Systematic approach to isolating a vague KeyError in a nested Python pipeline

[John.W]

I'm an intermediate Python developer working on a complex data pipeline that's throwing a vague `KeyError` deep within a chain of nested dictionaries and function calls, and my usual print statement debugging is becoming unmanageable. I know I should be using more advanced Python debugging techniques but I've never properly learned to use pdb or the debugger in my IDE. For experienced developers, what is your systematic approach to isolating such an error? Do you prefer using an integrated debugger to step through code, or do you rely more on strategic logging? What are your best practices for tracing variable state and execution flow in a large, unfamiliar codebase when the error isn't immediately reproducible?