Python headers -- What is the purpose of the $ ( dollar sign ) encapsulation?

Question

Apologies in advance, this is something that I really figured I could find on the web, but I was having troubles.

In Python, in the header portion, I see that sometimes people will wrap their literals with a dollar sign ( $ ). It seems, judging from examples, that this is a pointer to fill in information that is automatically updated (maybe via source control?), but I do not understand how it works.

For instance:

__version__ = '$Revision: 4799 $'.split()[1]
__date__ = '$Date: 2006-09-25 11:09:02 -0400 (Mon, 25 Sep 2006) $'.split()[1]
__author__ = 'John Doe FIX: put in the authors name'

is an example that I found at ( Python Example Documentation Template ). So the $ is wrapped around the version (which will alter over time), and the date (which will also alter). Assuming it is some sort of pointer to always capture up-to-date information regarding the version, date, etc., can someone point me to how this works? Which source control software use this syntax? Is it the same across languages?

Thank you!

Answer 1

This isn't specific to Python, it's something used by source control systems, going back to RCS and CVS.

See CVS Keywords for a list of the CVS syntax. While few people use CVS nowadays, many of the source control systems people do use (most notably, subversion) handle CVS syntax or a slight variation of it. (And some people also create build scripts that do similar substitutions at build time rather than checkin time.)