NameError: name is not defined

NAMEERROR in Python almost always indicates a virtual environment issue — either no environment is activated, the wrong environment is active, or the package was installed into a different Python than the one running the script. The diagnostic: run which python (macOS/Linux) or where python (Windows) and python --version to see which Python interpreter is active. If which python points to /usr/bin/python or similar system path rather than your project's .venv/bin/python, no environment is active. Activate with source .venv/bin/activate (bash/zsh) or .venvScriptsActivate.ps1 (PowerShell). A common mistake: using pip install without an active environment installs the package globally, then running the script inside the environment cannot find it. Always use python -m pip install to ensure pip installs into the same Python that runs python — this prevents the interpreter/pip mismatch that causes NAMEERROR.