pypddl 1.0.9

Python-distributed native package for the Loki PDDL parser library

Loki: PDDL Parsing and Translation in C++ and Python

Loki is a C++20 library with Python bindings for parsing and translation of PDDL files. Loki separates the parsing and translation of domain and problem files, allowing users to effectively work with collection of problems.

The parser is based on the canonical parser structure proposed in the Boost Spirit X3 library.

The translator is based on the method presented in section four of the paper "Concise finite-domain representations for PDDL planning tasks by Malte Helmert (AIJ 2009)".

Supported PDDL Requirements

• :strips
• :typing
• :negative-preconditions
• :disjunctive-preconditions
• :equality
• :existential-preconditions
• :universal-preconditions
• :quantified-preconditions
• :conditional-effects
• :numeric-fluents
• :adl
• :derived-predicates
• :action-costs
• :non-deterministic (unsupported in the translator)
• :probabilistic-effects (unsupported in the translator)

Dependencies

Loki depends on a fraction of Boost's header-only libraries (Fusion, Spirit x3, Container), its performance benchmarking framework depends on GoogleBenchmark, and its testing framework depends on GoogleTest.

Loki consumes native dependencies from Python packages:

• pyyggdrasil >= 0.0.14, < 0.1 for shared third-party native dependencies.

The shared workspace layout and general Python/CMake integration pattern are documented in the Planning and Learning build instructions.

For offline/local development, install pyyggdrasil from the sibling source checkout instead:

cd ../yggdrasil
uv pip install --python ../loki/.venv/bin/python .

Build C++

Install Loki's native dependency providers into the active Python environment, then configure CMake with their native prefixes:

python -m pip install 'pyyggdrasil>=0.0.14,<0.1'

cmake -S . -B build

CMake discovers the installed pyyggdrasil automatically through cmake/find_python_native_packages.cmake and links against the yggdrasil::yggdrasil target. To point at a different prefix explicitly:

cmake -S . -B build \
  -DCMAKE_PREFIX_PATH="$(python -m pyyggdrasil --prefix)"

cmake --build build -j4

CMake options:

Option	Default	Description
LOKI_BUILD_TESTS	OFF	Build Loki tests.
LOKI_BUILD_EXECUTABLES	OFF	Build Loki executables.
LOKI_BUILD_PROFILING	OFF	Build Loki profiling targets.
LOKI_BUILD_PYPDDL	OFF	Build Loki for the pypddl Python wheel.
LOKI_ENABLE_FMT_FORMATTERS	ON	Enable the public fmt::formatter specializations of Loki and yggdrasil (LOKI_/YGG_ENABLE_FMT_FORMATTERS macros, both default-on in the headers).

Run tests from a build configured with -DLOKI_BUILD_TESTS=ON:

ctest --test-dir build --output-on-failure

Install Loki from a configured build directory with:

cmake --install build --prefix=<path/to/installation-directory>

Build Python

python -m pip install .

Python API

The Python package exposes the semantic parser, translator, and reparseable PDDL formatter through pypddl.formalism:

from pypddl import formalism as pddl

parser = pddl.Parser("""
(define (domain ready-domain)
  (:predicates (ready))
)
""")

translation = pddl.translate_domain(parser.domain())
domain_text = pddl.format_domain(translation.translated_domain)
reparsed = pddl.Parser(domain_text)
assert reparsed.domain().get_name() == "ready-domain"

C++ API

The umbrella header exposes the semantic parser, translator, and reparseable PDDL formatter through the top-level loki namespace:

#include <loki/loki.hpp>

#include <string>

int main()
{
    auto parser = loki::Parser(std::string { "(define (domain ready-domain) (:predicates (ready)))" });
    const auto translation = loki::translate_domain(parser.get_domain());
    const auto domain_text = loki::format_domain(translation.get_translated_domain());
    auto reparsed = loki::Parser(domain_text);
    return reparsed.get_domain().get_name() == "ready-domain" ? 0 : 1;
}

CMake Integration

The Python package pypddl installs Loki's native headers, shared library, and CMake package config under pypddl.native_prefix(). It depends on pyyggdrasil>=0.0.14,<0.1 for third-party native dependencies:

import pypddl
import pyyggdrasil

print(pypddl.cmake_prefix())       # prefix to put on CMAKE_PREFIX_PATH
print(pypddl.cmake_dir())          # directory containing lokiConfig.cmake
print(pyyggdrasil.cmake_prefix())

The same paths are available from the shell via python -m pypddl --prefix, --cmake-dir, --include-dir, and --version.

Downstream CMake projects should include the native prefixes of pypddl and its native package dependencies in CMAKE_PREFIX_PATH:

cmake -S . -B build \
  -DCMAKE_PREFIX_PATH="$(python -m pypddl --prefix);$(python -m pyyggdrasil --prefix)"

Loki exports the loki::parsers target.

Running the Executables

The executable illustrates how to use Loki. It is disabled by default and can be enabled with -DLOKI_BUILD_EXECUTABLES=ON. Example PDDL inputs are available from the benchmark submodule:

git submodule update --init --recursive data/planning-benchmarks

./build/exe/loki \
  data/planning-benchmarks/tests/classical/gripper/domain.pddl \
  data/planning-benchmarks/tests/classical/gripper/test-1.pddl

Citing Loki

If you use Loki in your research, please cite it as follows:

@misc{drexler-zenodo2026,
  author =       "Dominik Drexler",
  title =        "{Loki}: A {PDDL} Parser and Normalizer",
  publisher =    "Zenodo",
  year =         "2026",
  doi =          "10.5281/zenodo.20081136",
  url =          "https://doi.org/10.5281/zenodo.20081136",
}

Acknowledgements

This work was partially supported by the Wallenberg AI, Autonomous Systems and Software Program (WASP) funded by the Knut and Alice Wallenberg Foundation.