Quick lookup.
SCRIPT WILL NOT RUN AT ALL
Syntax error, or the wrong interpreter. Read the first error.
INDENTATION ERROR
Mixed tabs and spaces. Configure the editor for spaces.
MODULE NOT FOUND
Not installed, installed elsewhere, or the virtual environment is not active.
Also: a file named after the library, shadowing it.
WORKS IN THE TERMINAL, FAILS UNDER CRON
Paths and environment. Use full paths and set what the script needs.
WORKS LOCALLY, FAILS ON THE SERVER
Python version, dependency versions, missing environment variable, or a path assumption.
CHANGES NOT TAKING EFFECT
The application was not restarted.
UNEXPECTED VALUE
Print the value and its type. It is frequently text where a number was assumed.
KEY OR INDEX ERROR
The data is not the shape assumed. Validate it.
SLOW ON REAL DATA, FINE ON SAMPLES
Nested loops, repeated searching, or per-item queries.
MEMORY ERROR OR UNEXPECTED RESTART
Everything loaded into memory, or something accumulating.
INTERMITTENT FAILURE
Timing, external service, or overlapping scheduled runs.
