Python KeyError Decoder

Python has thrown a KeyError and the traceback is not exactly chatty about why. This explains in plain English what the error means, why Python is raising it at your line, and gives you the fix, leading with the safest option when there is more than one way to sort it. For developers staring at a dictionary lookup that just blew up.

I'm getting this Python KeyError: {ERROR_MESSAGE}. Here is the relevant code: {CODE_SNIPPET}. Explain in plain English what this error actually means, why Python is throwing it here, and give me the fix. If there is more than one way to fix it, show me the safest one first.