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"""Class and Instance Variables.

@see: https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables

Generally speaking, instance variables are for data unique to each instance and class variables are

for attributes and methods shared by all instances of the class.

"""

def test_class_and_instance_variables():

"""Class and Instance Variables."""

# pylint: disable=too-few-public-methods

class Dog:

"""Dog class example"""

kind = 'canine' # Class variable shared by all instances.

def __init__(self, name):

self.name = name # Instance variable unique to each instance.

fido = Dog('Fido')

buddy = Dog('Buddy')

# Shared by all dogs.

assert fido.kind == 'canine'

assert buddy.kind == 'canine'

# Unique to fido.

assert fido.name == 'Fido'

# Unique to buddy.

assert buddy.name == 'Buddy'

# Shared data can have possibly surprising effects with involving mutable objects such as lists

# and dictionaries. For example, the tricks list in the following code should not be used as a

# class variable because just a single list would be shared by all Dog instances.

# pylint: disable=too-few-public-methods

class DogWithSharedTricks:

"""Dog class example with wrong shared variable usage"""

tricks = [] # Mistaken use of a class variable (see below) for mutable objects.

def __init__(self, name):

self.name = name # Instance variable unique to each instance.

def add_trick(self, trick):

"""Add trick to the dog

This function illustrate mistaken use of mutable class variable tricks (see below).

"""

self.tricks.append(trick)

fido = DogWithSharedTricks('Fido')

buddy = DogWithSharedTricks('Buddy')

fido.add_trick('roll over')

buddy.add_trick('play dead')

assert fido.tricks == ['roll over', 'play dead'] # unexpectedly shared by all dogs

assert buddy.tricks == ['roll over', 'play dead'] # unexpectedly shared by all dogs

# Correct design of the class should use an instance variable instead:

# pylint: disable=too-few-public-methods

class DogWithTricks:

"""Dog class example"""

def __init__(self, name):

self.name = name # Instance variable unique to each instance.

self.tricks = [] # creates a new empty list for each dog

def add_trick(self, trick):

"""Add trick to the dog

This function illustrate a correct use of mutable class variable tricks (see below).

"""

self.tricks.append(trick)

fido = DogWithTricks('Fido')

buddy = DogWithTricks('Buddy')

fido.add_trick('roll over')

buddy.add_trick('play dead')

assert fido.tricks == ['roll over']

assert buddy.tricks == ['play dead']