Dependencies

CPython is unusually self-contained for a project of its size, but it does depend on a handful of system and vendored libraries. This page lists them and where they live in the source tree.

System (linked dynamically)

These are required at build time and at runtime; missing ones make the corresponding stdlib module unbuildable. The detailed per-distro list is in the devguide build-setup page.

Library	Used by
libc + pthreads	The interpreter itself.
OpenSSL 1.1.1+	ssl, hashlib (digests when available). Modules/_ssl.c, Modules/_hashopenssl.c.
zlib	zlib, gzip. Modules/zlibmodule.c.
bzip2	bz2. Modules/_bz2module.c.
xz / liblzma	lzma, tarfile. Modules/_lzmamodule.c.
zstd	_zstd accelerator. Modules/_zstd/.
libffi	ctypes. Modules/_ctypes/.
Tcl/Tk	tkinter, IDLE. Modules/_tkinter.c.
ncurses	curses. Modules/_cursesmodule.c, Modules/_curses_panel.c.
readline (or editline)	Interactive REPL. Modules/readline.c.
sqlite3	sqlite3. Modules/_sqlite/.
gdbm, ndbm	dbm.gnu, dbm.ndbm. Modules/_gdbmmodule.c, Modules/_dbmmodule.c.
uuid	uuid. Modules/_uuidmodule.c.

A missing optional library is not fatal. The build records which modules failed in setup.py-style logs and prints a "modules that failed to build" summary at the end of make.

Vendored

Some libraries are kept in-tree to guarantee a known good version regardless of the OS:

Library	In-tree path	Why vendored
expat	Modules/expat/	XML parser; CPython is a major upstream user.
libmpdec + _decimal	Modules/_decimal/	The arbitrary-precision decimal library is co-maintained with CPython.
HACL* (formally verified crypto)	Modules/_hacl/	Provides hash digests when OpenSSL isn't available; kept verified-via-F*.
mimalloc	Objects/mimalloc/	Used by free-threaded builds. Pinned for ABI stability.
libb2 (BLAKE2)	Modules/blake2module.c (when not using OpenSSL)	Fallback BLAKE2 implementation.
Unicode Character Database	Generated tables under Modules/unicodedata_db.h, Modules/unicodename_db.h, Objects/unicodetype_db.h	Built from the UCD via Tools/unicode/.

./configure --with-system-<name> opts back to the system copy when one is available.

Build-time dependencies

These are needed to build CPython but not to run it:

Tool	Used by
C compiler (GCC/Clang/MSVC)	Everything.
make (GNU make on Unix)	Top-level build.
autoconf 2.71+	Regenerating configure from configure.ac.
pkg-config	Optional; used to locate system OpenSSL/Tk/etc.
Python 3.10+	Required to build from a fresh checkout — used to run the regenerators (PEG, Clinic, cases generator, etc.).
LLVM (clang + llvm-readobj)	Required only for --enable-experimental-jit: the Tools/jit/ build pipeline compiles uop stencils.
sphinx (and several plugins)	Building Doc/ HTML/man/pdf. Pinned via Doc/requirements.txt.
ruff, mypy	Lint and CI (.ruff.toml, Misc/mypy/).
blurb	NEWS-entry generation. ~`pipx run blurb` is fine if you don't have one in-tree.

Test-time dependencies

The default test run uses only the stdlib. Optional helpers:

• hypothesis — property-based tests gated by Tools/requirements-hypothesis.txt.
• coverage/gcov — for coverage reports (CI only).

Vendored Python packages in Lib/

The pure-Python stdlib is generally completely independent of third-party packages. Two historical exceptions:

• tomli — was vendored as the basis for Lib/tomllib/. Now inlined.
• pip is not vendored as Python source; it ships as a pre-built wheel embedded by ensurepip (see Lib/ensurepip/_bundled/).
• Test helpers in Lib/test/support/ implement minimal versions of common patterns rather than depending on external libraries.

Platform-specific code

Path	Platform
PC/	Windows-specific build helpers (winreg.c, getpathp.c, _msi.c, pyconfig.h, …).
PCbuild/	MSBuild project files for python.exe.
Mac/	macOS framework/installer scripts.
Tools/wasm/, Modules/_wasi/-style files	Emscripten/WASI cross-compile.
Lib/_apple_support.py, Lib/_ios_support.py, Lib/_android_support.py	Mobile platform shims.
Programs/_freeze_module.c	The freezer, run during cross-compile.