This module is a polyfill, implementing ChainMap for reasonably-recent versions of Python that do not have collections.ChainMap--namely, Python 2.6, Python 3.2, and PyPy3 releases based on Python 3.2. (It will also work as expected on Python 2.7, PyPy, and Python 3.3 and higher, but it is not needed there since those verions' collections modules contains a ChainMap implementation.)
The code for this package is closely derived from the Python 3.5 source code at hg.python.org, (especially the collections and reprlib modules). Several changes have been made to ensure Python 2.6 compatibility, and tests and packaging have been added.
from chainmap import ChainMap
If you prefer to preferentially import from the Python standard library when ChainMap is available there, this will do the trick:
try: from collections import ChainMap except ImportError: from chainmap import ChainMap
Please see the standard documentation for details on how to use ChainMap.
If you're not sure why someone would use a ChainMap, they are useful for managing "nested" contexts and "overlays." See e.g. my options, quoter, and say packages, which use them quite extensively for flexible option handling. You can find similar uses in the Context class of Django's template engine.
- Automated multi-version testing managed with pytest, pytest-cov, coverage, and tox. Packaging linting with pyroma.
- Successfully packaged for, and tested against, all late-model versions of Python: 2.6, 2.7, 3.2, 3.3, 3.4, and 3.5 as well as PyPy 2.6.1 (based on 2.7.10) and PyPy3 2.4.0 (based on 3.2.5).
- The author, Jonathan Eunice or @jeunice on Twitter welcomes your comments and suggestions.
To install or upgrade to the latest version:
pip install -U chainmap
To easy_install under a specific Python version (3.3 in this example):
python3.3 -m easy_install --upgrade chainmap
(You may need to prefix these with sudo to authorize installation. In environments without super-user privileges, you may want to use pip's --user option, to install only for a single user, rather than system-wide.)