Modern Python Standard Library Cookbook

Ordered dictionaries

One of the most surprising aspects of Python dictionaries for new users is that their order is unpredictable and can change from environment to environment. So, the order of keys you expected on your system might be totally different on your friend's computer.

This frequently causes unexpected failures during tests; if a continuous integration system is involved, the ordering of dictionary keys on the system running the tests can be different from the ordering on your system, which might lead to random failures.

Suppose you have a snippet of code that generates an HTML tag with some attributes:

>>> attrs = dict(style="background-color:red", id="header") >>> '<span {}>'.format(' '.join('%s="%s"' % a for a in attrs.items())) ...