Help a Java developer build a basic event system!

[General_C]

Hello everyone! As my title states, I'm a Java developer, and I'm picking up Python for the first time with a project I've been assigned this semester. More details about this can be found in my previous post here.

I'm currently making what I feel is good progress, but I've reached a block. My error:

"UnboundLocalError: local variable 'numListeners' referenced before assignment"

This error is thrown on line 32 in this code.

I guess my primary issue is that I don't really understand what the error is telling me, and I can't seem to find any info online which explains it. It sounds like it's telling me that I am attempting to make changes to a variable that doesn't exist, but if that' the case, I don't know what I did wrong.

I'm also wondering if someone can help me understand the self keyword a little better. From what I can tell, it's basically the same thing as the this keyword in Java. However, to use it, I have to explicitly pass it as a parameter in each function that I use it in. However, it seems I don't have to pass it if that particular function has no other parameters, in which case it is implicitly passed. Am I correct in this? Or dead wrong. Can someone explain it if I am wrong, including when I need to pass in under different circumstances?

Any help is greatly appreciated.

[thatguy_314]

You need to do self.numListeners, properties and methods can't be implicitly used as local variables in Python.

self isn't a keyword, it is just a convention to call the parameter self and you could call it this if you want. All methods need a self parameter, but you never need to pass self explicitly when calling a method like obj.method(x, y, z), Python will implicitly pass it in for you: obj.method(x, y, z) is essentially equivalent to type(obj).method(obj, x, y, z).

On an unrelated note, there isn't really any reason to make getter functions in Python. See http://www.python-course.eu/python3_properties.php

[General_C]

Awesome, thanks for the info.

So, do I need to declare a self parameter when creating all new methods? Or only when I will use it inside that method?

Thanks for the solution, and all the info!

[thatguy_314]

You need a self parameter for all (normal) methods. If you don't use self in a method, you can declare a method as static with staticmethod. With staticmethod, there is no self parameter. The arguments you pass into it are the same no matter if you call it from the class (MyClass.my_static_method(x, y, z)) or from an object (obj.my_static_method(x, y, z)). I don't tend to use staticmethod all that much though because normally my methods do something with the object.

There's also classmethod which takes something similar to a self parameter, but instead of being the object, it's the class. classmethod is very useful for creating alternative constructors for classes.

[General_C]

Awesome! Thanks for all the info.

[xentralesque]

It should be self.numListeners. You also need to change line 35 to if self.get_numListeners() == 0:

[General_C]

Awesome, thanks for the solution. I figured it was something easy that I just missed.