Mastering Python

Storing class attributes in definition order

There are cases where the definition order makes a difference. For example, let's assume we are creating a class that represents a CSV (Comma Separated Values) format. The CSV format expects the fields to have a particular order. In some cases this will be indicated by a header but it's still useful to have a consistent field order. Similar systems are using in ORM systems such as SQLAlchemy to store the column order for table definitions and for the input field order within forms in Django.

The classic solution without metaclasses

An easy way to store the order of the fields is by giving the field instances a special __init__ method which increments for every definition, so the fields have an incrementing ...