On Thu, 27 Jun 2013 01:34:34 -0700, Russel Walker wrote:
> 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'>
--
Steven
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