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What is the purpose of type() and the types module in python

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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?

>>> import types
>>> a = type(1)
>>> b = (1).__class__
>>> c = int
>>> d = types.IntType
>>> a is b is c is d
True
>>> 

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.

>>> class xint(int):
 def __init__(self):
 pass

>>> x = xint()
>>> type(x) is int
False
>>> isinstance(x, int)
True
>>> 
posted Jun 27, 2013 by anonymous

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1 Answer

0 votes

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).

answer Jun 27, 2013 by anonymous
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