Origin of metaclasses in Python

October 24, 2013 — originally posted on The History of Python

There was some speculation on python-ideas today on whether Python's metaclass design came from Ruby. It did not. And as long as we are speculating about the origins of language features, I feel the need to set the record straight.

I was not inspired by Ruby at that point (or ever :-). Ruby was in fact inspired by Python. Mats once told me that his inspiration was 20% Python, 80% Perl, and that Larry Wall is his hero.

I wrote about metaclasses in Python in 1998: http://www.python.org/doc/essays/metaclasses/.

New-style classes were just the second or third iteration of the idea.

I was inspired to implement new-style classes by a very specific book, "Putting Metaclasses to Work" by Ira Forman and Scott Danforth (http://www.amazon.com/Putting-Metaclasses-Work-Ira-Forman/dp/0201433052).

But even Python's original design (in 1990, published in 1991) had the notion that 'type' was itself an object. The type pointer in any object has always been a pointer to a special object, whose "data" was a bunch of C function pointers implementing the behavior of other objects, similar to a C++ vtable. The type of a type was always a special type object, which you could call a meta-type, to be recognized because it was its own type.

I was only vaguely aware of Smalltalk at the time; I remember being surprised by its use of metaclasses (which is quite different from that in Python or Ruby!) when I read about them much later. Smalltalk's bytecode was a bigger influence of Python's bytecode though. I'd read about it in a book by Adele Goldberg and others, I believe "Smalltalk-80: The Language and its Implementation" (http://www.amazon.com/Smalltalk-80-The-Language-its-Implementation/dp/0201113716).

Comments (1)

Guido van Rossum — October 24, 2013

Also note that that 1998 essay contains a fully-functional Enum implementation that has many of the same features as PEP 435. (Via Christian Heimes.)