Python

Table of contents

  1. No module named ‘_ctypes’
    1. Remove the current version of Python 3.13 (if already installed)
    2. Install necessary dependencies
    3. Download Python 3.13 from source
    4. Compile and install Python
    5. Verify the installation
    6. Recreate the virtual environment

The project targets Python 3.13

The root pyproject.toml declares requires-python = ">=3.13", the Docker images are built from python:3.13-slim, the Vagrant machine installs python3.13, and the CI workflows set up 3.13. Anything older will fail to install the dependencies.

No module named ‘_ctypes’

This is caused by an incorrect installation of Python. The best thing to do is to delete the current Python 3.13 version from the system, purge it and reinstall Python 3.13 from source, going through the build.

Compiling takes a few minutes, but it is effective.

Remove the current version of Python 3.13 (if already installed)

It is important not to remove the default versions of Python that come with Ubuntu, as they are necessary for the system to function properly. Be sure not to remove critical versions such as the distribution’s own python3, which may be part of the system. If you already installed Python 3.13 from external sources or a PPA and want to reinstall it, you can remove it like this:

sudo apt remove --purge python3.13
sudo apt autoremove

Install necessary dependencies

Before installing Python 3.13, make sure you have the necessary tools to compile Python from source:

sudo apt update
sudo apt install -y build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm libncurses5-dev libncursesw5-dev xz-utils tk-dev libffi-dev liblzma-dev python3-openssl git

The package that fixes No module named '_ctypes' specifically is libffi-dev. It must be present before you configure and build, otherwise the _ctypes extension module is silently skipped.

Download Python 3.13 from source

Download Python 3.13 from the official Python archives:

wget https://www.python.org/ftp/python/3.13.14/Python-3.13.14.tgz
tar -xvf Python-3.13.14.tgz

Any 3.13.x release works. Replace the version number in both the URL and the commands below if you prefer a different patch release from python.org/ftp/python.

Compile and install Python 

cd Python-3.13.14
./configure --enable-optimizations
make -j$(nproc)
sudo make altinstall

Note: Use altinstall instead of install to avoid overwriting the default version of Python on your system.

Verify the installation

After installation, verify that Python 3.13 is correctly installed by running:

python3.13 --version

And verify that the module that failed is now importable:

python3.13 -c "import ctypes; print('ok')"

Recreate the virtual environment

A virtual environment created with the broken interpreter stays broken. Once Python 3.13 is rebuilt, throw it away and start again:

rm -r venv
python3.13 -m venv venv
source venv/bin/activate
pip install --upgrade pip
pip install -r requirements.txt
pip install -e ./rosemary