What is the purpose of type() and the types module and what is a type?

[Russel Walker]

The type() builtin according to python docs, returns a "type object".
http://docs.python.org/2/library/types.html

And in this module is bunch of what I assume are "type objects". Is this correct?
http://docs.python.org/2/library/functions.html#type

And type(), aside from being used in as an alternative to a class statement to create a new type, really just returns the object class, doesn't it?



If type() didn't exist would it be much more of a matter than the following?:


def type(x):
return x.__class__


What is the purpose of type()?
What exactly is a "type object"? Is it a "class"?
What is the purpose of the types module?

I understand the purpose of isinstance and why it's recommended over something like (type(1) is int). Because isinstance will also return True if the object is an instance of a subclass.

def __init__(self):
pass

 

[Steven D'Aprano]

The type() builtin according to python docs, returns a "type object".
http://docs.python.org/2/library/types.html

And in this module is bunch of what I assume are "type objects". Is this
correct? http://docs.python.org/2/library/functions.html#type

And type(), aside from being used in as an alternative to a class
statement to create a new type, really just returns the object class,
doesn't it?

type() does two things:

- with a single argument, it returns the actual type of an object;

- with three arguments, it creates new types.

For example, `type(42)` returns int, and type([]) returns list. Not the
*strings* "int" and "list", but the actual int object and list object.

py> x = 42
py> type(x)("23")
23


The three argument form of type() creates new types, also known as
classes. The class statement is just syntactic sugar, under the hood it
calls type.

(Note: in Python 2 there is a slight complication due to the existence of
so-called "old-style classes", also known as "classic classes". They are
a bit of a special case, but otherwise don't really make any difference
to what I'm saying.)

For example, the following:

class Spam(object):
a = 42
def method(self, x):
return self.a + x


is syntactic sugar for the much longer:


def method(self, x):
return self.a + x

d = {'method': method, 'a': 42}
Spam = type('Spam', (object,), d)
del d, method


(more or less, there may be slight differences).

If type() didn't exist would it be much more of a matter than the
following?:


def type(x):
return x.__class__

That only covers the one-argument form of type(). The three-argument form
is fundamental to Python's workings, Python cannot operate without it, or
something like it.

What is the purpose of type()?
What exactly is a "type object"? Is it a "class"? What is the purpose of
the types module?

"Type" is a synonym for "class". (To be precise, "new-style class" only,
classic classes are different, but the precise difference is no longer
important, and goes away in Python 3.)

The types module is just a collection of useful names for built-in types
that otherwise don't exist in the default namespace. E.g. functions are
objects, and have a type, but that type is not available by default:

py> FunctionType
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'FunctionType' is not defined


You need to import it first:

py> from types import FunctionType
py> FunctionType
<type 'function'>