fehler-python-modul-nicht-gefunden.md

title	ModuleNotFoundError in Python trotz pip install
domain	python
tags	python pip module path virtualenv
language	de
status	published
source	https://docs.python.org/3/tutorial/venv.html
created	2026-07-29
confidence	0.9
verified_date	2026-07-29

Problem

This error occurs when a Python module cannot be found at runtime even though pip install succeeded. Running a script fails with:

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

But pip list shows the package as installed. This failure often happens after switching between projects or installing new packages.

Root Cause

The root cause is almost always a mismatch between the Python environment where pip install was run and the environment executing python script.py.

The three most common scenarios:

1. Virtual environment not activated: pip install ran inside an active venv, but python script.py runs outside it.
2. Multiple Python installations: The system has several Python versions (3.9, 3.10, 3.11). pip installed into one, python uses another.
3. pip install --user vs. system-wide: The package was installed with --user into a local path, but PYTHONPATH does not include that path.

Solution

1. Identify which Python and pip are in use

which python
python --version
which pip
pip --version

Ensure python and pip point to the same version.

2. Use a virtual environment correctly

python -m venv .venv
source .venv/bin/activate      # Linux/Mac
.venv\Scripts\activate          # Windows

Then install and run:

pip install requests
python script.py

3. With multiple Python versions, use the explicit interpreter

python3.11 -m pip install requests
python3.11 script.py

Or use pip3:

pip3 install requests
python3 script.py

4. Check PYTHONPATH

python -c "import sys; print('\n'.join(sys.path))"

Confirm that the site-packages path for your Python installation appears in the list.

5. Install with --target as a last resort

python -m pip install --target=$HOME/.local/lib/python3.11/site-packages requests
export PYTHONPATH=$HOME/.local/lib/python3.11/site-packages:$PYTHONPATH

Verification

• Run which python and which pip — both should point to the same installation
• Execute python -c "import requests; print(requests.__version__)" without errors
• Inside a virtual environment, confirm pip list shows the package
• Test with a full script that uses the imported package

Notes

• Always create a dedicated virtual environment per project with python -m venv
• Do not commit .venv/ to your repository (add it to .gitignore)
• On Windows, use py -3.11 instead of python3.11
• Re-run pip install -r requirements.txt whenever requirements.txt changes
• Reference: Python venv documentation