since if you chose to keep no context or no binding to the instance  all that was needed was to write a class to wrap the descriptor CPO and override its   get     method to require no context   This new class is what we call a decorator and is applied via the keyword  staticmethod  class C object      staticmethod   def foo       pass   The absence of context in the new wrapped CPO foo doesnt throw an error and can be verified as follows    gt  gt  gt  C   dict    foo     get   None  C   lt function foo at 0x17d0c30 gt    Use case of a static method is more of a namespacing and code maintainability one taking it out of a class and making it available throughout the module etc     It maybe better to write static methods rather than instance methods whenever possible  unless ofcourse you need to contexualise the methods like access instance variables  class variables etc   One reason is to ease garbage collection by not keeping unwanted reference to objects

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Please read this docs from the Guido First Class everything Clearly explained how Unbound  Bound methods are born

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The definition of method two is invalid  When you call method two  you ll get TypeError  method two   takes 0 positional arguments but 1 was given from the interpreter    An instance method is a bounded function when you call it like a test method two    It automatically accepts self  which points to an instance of Test  as its first parameter  Through the self parameter  an instance method can freely access attributes and modify them on the same object

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The call to method two will throw an exception for not accepting the self parameter the Python runtime will automatically pass it   If you want to create a static method in a Python class  decorate it with the staticmethod decorator   Class Test Object      staticmethod   def method two        print  Called method two   Test method two

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When you call a class member  Python automatically uses a reference to the object as the first parameter  The variable self actually means nothing  it s just a coding convention  You could call it gargaloo if you wanted  That said  the call to method two would raise a TypeError  because Python is automatically trying to pass a parameter  the reference to its parent object  to a method that was defined as having no parameters   To actually make it work  you could append this to your class definition   method two   staticmethod method two    or you could use the  staticmethod function decorator

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The second one won t work because when you call it like that python internally tries to call it with the a test instance as the first argument  but your method two doesn t accept any arguments  so it wont work  you ll get a runtime error  If you want the equivalent of a static method you can use a class method  There s much less need for class methods in Python than static methods in languages like Java or C   Most often the best solution is to use a method in the module  outside a class definition  those work more efficiently than class methods

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gt  gt  gt  class Class object           def   init   self               self i   0         def instance method self               self i    1             print self i         c   0          classmethod         def class method cls               cls c    1             print cls c          staticmethod         def static method s               s    1             print s       gt  gt  gt  a   Class    gt  gt  gt  a class method   1  gt  gt  gt  Class class method        The class shares this value across instances 2  gt  gt  gt  a instance method   1  gt  gt  gt  Class instance method     The class cannot use an instance method Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unbound method instance method   must be called with Class instance as first argument  got nothing instead   gt  gt  gt  Class instance method a  2  gt  gt  gt  b   0  gt  gt  gt  a static method b  1  gt  gt  gt  a static method a c    Static method does not have direct access to   gt  gt  gt                         class or instance properties  3  gt  gt  gt  Class c          a c above was passed by value and not by reference  2  gt  gt  gt  a c 2  gt  gt  gt  a c   5          The connection between the instance  gt  gt  gt  Class c          and its class is weak as seen here  2  gt  gt  gt  Class class method   3  gt  gt  gt  a c 5

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method two won t work because you re defining a member function but not telling it what the function is a member of  If you execute the last line you ll get    gt  gt  gt  a test method two   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  method two   takes no arguments  1 given    If you re defining member functions for a class the first argument must always be  self

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In Python  there is a distinction between bound and unbound methods    Basically  a call to a member function  like method one   a bound function  a test method one     is translated to  Test method one a test    i e  a call to an unbound method  Because of that  a call to your version of method two will fail with a TypeError   gt  gt  gt  a test   Test     gt  gt  gt  a test method two   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  method two   takes no arguments  1 given     You can change the behavior of a method using a decorator  class Test object       def method one self           print  Called method one        staticmethod     def method two            print  Called method two    The decorator tells the built-in default metaclass type  the class of a class  cf  this question  to not create bound methods for method two   Now  you can invoke static method both on an instance or on the class directly    gt  gt  gt  a test   Test    gt  gt  gt  a test method one   Called method one  gt  gt  gt  a test method two   Called method two  gt  gt  gt  Test method two   Called method two

[python] Class method differences in Python: bound, unbound and static

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