[l0nwlf]

There is a reason Python is paired with Mercurial.

Hg is written in Python. As for why Mercurial was chosen over Bazaar, it came down to popularity. As the core developer survey shows, hg was preferred over bzr.

[briancurtin]

That's not really the reason, and I believe it's even stated in the PEP.

[sunqiang]

yeah, http://www.python.org/dev/peps/pep-0374/