ModuleNotFoundError: No module named 'Cython' in Python

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Borislav Hadzhiev

Last updated: Apr 20, 2022

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"ModuleNotFoundError: No module named 'Cython' in Python" #

The Python "ModuleNotFoundError: No module named 'Cython'" occurs when we forget to install the Cython module before importing it or install it in an incorrect environment. To solve the error, install the module by running the pip install Cython command.

no module named cython

Open your terminal in your project's root directory and install the Cython module.

shell
# 👇️ in a virtual environment or using Python 2 pip install Cython # 👇️ for python 3 (could also be pip3.10 depending on your version) pip3 install Cython # 👇️ if you get permissions error sudo pip3 install Cython pip install Cython --user # 👇️ if you don't have pip in your PATH environment variable python -m pip install Cython # 👇️ for python 3 (could also be pip3.10 depending on your version) python3 -m pip install Cython # 👇️ using py alias (Windows) py -m pip install Cython # 👇️ for Anaconda conda install -c anaconda cython # 👇️ for Jupyter Notebook !pip install Cython

After you install the Cython package, try importing it like:

main.py
from Cython.Build import cythonize print(dir(cythonize))

The Python error "ModuleNotFoundError: No module named 'Cython'" occurs for multiple reasons:

  1. Not having the Cython package installed by running pip install Cython.
  2. Installing the package in a different Python version than the one you're using.
  3. Installing the package globally and not in your virtual environment.
  4. Your IDE running an incorrect version of Python.
  5. Naming your module Cython.py which would shadow the official module.
  6. Declaring a variable named Cython which would shadow the imported variable.

If the error persists, get your Python version and make sure you are installing the package using the correct Python version.

shell
python --version

get python version

For example, my Python version is 3.10.4, so I would install the Cython package with pip3.10 install Cython.

shell
pip3.10 install Cython # 👇️ if you get permissions error use pip3 (NOT pip3.X) sudo pip3 install Cython

Notice that the version number corresponds to the version of pip I'm using.

If the PATH for pip is not set up on your machine, replace pip with python3 -m pip:

shell
# 👇️ make sure to use your version of Python, e.g. 3.10 python3 -m pip install Cython

If the "No module named 'Cython'" error persists, try restarting your IDE and development server/script.

You can check if you have the Cython package installed by running the pip show Cython command.

shell
# 👇️ check if you have Cython installed pip3 show Cython # 👇️ if you don't have pip set up in PATH python3 -m pip show Cython

The pip show Cython command will either state that the package is not installed or show a bunch of information about the package, including the location where the package is installed.

If the package is not installed, make sure your IDE is using the correct version of Python.

If you have multiple Python versions installed on your machine, you might have installed the Cython package using the incorrect version or your IDE might be set up to use a different version.

For example, In VSCode, you can press CTRL + Shift + P or ( + Shift + P on Mac) to open the command palette.

Then type "Python select interpreter" in the field.

python select interpreter

Then select the correct python version from the dropdown menu.

select correct python version

Your IDE should be using the same version of Python (including the virtual environment) that you are using to install packages from your terminal.

If you are using a virtual environment, make sure you are installing Cython in your virtual environment and not globally.

You can try creating a virtual environment if you don't already have one.

shell
# 👇️ use correct version of Python when creating VENV python3 -m venv venv # 👇️ activate on Unix or MacOS source venv/bin/activate # 👇️ activate on Windows (cmd.exe) venv\Scripts\activate.bat # 👇️ activate on Windows (PowerShell) venv\Scripts\Activate.ps1 # 👇️ install Cython in virtual environment pip install Cython

If the python3 -m venv venv command doesn't work, try the following 2 commands:

  • python -m venv venv
  • py -m venv venv

Your virtual environment will use the version of Python that was used to create it.

If the error persists, make sure you haven't named a module in your project as Cython.py because that would shadow the original Cython module.

You also shouldn't be declaring a variable named Cython as that would also shadow the original module.

If the error is not resolved, try to uninstall the Cython package and then install it.

shell
# 👇️ check if you have Cython installed pip3 show Cython # 👇️ if you don't have pip set up in PATH python3 -m pip show Cython # 👇️ uninstall Cython pip3 uninstall Cython # 👇️ if you don't have pip set up in PATH python3 -m pip uninstall Cython # 👇️ install Cython pip3 install Cython # 👇️ if you don't have pip set up in PATH python3 -m pip install Cython

Try restarting your IDE and development server/script.

You can also try to upgrade the version of the Cython package.

shell
pip3 install Cython --upgrade # 👇️ if you don't have pip set up in PATH python3 -m pip install Cython --upgrade
If the error persists, I would suggest watching a quick video on how to use Virtual environments in Python.

This one is for using virtual environments (VENV) on Windows:

This one is for using virtual environments (VENV) on MacOS and Linux:

Conclusion #

The Python "ModuleNotFoundError: No module named 'Cython'" occurs when we forget to install the Cython module before importing it or install it in an incorrect environment. To solve the error, install the module by running the pip install Cython command.

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