Creating a type class using type class in python

Question

I was reading this interesting post on metaclasses What is a metaclass in Python?. The accepted answer shows how to create a class using type with the following signature.

type(name of the class, tuple of the parent class (for inheritance, can be empty), dictionary containing attributes names and values)

I tried to create 'type' class using the above signature and I was surprised that I was allowed to create one in the first place! Your views are appreciated!

type = type('type',(),{});

Second, after I created a type class using the above syntax, I was not able to do

myclass = type('myclass',(),{});

and type = type('type',(),{});

I got an error saying

Traceback (most recent call last): File "", line 1, in TypeError: object.new() takes no parameters

But, when I tried to the following, I could succeed.

class myclass(object):
    pass

I am puzzled coz, according to my understanding the above snippet should invoke type in an attempt to create the class 'myclass'. So whats going on!? Am I missing some detail?

For the record, type classes used to refer to metaclasses here are a misnomer; type classes are a widely used concept in Functional Programming languages such as Haskell and Scala and have nothing to do with metaclasses. — Erik Kaplun
– Erik Kaplun, Commented Apr 13, 2014 at 17:07

Jon Clements · Accepted Answer · 2012-06-25 20:59:29Z

6

You've shadowed type with type with type = type('type',(),{})

Note that semi-colons are redundant in Python.

So you're creating a type that can't do anything.

That's why the later type breaks.

answered Jun 25, 2012 at 20:59

Jon Clements

143k34 gold badges254 silver badges288 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Karthick Over a year ago

Yes.. I agree.. I understand thats what is happening. But my question is Why does python even allow that and type being shadowed, how is class myclass(object) successfull? shouldnt it try to use the type class that I have created and therefore fail?

Jon Clements Over a year ago

@Karthick Because it has a philosophy of "consenting adults" - you're allowed to over-ride any builtins... I could write a module that says str = int, hex = lambda L: return 'FF' - etc... Python is a language that is flexible and allows control if you explicitly wish to do so - this is almost always not what is required though, but the black magic is there if you find a need to use it

Karthick Over a year ago

I can agree with the philosophy, and then again, I still don understand how type('myclass', (), {}) fails but Class myclass(Object) : pass succeeds

Jon Clements Over a year ago

Because it's the difference between a definition and a construction

Karthick Over a year ago

Well if I am overshadowing it, I should be able to do it INSTEAD of the default type rite and isnt it the purpose of over shadowing? Like for example, when you overload operator '+' in C++, you see only overloaded '+' not the default one.. You could either invoke it explicitly with a function or just say + . both would invoke my function not the default.

|

Community · Accepted Answer · 2017-05-23 10:26:41Z

0

Definition

>>> shadow = type("type",(),{})
>>> shadow
<class '__main__.type'>
>>> isinstance(shadow,type)
True
>>> issubclass(shadow,type)
False
>>> shadow(int)

Traceback (most recent call last):
  File "<pyshell#7>", line 1, in <module>
    shadow(int)
TypeError: object.__new__() takes no parameters

Construction

>>> class shadow(type):
    pass

>>> shadow
<class '__main__.shadow'>
>>> isinstance(shadow,type)
True
>>> issubclass(shadow,type)
True
>>> shadow(int)
<type 'type'>
>>> shadow(type)
<type 'type'>
>>> shadow(1)
<type 'int'>

Python (and Python C API): __new__ versus __init__

edited May 23, 2017 at 10:26

CommunityBot

11 silver badge

answered Jun 25, 2012 at 21:20

Paul Seeb

6,1963 gold badges30 silver badges38 bronze badges

Collectives™ on Stack Overflow

Creating a type class using type class in python

2 Answers 2

8 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related