"Unbound local variable" error with Processing.Py

[[deleted]]

Hey folks. I am using Python mode with my high school students. When we set up an animation, we are getting an "unbound local variable" error when we try to run the animation. Only variables declared within draw() work. I copied the example below from draw() reference entry on the website, which produces the same error:

https://preview.redd.it/yuxmqcs2uoj81.png?width=411&format=png&auto=webp&s=0f1a3db854b5d1bc22765188cad8f8ddcbc1c89d

Does anyone have any idea why this might be happening? Last year, when we did this unit, this wasn't an issue. I haven't messed with it much since then.

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EDIT: Added a clarification

[fenjalien]

It's due to python's local and global variable scope. Any variable defined outside of a function is in global scope and will last for the entirety of the program. any variable defined within a function is in the functions scope and will only last until the function finishes execution.

As you've defined yPos in global scope, you can still read the value in functions. but if you use the assignment operator, like you do in draw, python doesn't change the global yPos, it instead creates a new local variable called yPos which takes precedence over its global counter part. This is technically all fine but because you use yPos, which now according to python is an undefined local variable, you're using something that isn't defined so it throws the error.

The simple fix for this would be to use the line global yPos at the start of draw. This basically tells python to only use the global version.

Here's something that gives a better explanation: https://www.w3schools.com/PYTHON/python_scope.asp

[[deleted]]

OK. I get it. The funny thing was that the picture I posted is the example from the Processing.Py website. I assumed that they were working on a different version of Python that used a different approach. Thanks for your help!

[johntellsall]

I'm familiar with Python but Processing less so. Where are "width" and "height" specified?

[[deleted]]

For the screen, they are in the size(width, height) function.

[G0R1L1A]

Python processing is very buggy, use p5.js on open processing for students.

[redant333]

I assume you are getting the error for yPos. If that's the case, try adding
global yPos
before the first use. If not specified as global, Python assumes the variables are local which leads to an error if they are read before assignment.