ModuleNotFoundError: No module named 'X'

What this error means

When you see it

Traceback (most recent call last):
  File "app.py", line 1, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'

Why it happens

How to fix it

Option 1: install it in the right interpreter. Always install with python -m pip. This makes sure pip belongs to the Python you're running.

python -m pip install requests

Compare with the wrong way (pip install requests), which may install into a completely different Python.

Option 2: check that the virtual environment is active. A bare pip install outside an activated venv often goes into the system Python while your editor runs the venv's Python.

# Windows
.venv\Scripts\activate
# macOS / Linux
source .venv/bin/activate

python -m pip install requests

Option 3: check the import name. The name you install from PyPI and the name you import are often different:

Option 4: check which Python you're using. On Windows, the most common cause is that the python in your shell is not the Python your editor runs.

python -c "import sys; print(sys.executable)"
python -m pip show requests

If the second command says "WARNING: Package(s) not found", requests is not installed in that Python, whatever other copies of pip say.

When you'd see this in real code

• You cloned a repo, ran python app.py, and forgot to install requirements.txt.
• You installed scikit-learn but imported it as import scikit_learn (its import name is sklearn).
• VS Code is running the global Python but your terminal has the venv activated, or vice versa.
• You're inside a Jupyter kernel that points at a different Python than pip install used.

Related errors

• ImportError: cannot import name 'X' from 'Y': the module exists, but the specific name inside it doesn't. See error-importerror-cannot-import.
• ModuleNotFoundError: No module named 'X.Y'; 'X' is not a package: X is a single module, not a package, so it has no submodules.

See Also

• All Python errors: the full index, by type and by when it happens.
• Virtual environments: isolating dependencies.
• pip and the standard library: installing packages correctly.
• Modules and imports: how sys.path and import resolution work.
• cheat-imports: fast lookup.