>>> class Pizza(object):
...   def __init__(self, size):
...     self.size = size
...   def get_size(self):
...     return self.size
...
>>> Pizza.get_size
<unbound method Pizza.get_size>

>>> Pizza.get_size()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unbound method get_size() must be called with Pizza instance as first argument (got nothing instead)

>>> Pizza.get_size(Pizza(42))

>>> Pizza(42).get_size
<bound method Pizza.get_size of <__main__.Pizza object at 0x7f3138827910>>
>>> Pizza(42).get_size()
42

>>> m = Pizza(42).get_size
>>> m()
42

>>> m = Pizza(42).get_size
>>> m.__self__
<__main__.Pizza object at 0x7f3138827910>
>>> 
# You could guess, look at this:
...
>>> m == m.__self__.get_size
True

>>> class Pizza(object):
...   def __init__(self, size):
...     self.size = size
...   def get_size(self):
...     return self.size
...
>>> Pizza.get_size
<function Pizza.get_size at 0x7f307f984dd0>

class Pizza(object):
  @staticmethod
  def mix_ingredients(x, y):
    return x + y
 
  def cook(self):
    return self.mix_ingredients(self.cheese, self.vegetables)

>>> Pizza().cook is Pizza().cook
False
>>> Pizza().mix_ingredients is Pizza.mix_ingredients
True
>>> Pizza().mix_ingredients is Pizza().mix_ingredients
True

>>> class Pizza(object):
...   radius = 42
...   @classmethod
...   def get_radius(cls):
...     return cls.radius
... 
>>> 
>>> Pizza.get_radius
<bound method type.get_radius of <class '__main__.Pizza'>>
>>> Pizza().get_radius
<bound method type.get_radius of <class '__main__.Pizza'>>
>>> Pizza.get_radius is Pizza().get_radius
True
>>> Pizza.get_radius()
42

class Pizza(object):
  def __init__(self, ingredients):
    self.ingredients = ingredients
 
  @classmethod
  def from_fridge(cls, fridge):
    return cls(fridge.get_cheese() + fridge.get_vegetables())

class Pizza(object):
  def __init__(self, radius, height):
    self.radius = radius
    self.height = height
 
  @staticmethod
  def compute_area(radius):
     return math.pi * (radius ** 2)
 
  @classmethod
  def compute_volume(cls, height, radius):
     return height * cls.compute_area(radius)
 
  def get_volume(self):
    return self.compute_volume(self.height, self.radius)

class Pizza(object):
  def get_radius(self):
    raise NotImplementedError

>>> Pizza()
<__main__.Pizza object at 0x7fb747353d90>
>>> Pizza().get_radius()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 3, in get_radius
NotImplementedError

import abc
 
class BasePizza(object):
  __metaclass__ = abc.ABCMeta
 
  @abc.abstractmethod
  def get_radius(self):
     
"""Method that should do something."""

>>> BasePizza()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class BasePizza with abstract methods get_radius

import abc
 
class BasePizza(object):
  __metaclass__ = abc.ABCMeta
 
  @abc.abstractmethod
  def get_ingredients(self):
     
"""Returns the ingredient list."""
 
class Calzone(BasePizza):
  def get_ingredients(self, with_egg=False):
    egg = Egg() if with_egg else None
    return self.ingredients + egg

import abc
 
class BasePizza(object):
  __metaclass__ = abc.ABCMeta
 
  @abc.abstractmethod
  def get_ingredients(self):
     
"""Returns the ingredient list."""
 
class DietPizza(BasePizza):
  @staticmethod
  def get_ingredients():
    return None

import abc
 
class BasePizza(object):
  __metaclass__ = abc.ABCMeta
 
  ingredient = ['cheese']
 
  @classmethod
  @abc.abstractmethod
  def get_ingredients(cls):
     
"""Returns the ingredient list."""
     return cls.ingredients

import abc
 
class BasePizza(object):
  __metaclass__ = abc.ABCMeta
 
  default_ingredients = ['cheese']
 
  @classmethod
  @abc.abstractmethod
  def get_ingredients(cls):
     
"""Returns the ingredient list."""
     return cls.default_ingredients
 
class DietPizza(BasePizza):
  def get_ingredients(self):
    return ['egg'] + super(DietPizza, self).get_ingredients()