Python Event Dispatcher
Checking items in dictionnary
There is no need to call keys when checking for dictionnary membership. Much like there is no need of True in if my_bool == True.
on
Checking if a key exist and set it to a default value otherwise is so common that you have 2 standard ways of doing it: dict.setdefault(...) or using a collections.defaultdict.
trigger
Since KeyError should normaly not happen, you can use EAFP approach by using try...except instead of if. It's a tiny bit faster if exceptions are quite rare. Moreover, by using defaultdict you are guaranteed that every access to an item will be valid, thus eliminating the need to check membership of any key.
class
Since the only state you need is the one of the class, and in order to avoid instanciating to call methods, you can use @staticmethods and call them with Event.on(...) and Event.trigger(..).
Proposed alternative
from collections import defaultdict

class Event():
    __events = defaultdict(list)

    @staticmethod
    def on(event, func):
        Event.__events[event].append(func)

    @staticmethod
    def trigger(event, *args, **namedArgs):
        for func in Event.__events[event]:
            func(*args, **namedArgs)