Arguments from the command line

[aug dawg]

I've seen Python programs that can be activated from the command line. For
example:

hg

This displays a list of commands for the Mercurial revision control system.
But another command is this:

hg commit "This is a commit name"

Mercurial is written in Python. I know that commit is a function that
commits to a repo, but what command does the program use in order to get the
commit name, like "This is a commit name" (This would make a commit with
"This is a commit name" as the commit name)
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