[Python-Dev] Type checks of instance variables (original) (raw)

Greg Ward gward@python.net
Wed, 10 Oct 2001 22:47:36 -0400

On 10 October 2001, Andy Robinson said:

> class Color(object): > slots =TypedSlots({ "r":(unsignedByte,unsignedByte(0), > "g":(unsignedByte,unsignedByte(0), > "b":(unsignedByte,unsignedByte(0), > "name":str}) >

This sounds great. In fact, it's almost exactly what we have been trying to do in reportlab/graphics for the last six months: a graphical widget hierarchy which has to check the type of every assignment and maybe trigger recalculations, because it supports a GUI. It was one of the few occasions when I wished I was using Delphi instead of Python :-) Reportlab would certainly be interested in a standardised framework for property-checking.

Allow me to plug Grouch, my attempt at a type-description language and type-enforcement tool for Python. See http://www.mems-exchange.org/software/grouch/ for some mild hype.

Grouch works by parsing type descriptions (currently found in class docstrings) and creating an ObjectSchema instance. An object schema consists of:

A class definition, in turn, consists (mainly) of a list of instance attributes and the type of each instance attribute.

One of the things you can do with an object schema is use it to walk an object graph, enforcing the attribute types in the class definitions in the schema. The two scripts currently distributed with Grouch do just that: gen_schema parses docstrings, creates an ObjectSchema, and pickles it; check_data loads the pickled schema, and uses it to typecheck some stored object graph (currently either another pickle or a ZODB).

The major things missing from Grouch right now are:

I would love for some other python-dev'ers to download Grouch and give it a look. In my non-at-all-humble opinion, Grouch is damned neat and provides a lot of cool ideas for anyone interested in deep, rigorous type-checking of Python objects.

    Greg

-- Greg Ward - Unix bigot gward@python.net http://starship.python.net/~gward/ All programmers are playwrights and all computers are lousy actors.