[Python-Dev] Assignment to class (original) (raw)

Guido van Rossum guido@python.org
Thu, 09 Jan 2003 10:31:29 -0500

I'm testing large chunks of our code base with Python 2.3a1 and have run into another minor snag. Five months ago Guido committed a patch to prevent assigning class for non-heap-types, which was backported to 2.2-maint two weeks ago in response to SF #658106. This is a great idea for preventing nonsensical assignments to None.class, or 2.class, but it is too heavy handed in preventing assignments to [1,2,3].class, (1,2,3).class or {1:2,3:4}.class.

My specific use-case involves dictionary and list objects. I define a classes that inherits from list or dict and add specialized algebraic, vector and tensor functions over the range and domain of the data in the list or dictionary. I could just copy the data into my new objects, but it is wasteful since these structures can be very large and deeply nested. I suspect that it is possible to come up with better criteria for allowing safe assignment to class that will still allow the useful technique I describe above.

You can only set class when the old and new class instance have the same instance layout at the C level. Changing this is impossible given the way objects are implemented in C. This means you can never change a list into a dict or vice versa, because the C structs are different.

Or do I misunderstand you? Can you give an example of something you think should be allowed but currently isn't?

--Guido van Rossum (home page: http://www.python.org/~guido/)