Pywikibot is a Python-based framework to write bots for MediaWiki (more information).
Thanks to work in Google Code-in, Pywikibot now has a script called download_dump.py. It downloads a Wikimedia database dump from http://dumps.wikimedia.org/ , and places the dump in a predicable directory for semi-automated use by other scripts and tests.
You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.