Accessing a class member variables in Python

Question

class Example object       def the example self           itsProblem    problem   theExample   Example   print theExample itsProblem    How do I access a class s variable   I ve tried adding this definition   def return itsProblem self       return itsProblem   Yet  that fails also

User · Answer

If you have an instance function  i e  one that gets passed self  you can use self to get a reference to the class using self   class   For example in the code below tornado creates an instance to handle get requests  but we can get hold of the get handler class and use it to hold a riak client so we do not need to create one for every request  import tornado web import riak  class get handler tornado web requestHandler       riak client   None      def post self           cls   self   class           if cls riak client is None              cls riak client   riak RiakClient pb port 8087  protocol  pbc             Additional code to send response to the request

User · Answer

You are declaring a local variable  not a class variable  To set an instance variable  attribute   use  class Example object       def the example self           self itsProblem    problem      lt -- remember the  self    theExample   Example   theExample the example   print theExample itsProblem    To set a class variable  a k a  static member   use  class Example object       def the example self           Example itsProblem    problem            or  type self  itsProblem    problem            depending what you want to do when the class is derived

User · Answer

Implement the return statement like the example below  You should be good  I hope it helps someone    class Example object       def the example self           itsProblem    problem          return itsProblem    theExample   Example   print theExample the example

User · Answer

The answer  in a few words  In your example  itsProblem is a local variable   Your must use self to set and get instance variables  You can set it in the   init   method  Then your code would be   class Example object       def   init   self           self itsProblem    problem    theExample   Example   print theExample itsProblem    But if you want a true class variable  then use the class name directly   class Example object       itsProblem    problem    theExample   Example   print theExample itsProblem  print  Example itsProblem    But be careful with this one  as theExample itsProblem is automatically set to be equal to Example itsProblem  but is not the same variable at all and can be changed independently   Some explanations  In Python  variables can be created dynamically  Therefore  you can do the following   class Example object       pass  Example itsProblem    problem   e   Example   e itsSecondProblem    problem   print Example itsProblem    e itsSecondProblem    prints      True   Therefore  that s exactly what you do with the previous examples   Indeed  in Python we use self as this  but it s a bit more than that  self is the the first argument to any object method because the first argument is always the object reference  This is automatic  whether you call it self or not   Which means you can do   class Example object       def   init   self           self itsProblem    problem    theExample   Example   print theExample itsProblem    or   class Example object       def   init   my super self           my super self itsProblem    problem    theExample   Example   print theExample itsProblem    It s exactly the same  The first argument of ANY object method is the current object  we only call it self as a convention  And you add just a variable to this object  the same way you would do it from outside   Now  about the class variables    When you do   class Example object       itsProblem    problem    theExample   Example   print theExample itsProblem    You ll notice we first set a class variable  then we access an object  instance  variable  We never set this object variable but it works  how is that possible   Well  Python tries to get first the object variable  but if it can t find it  will give you the class variable  Warning  the class variable is shared among instances  and the object variable is not   As a conclusion  never use class variables to set default values to object variables  Use   init   for that   Eventually  you will learn that Python classes are instances and therefore objects themselves  which gives new insight to understanding the above  Come back and read this again later  once you realize that

[python] Accessing a class' member variables in Python?

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