Swiss Ephemeris Python FFI

Pure Python ctypes runtime-FFI binding for the Swiss Ephemeris C library.

This project is the Python sibling of Swiss-Ephemeris-PHP: the wrapper loads the native Swiss Ephemeris shared library at runtime and exposes the C API as-is.

Latest Upstream Status

Checked against upstream on April 25, 2026.

• Latest upstream release tag: v2.10.3final released on April 14, 2026.
• Current upstream master checked: commit 2f18c14 from April 18, 2026 (fixed bug in semo4200.se1).
• Internal Swiss Ephemeris version string: the upstream C header still defines SE_VERSION as 2.10.03.
• Bundled native libraries: sourced from the sibling Swiss-Ephemeris-PHP prebuilt libraries.

See VERSION.md and UPSTREAM_SYNC.md for detailed version tracking.

Design Contract

The raw binding does not:

• recalculate anything;
• round numeric outputs;
• normalize angles;
• reshape output arrays;
• hide return flags;
• convert C output arrays into Python arrays;
• drop serr parameters where the C API has them;
• change house cusp indexing;
• create a higher-level astrology API.

Callers pass ctypes buffers and pointers exactly as the C API requires.

Python Support

The project baseline is Python 3.10 and the target support range is Python 3.10 through Python 3.14.

Installation

pip install swisseph-ffi

Native Libraries

The package ships the same prebuilt Swiss Ephemeris binaries as Swiss-Ephemeris-PHP:

• linux-x64/libswe.so
• linux-arm64/libswe.so
• macos-x64/libswe.dylib
• macos-arm64/libswe.dylib
• windows-x64/swe.dll

You can override discovery with SWISSEPH_LIBRARY_PATH.

Library search order:

1. SWISSEPH_LIBRARY_PATH
2. bundled swisseph_ffi/libs/<os-arch>/
3. common system library paths on Linux/macOS

Why Another Python Package?

Python already has mature Swiss Ephemeris bindings such as pyswisseph and pysweph. They are useful and work well for many CPython users.

This package takes a different approach: the Python layer is pure Python and uses runtime FFI through ctypes. It does not compile a CPython extension module and does not reshape the raw C API into a Pythonic convenience API.

Quick Start

from swisseph_ffi import SwissEph, SE_GREG_CAL, SE_SUN, SEFLG_SPEED
from swisseph_ffi import c_double, create_string_buffer

swe = SwissEph()

jd = swe.swe_julday(2000, 1, 1, 12.0, SE_GREG_CAL)
xx = (c_double * 6)()
serr = create_string_buffer(256)

ret = swe.swe_calc_ut(jd, SE_SUN, SEFLG_SPEED, xx, serr)
print(ret, list(xx), serr.value)

Scope

This package intentionally starts with the raw 1:1 layer only. A convenience API can be built separately later without changing the raw ABI binding.

Development Checks

python -m pip install -e ".[dev]"
python -m ruff check .
python -m ruff format . --check
python -m pyright
python -m pytest
python -m build
python -m twine check dist/*

The test suite includes:

• all 106 C functions loaded and signature-checked;
• runtime behavior tests mirrored from Swiss-Ephemeris-PHP;
• optional swetest parity tests when the CLI and ephemeris files are available.

License

This package metadata declares AGPL-3.0-or-later.

The upstream Swiss Ephemeris C library and ephemeris data are distributed under Astrodienst's dual licensing model: AGPL or Swiss Ephemeris Professional License. If you use Swiss Ephemeris in commercial, closed-source, SaaS, or public web-service software, review Astrodienst's license terms before use.