Python in Detail: Even Classes Are Objects

• In an earlier lesson, we saw that every Python value has attributes. In this lesson, we'll see that every value in Python is an instance of some class.

• We've already used the type(...) function to get a value's class.

• >

  type(5).__name__

  Result:

  Run Code

• When we call type(x), it returns x.__class__. We can also access that attribute directly.

• >

  (5).__class__.__name__

  Result:

  Run Code

• >

  ("Amir").__class__.__name__

  Result:

  'str'

• Our functions are instances of the function class.

• >

  def double(x):
    return x * 2
  
  double.__class__.__name__

  Result:

  'function'

• In Python, classes themselves are also values: we can put them in variables, pass them as functions, etc.

• >

  class Cat:
    def __init__(self, name):
      self.name = name
  
  some_class = Cat
  cat = some_class("Keanu")
  cat.name

  Result:

  'Keanu'

• If classes are values, and all values are instances of a class, then what's the class of a class?

• >

  class Cat:
    pass
  
  Cat.__class__.__name__

  Result:

  Run Code

• This may seem strange, but it's also consistent! In Python, "type" is sometimes used as a synonym for "class". A class like Cat is an instance of type.

• In that example, we accessed type's .__name__ attribute. type itself is a class, which must be a value, so it must be an object. What's the class of type?

• >

  type.__class__.__name__

  Result:

  Run Code

• Classes are instances of the type class. But type itself is also a class, so type is an instance of itself.

• If type is a class, then we can instantiate it like any other class. Instantiating type is a way to build a new type (a new class). The next example defines a Point class. But instead of using the regular class syntax, we build the class up piece by piece, starting with a call to type(...).

• >

  # This function will become our Point class's constructor.
  def __init__(self, x, y):
    self.x = x
    self.y = y
  
  # We build the `Point` class by instantiating the `type` class. It takes a
  # name for the new class, a tuple of base classes, and a dictionary of
  # attributes for the class.
  Point = type(
    "Point",
    (object, ),
    {
      "__init__": __init__,
    }
  )
  
  # Now we have our `Point` class, which works like any other class!
  p = Point(2, 3)
  (p.x, p.y)

  Result:

  (2, 3)

• This isn't a good way to define a new class in practice. But it shows us how far Python goes in exposing its internals, and in treating everything as an object. Even the class syntax is just a more convenient way to instantiate the type class.

• You've reached the end of this course! We thought that this was a good place to end, since it highlights how dynamic Python is. We hope that you enjoyed it!