Library has only one way for setting api key and it's a "monkey patching"(look at readme). I think it's a really big problem for payment gateway client, because client could operate with many accounts and it could be an reason of many problems.

I suggest to implement something like this:

moyasar_client = moyasar.Client('<api key>')
moyasar_client.Payment.fetch('<payment id>')
moyasar_client.Invoice.fetch('<invoice id>')

I suggest you to raise moyasar custom exception (for payment requests) instead of base exceptions. It's a really good mechanism for separating user exception and moyasar exceptions. More details about this can be found at python doc.