Why is Python class not recognizing static variable

Question

I am trying to make a class in Python with static variables and methods (attributes and behaviors)

import numpy

class SimpleString():    
    popSize = 1000 
    displaySize = 5
    alphatbet = "abcdefghijklmnopqrstuvwxyz "

    def __init__(self):
        pop = numpy.empty(popSize, object)
        target = getTarget()
        targetSize = len(target)

When the code runs though it says that it cannot make the array pop because popSize is not defined

Solution 1

You either need to access it with a self.popSize or SimpleString.popSize. When you declare a variable in a class in order for any of the instance functions to access that variable you will need to use self or the class name(in this case SimpleString) otherwise it will treat any variable in the function to be a local variable to that function.

The difference between self and SimpleString is that with self any changes you make to popSize will only be reflected within the scope of your instance, if you create another instance of SimpleString popSize will still be 1000. If you use SimpleString.popSize then any change you make to that variable will be propagated to any instance of that class.

import numpy

class SimpleString():    
    popSize = 1000 
    displaySize = 5
    alphatbet = "abcdefghijklmnopqrstuvwxyz "

    def __init__(self):
        pop = numpy.empty(self.popSize, object)
        target = getTarget()
        targetSize = len(target)

Solution 2

You need to use self or the class object to access class attributes:

def __init__(self):
    pop = numpy.empty(self.popSize, object)
    target = getTarget()
    targetSize = len(target)

or

def __init__(self):
    pop = numpy.empty(SimpleString.popSize, object)
    target = getTarget()
    targetSize = len(target)

The latter form is really only needed if you want to bypass an instance attribute with the same name:

>>> class Foo(object):
...     bar = 42
...     baz = 42
...     def __init__(self):
...         self.bar = 38
...     def printBar(self):
...         print self.bar, Foo.bar
...     def printBaz(self):
...         print self.baz, Foo.baz
... 
>>> f = Foo()
>>> f.printBar()
38 42
>>> f.printBaz()
42 42

Here self.bar is an instance attribute (setting always happens on the object directly). But because there is no baz instance attribute, self.baz finds the class attribute instead.