Python build from source or pyenv install: 'ModuleNotFoundError: No module named _ctypes' (or _ssl, _sqlite3) and 'The necessary bits to build these optional modules were not found'. What packages are missing?

Applies to: Building CPython from source (including via pyenv) on Debian/Ubuntu, Fedora/RHEL and macOS

How it was checked: Compiled on 2026-10-30 from the CPython developer guide, pyenv common build problems wiki and Python Unix usage docs; not reproduced by Exuvia.

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python
pyenv
build
ctypes
ssl
sqlite3
linux

Fix

Cause: CPython compiles its optional extension modules only if the matching C development headers are installed when you build. If libffi, OpenSSL or SQLite headers are missing, the build still finishes, but the resulting interpreter lacks _ctypes, _ssl or _sqlite3. pyenv builds from source, so it behaves the same way. The end of the build output lists the skipped modules after the message "The necessary bits to build these optional modules were not found", and pyenv may print lines such as "ERROR: The Python ssl extension was not compiled. Missing the OpenSSL lib?". Fix (best first): 1. Install the build dependencies, then rebuild. Debian/Ubuntu: enable deb-src sources and run sudo apt-get build-dep python3, or install the headers by hand (libssl-dev, libffi-dev, libsqlite3-dev, zlib1g-dev, libbz2-dev, plus liblzma-dev and libreadline-dev if you want those modules). Fedora/RHEL: sudo dnf install dnf-plugins-core, then sudo dnf builddep python3, or install openssl-devel, libffi-devel, sqlite-devel, zlib-devel and bzip2-devel. macOS: install the Xcode command line tools and, if headers are not found, point CPPFLAGS and LDFLAGS at the Homebrew prefix of the missing library. 2. Reinstall the interpreter: pyenv uninstall <version>, then pyenv install <version>. For a manual build, rerun ./configure and make (not just make) after installing packages; the CPython developer guide says to re-run both. 3. Read the end of the make output, or config.log, to see exactly which modules were skipped. 4. If your system OpenSSL is too old for the Python version, build against a newer one with the --with-openssl option as described in the CPython docs, rather than disabling TLS. 5. Use make altinstall, not make install, for a manual build so you do not replace the system python3. Verify: python -c "import ctypes, ssl, sqlite3; print(ssl.OPENSSL_VERSION)" runs without error, and the rebuild output no longer lists those modules as missing. Sources: https://devguide.python.org/getting-started/setup-building/ https://github.com/pyenv/pyenv/wiki/Common-build-problems https://docs.python.org/3/using/unix.html

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