Simple OOP currency converter
First: welcome to CodeReview! I hope that you get good feedback.

About your code. Your usage of Python is not terrible; most of your problems are conceptual, about the decisions you've made about your class. Looking at the method signatures from the outside, one would guess that addRate() can be called multiple times per single Bank instance, and that convert would then allow for any currency to be converted. That isn't the case; instead, your "Bank" class is actually closer to an "ExchangeRate" class.

There are two ways to go, here. Either make your class an ExchangeRate, in which case


you need to rename the class
convert, rather than accepting the name of a currency, should perhaps accept a forward boolean. Or, if you want to keep passing the name of a currency, it's important to make the argument name more clear, i.e. dest_currency.
Rename your addCommission to setCommission. "add" implies having more than one.
Rename your addRate to setRate. "add" implies having more than one.


Or, keep a "Bank" class, in which case:


you need to change the way that you store your exchange rates, probably to a dictionary-based system instead of a single float member
convert would accept both a source and destination currency
addRate's signature would stay the same, but its contents would change to use your dict.