An ImportError means Python found a module but could not import the requested name; ModuleNotFoundError usually means it could not find the module at all. Start by reading the complete traceback and checking which interpreter is running.
Five common causes are: the package is not installed in the active environment; the wrong virtual environment is selected; a local file shadows a package; the installed version changed an API; or a circular import exists.
python -c "import sys; print(sys.executable)"
python -m pip show package-name
python -c "import package_name; print(package_name.__file__)"Use python -m pip so pip belongs to the same interpreter as Python. Rename files such as requests.py, random.py, or json.py if they shadow standard or third-party modules. For version-related failures, check the package changelog and pin a compatible version while planning an upgrade.
Circular imports often improve when shared types or constants move into a small third module. Avoid fixing imports by randomly changing sys.path; that hides the real environment problem and makes deployment less predictable.