Handle bad input

[mnot]

We need to figure out the right thing to do when we get bad templates, e.g., "{foo", "}foo{", and so forth.

[tadeck]

Maybe you should check it (using regular expression) before using the template and if it is incorrect, you should probably throw your own exception inheriting from ValueError like this:

class BadTemplate(ValueError):
    """
    Exception thrown when the template is incorrect and cannot be supported.
    """
    pass