type() takes one or *three* arguments!?

[jamesthiele.usenet]

I was looking at Simon Burton's Povray.py code (part of pypov) and saw
this line:
globals()[name] = type( name, (KWItem,), {} ) # nifty :)

where 'KWItem' was a class. It did seem nifty, but it was unclear to me
what was happening.

I went to python.org's online documentation which said that type()
takes one argument. So I fired up python:

type(42) <type 'int'> type("x", (type(42),), {})

<class '__main__.x'>

OK, It appears that type() with 3 arguments constructs a class. Is this
documented somewhere? If not can someone explain what is going on?
james

[Pedro Werneck]

Hi,

Up to Python 2.2, type() was just a function to return an object type.

From 2.2 on, type have this behavior when called with only one argument and is used to create a new type when called with 3 arguments.

From http://www.python.org/2.2/descrintro.html :
"The signature of type() requires an explanation: traditionally, type(x)
returns the type of object x, and this usage is still supported.
However, type(name, bases, methods) is a new usage that creates a brand
new type object. (This gets into metaclass programming, and I won't go
into this further here except to note that this signature is the same as
that used by the Don Beaudry hook of metaclass fame.)"

So, an example:

Foo = type('Foo', (object,), {})

and...

class Foo(object):
pass

Are the same thing...


On 30 Jan 2005 11:57:23 -0800
ja****************@gmail.com wrote:
I was looking at Simon Burton's Povray.py code (part of pypov) and saw
this line:
globals()[name] = type( name, (KWItem,), {} ) # nifty :)

where 'KWItem' was a class. It did seem nifty, but it was unclear to
me what was happening.

I went to python.org's online documentation which said that type()
takes one argument. So I fired up python:

type(42) <type 'int'> type("x", (type(42),), {})

<class '__main__.x'>

OK, It appears that type() with 3 arguments constructs a class. Is
this documented somewhere? If not can someone explain what is going
on? james

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http://mail.python.org/mailman/listinfo/python-list

[jamesthiele.usenet]

Thank you - that explains everything quite nicely.