How to properly use attributes in Python

example.py

class FaultyFoo (object):

    bar = []


class GoodFoo (object):

    def __init__(self):
        self.bar = []


foo_one = FaultyFoo()
foo_one.bar.append('foo 1')

foo_two = FaultyFoo()
foo_two.bar.append('foo 2')


print "Faulty Foo one =>", foo_one.bar  # Prints ['foo 1', 'foo 2']
print "Faulty Foo two =>", foo_two.bar  # Prints ['foo 1', 'foo 2']
print "Faulty Foo one bar memory address =>", hex(id(foo_one.bar))  # Prints 0X1234
print "Faulty Foo two bar memory address =>", hex(id(foo_two.bar))  # Prints 0x1234

"""
Notice how in this example self.bar points to the same memory address.
"""

foo_one = GoodFoo()
foo_one.bar.append('foo 1')

foo_two = GoodFoo()
foo_two.bar.append('foo 2')


print "Good Foo one =>", foo_one.bar  # Prints ['foo 1']
print "Good Foo two =>", foo_two.bar  # Prints ['foo 2']
print "Good Foo one bar memory address =>", hex(id(foo_one.bar))  # Prints 0x1234
print "Good Foo two bar memory address =>", hex(id(foo_two.bar))  # Prints 0x5678

"""
Why?

When we create a class, it's attributes are mutable by every instance of the class.

This is why you have to set the attributes of a class in the __init__ function.

The init function gets called automatically when you instantiate a class.
"""