Need a good way to detect invalid tracking codes

[mw44118]

I'm using your package to check if packages have been delivered. The tracking codes I get are manually entered and sometimes they are entered with errors.

What's the right way to check if a tracking code is valid?

[poundifdef]

Right now, if you enter an invalid tracking number, you'll get a KeyError exception when invoking tracking_info.delivered because the XML returned doesn't have the expected tags.

However, I realize that this is cumbersome. Do you have any thoughts on how to make this better?

[mw44118]

I would either make a custom exception class, or explicitly return None.