'lil event emitter in python!

ee.py

class EventEmitter:
    """Create listeners on an object"""
    
    def __init__(self):
        self._events = {}
        
    def on(self, event, callback):
        """Execute callback when event is emitted. Returns an id for deletion.
        
        event {String}
        callback {function}
        """
        if event not in self._events:
            self._events[event] = []
        self._events[event].append(callback)
        return self._events[event].index(callback)

    def once(self, event, callback):
        """Execute callback only once at the event, then destroys itself. Returns an id for deletion.
        """
        i = self.on(event, callback)
        def self_destruct():
            callback()
            self._events[event][i] = None
        self._events[event][i] = self_destruct
        return i
    
    def emit(self, event):
        """Execute every callback tied to the event.
        """
        if event in self._events:
            # Execute every non-null callback
            [cb() for cb in self._events[event] if cb]
    
    def off(self, event, i):
        """Remove a specific callback by id.
        """
        if (event in self._events and 
            len(self._events[event]) > i):
            self._events[event][i] = None


#### Testing ####

evan = EventEmitter()

def ouch():
    print("Ow!")


# Every time 'punch' occurs, ouch will be called
# The integer e stores the index of our callback for removal later

print("\n Evan will react to each punch")
e = evan.on('punch', ouch)
evan.emit('punch')
evan.emit('punch')


# Protest will be added to the execution list on 'punch' events
# But the protest will only execute once, then delete itself
# Notice lambda syntax is equivalent to function definition

print("\n Evan will protest once:")
evan.once('punch', lambda: print("Hey!"))
evan.emit('punch')   # ouch and protest occur
evan.emit('punch')   # only ouch occurs


print("\n Evan will no longer react:")
evan.off('punch', e)
evan.emit('punch')


print("\nSorry, Evan\n")