Meaning of "a type/class/function/module/... object"?

Question

When I read Python in a Nutshell, I saw a "type object", "class object", "function object", "module object", ....

Does a "xxx object" mean an instance of type "xxx"?

For example,

• does a type object mean an instance of type type? For example, is int a type object? is a specific class a type object?

• does a class object mean an instance of what? Is a specific class a class object? For example,

  A class is a Python object with several characteristics:

  • You can call a class object as if it were a function. The call, often known as instantiation, returns an object known as an instance of the class; the class is known as the type of the instance.

  • A class can inherit from other classes, meaning it delegates to other class objects the lookup of attributes that are not found in the class itself.

• does a function object mean an instance of types.FunctionType? Is a specific function a function object?

• does a module object mean an instance of types.ModuleType? Is a specific module a module object?

• is there an int object? If yes, does it mean an integer value (e.g. 12) of type int?

Thanks.

Answer 1

Does a type object mean an instance of type type? For example, is int a type object? Ιs a specific class a type object?

Yes, a "type" object means an object of type "type", ergo an instance of that type. Information on what type something is can always be checked with isinstance. To answer your second question, is int an object of type type:

>>> isinstance(int, type) # True

Are custom classes type objects:

>>> class Spammy: 
...     pass
>>> isinstance(Spammy, type)
True

Does a class object mean an instance of what? Is a specific class a class object?

The use of class object is interchangeable with the term type object after the Unification of types and classes in Python 2.2. The term 'classes' is mainly used to refer to user defined classes (the ones you create with the class statement) but it really means the same thing.

This did make a difference in Python 2 that has old style classes. Not inheriting from object in a class definition gave an object of type classobj:

>>> class Foo:
...     pass
>>> type(Foo)
<type 'classobj'>

This just doesn't exist in Python 3 where all classes are objects of type type.

Does a function object mean an instance of types.FunctionType? Is a specific function a function object?

Does a module object mean an instance of types.ModuleType? Is a specific module a module object?

Yes to all of these:

>>> def foo(): pass
>>> type(foo)  
function 
>>> import collections    
>>> type(collections)
module

Is there an int object? If yes, does it mean an integer value (e.g. 12) of type int?

There is an int type, which is a type object and an int object, which is an object of type int. An example of that is 12 since:

>>> type(12)
int 

While, the same thing on int yields:

>>> type(int)
type

Answer 2

See the documentation of type():

With one argument, return the type of an object. The return value is a type object and generally the same object as returned by object.__class__.

The isinstance() built-in function is recommended for testing the type of an object, because it takes subclasses into account.

Let's see. Is int a type object?

>>> type(int)
<class 'type'>

A class object means an instance of what? In Python 3, classes are always new-style classes so they constitute a type of themselves:

>>> class X:
...     pass
... 
>>> y = X()
>>> type(y)
<class '__main__.X'>

But with Python 2.x's old-style classes:

>>> class X:
...     pass
... 
>>> y = X()
>>> type(y)
<type 'instance'>
>>> type(X)
<type 'classobj'>

Here, the class has types.ClassType and the instance has types.InstanceType (neither of which exists in Python 3.x)

A function object is a types.FunctionType:

>>> def foo():
...    return 42
>>> type(foo) is types.FunctionType
True
>>> type(lambda x: x + 2) is types.FunctionType
True

A module object is a types.ModuleType:

>>> import sys
>>> type(sys) is types.ModuleType
True

And yes, there are int objects. Consider type(12).

Answer 3

In Python you can represent types as objects themselves. So yes, they're type objects. For every type there is an object representing that type, like int. It's an instance of the type itself.

It's like .getClass() and Class<?> objects in Java: "some string".getClass() == String.class. Java doesn't treat functions as first class values so there's types like Callable, Consumer<T>, etc., but in Python there's a specific type for functions, and with that a specific object representing that type.