Populate once static variable from static method Python within a class

Question

I have a Class A in Python and I would like to populate the a static variable calling a static method like:

Class A:
 arr = []
 @staticmethod
 def FillArr():
 #do more stuff but for semplicity...
 A.arr = [[2,2,2,]]
 FillArr.__func__()

when I run the code I got 'NameError: name A not defined' so essentially I can't initialize the arr static variable. Essentially once the the class has been instantiated once I would like to populate the static variable

Answer 1

This runs flawlessly on Python 3.6:

class A:
 arr = []
 @staticmethod
 def fillArr():
 #do more stuff but for simplicity...
 A.arr = [[2,2,2,]]
A.fillArr()
print (A.arr)

Or, with the extra info in your comment:

class A:
 arr = []
 @staticmethod
 def fillArr():
 #do more stuff but for simplicity...
 A.arr = [[2,2,2,]]
 def __init__ (self):
 if not A.arr:
 A.fillArr ()
A ()
print (A.arr)

• ai2016: ok I accepted but I needed to add a boolean to make it execute once
• Jacques de Hooge: I've retested it, but on my system (win10, python3.6 it runs unmodified, printing [[2, 2, 2]]. Curious where you had to add the boolean...
• ai2016: yes because your code has only one instance of A , if you have multiple you should notice the diff
• Jacques de Hooge: Ok, you want to populate it only once. Understand. Will edit.

Answer 2

Use @classmethod:

class A(object):
 arr = []
 @classmethod
 def FillArr(cls):
 cls.arr = [[2,2,2]]
A.FillArr()
print A.arr

This will result in: [[2,2,2]]

/edit/ the using normal method example I mention in my comment below (based on Jacques explanation):

class A
 arr=[]
 def FillArr(self):
 self.arr = [[2,2,2,]]
 def __init__(self):
 self.FillArr()
a = A()
print a.arr

• ai2016: nope.. I need to populate within the class not out of it... essentially once the the class has been instantiated once I would like to populate the static variable
• Nemelis: @Jacques: lol I was already looking surprised when I suddenly saw the edit on my own post without doing a thing ;-)
• Nemelis: @ai2016: What Jacques posts with his staticmethod solution can also done with classmethod. It is just what you like: Do you want to use the name of the class in the method explictly than use staticmethod, if instead you want to use cls (not to be confused with self, which is reserved for instanciated classes), use classmethod. But you can even do it with a normal method by replacing cls with self and call self.FillArr in init
• Jacques de Hooge: To be honest I like the classmethod better, since it doesn't depend upon the name of the class, which may easily change, while the first param is by convention almost always named cls. Just took the question literally.