Meaning of $ in a string?

Question

I came along this

__date__ = "$Date: 2011/06$"

and found this in the docs

• $$ is an escape; it is replaced with a single $.
• $identifier names a substitution placeholder matching a mapping key of "identifier". By default, "identifier" must spell a Python identifier. The first non-identifier character after the $ character terminates this placeholder specification.
• ${identifier} is equivalent to $identifier. It is required when valid identifier characters follow the placeholder but are not part of the placeholder, such as "${noun}ification".

but I don't understand it.

Could someone explain in plain english what's the $ for and give some examples preferably?

Answer 1

To Python, those dollar signs mean nothing at all. Just like the 'D' or 'a' that follow, the dollar sign is merely a character in a string.

To your source-code control system, the dollar signs indicate a substitution command. When you check out a new copy of your source code, that string is replaced with the timestamp of the last committed change to the file.

Reference:

• http://svnbook.red-bean.com/en/1.6/svn.advanced.props.special.keywords.html
• http://www.badgertronics.com/writings/cvs/keywords.html