Python 3 gained syntax for keyword-only arguments.
Try "foo(1)" and it will fail -- "bar" needs to be given as a keyword.
This is because it comes after a *-expression. You can also do "def
foo(*, bar=1)" if you want bar to be keyword-only without accepting
any number of positional arguments. Python 2 does not have these, and
doesn't understand the syntax for them.