Instance variables vs. class variables in Python

Question

I have Python classes, of which I need only one instance at runtime, so it would be sufficient to have the attributes only once per class and not per instance. If there woul

Answer 1

Same question at Performance of accessing class variables in Python - the code here adapted from @Edward Loper

Local Variables are the fastest to access, pretty much tied with Module Variables, followed by Class Variables, followed by Instance Variables.

There are 4 scopes you can access variables from:

1. Instance Variables (self.varname)
2. Class Variables (Classname.varname)
3. Module Variables (VARNAME)
4. Local Variables (varname)

The test:

import timeit

setup='''
XGLOBAL= 5
class A:
    xclass = 5
    def __init__(self):
        self.xinstance = 5
    def f1(self):
        xlocal = 5
        x = self.xinstance
    def f2(self):
        xlocal = 5
        x = A.xclass
    def f3(self):
        xlocal = 5
        x = XGLOBAL
    def f4(self):
        xlocal = 5
        x = xlocal
a = A()
'''
print('access via instance variable: %.3f' % timeit.timeit('a.f1()', setup=setup, number=300000000) )
print('access via class variable: %.3f' % timeit.timeit('a.f2()', setup=setup, number=300000000) )
print('access via module variable: %.3f' % timeit.timeit('a.f3()', setup=setup, number=300000000) )
print('access via local variable: %.3f' % timeit.timeit('a.f4()', setup=setup, number=300000000) )

The result:

access via instance variable: 93.456
access via class variable: 82.169
access via module variable: 72.634
access via local variable: 72.199