KeyError:

Python's import system causes Keyerror Dictionary in patterns that are not immediately obvious: circular imports, module shadowing, and the difference between running a file as a script versus as a module. Circular imports (module A imports B, B imports A) cause attribute errors rather than import errors because Python partially executes the first module before returning to finish the second. The fix: move shared code to a third module that neither A nor B imports from, or use import-inside-function for the circular dependency. Module shadowing: naming a local file the same as a stdlib or installed package (e.g., creating json.py in your project root) makes Python import the local file instead of the standard library, causing Keyerror Dictionary with confusing error messages. Check for this with python -c "import <module>; print(<module>.__file__)". The script-vs-module distinction: python script.py adds the script's directory to sys.path; python -m package.script does not — this changes which relative imports resolve correctly and which trigger Keyerror Dictionary.