py2pddl (Python to PDDL)

Write your planning task as Python classes, then translate them to PDDL files. We will use PDDL 1.2 with (:requirements :strips :typing), i.e. the actions only use positive preconditions and deterministic effects, and we use 'types' like in OOP to represent sets of objects.

The library is written with these considerations:

• Type checking
• To define the domain and problem as Python but yet still look similar to PDDL
• To quickly define the domain and problem using some boilerplate
• To define ground predicates cleanly
• To reduce the use of strings while defining ground predicates

• Features
• Requirements
• Installation
• Quick start in 5 steps
• Examples
• Resources

Features

This library allows you to define:

• Domain: requirements (strips and typing), types, predicates, action
• Problem: objects, init, goal

Negation is done using the ~ operator.

Requirements

• Python 3.6
• python-fire (pip install fire)

Installation

pip install git+https://github.com/remykarem/py2pddl#egg=py2pddl

Quick start in 5 steps

We will use the following air cargo problem:

1. Set up boilerplate

Create an aircargo.py file by running:

python -m py2pddl.init aircargo.py

and enter the following:

Name: AirCargo
Types (separated by space): cargo airport plane
Predicates (separated by space): plane_at cargo_at in_
Actions (separated by space): load unload fly

2. Define the domain

In the aircargo.py source file, the AirCargoDomain class has been created. The structure of the class is similar to how a PDDL domain should be defined.

• Name of the domain is the name of the Python class (AirCargoDomain).
• Types are defined as class variables at the top (Plane, Cargo, Airport).
• Predicates are defined as instance methods decorated with @predicate.
• Actions are defined as instance methods decorated with @action.

Now, complete the class definition such that it looks like this:

from py2pddl import Domain, create_type
from py2pddl import predicate, action

class AirCargoDomain(Domain):

    Plane = create_type("Plane")
    Cargo = create_type("Cargo")
    Airport = create_type("Airport")

    @predicate(Cargo, Airport)
    def cargo_at(self, c, a):
        """Complete the method signature and specify
        the respective types in the decorator"""

    @predicate(Plane, Airport)
    def plane_at(self, p, a):
        """Complete the method signature and specify
        the respective types in the decorator"""

    @predicate(Cargo, Plane)
    def in_(self, c, p):
        """Complete the method signature and specify
        the respective types in the decorator"""

    @action(Cargo, Plane, Airport)
    def load(self, c, p, a):
        precond = [self.cargo_at(c, a), self.plane_at(p, a)]
        effect = [~self.cargo_at(c, a), self.in_(c, p)]
        return precond, effect

    @action(Cargo, Plane, Airport)
    def unload(self, c, p, a):
        precond = [self.in_(c, p), self.plane_at(p, a)]
        effect = [self.cargo_at(c, a), ~self.in_(c, p)]
        return precond, effect

    @action(Plane, Airport, Airport)
    def fly(self, p, orig, dest):
        precond = [self.plane_at(p, orig)]
        effect = [~self.plane_at(p, orig), self.plane_at(p, dest)]
        return precond, effect

Note:

• To create a new type Car, simply add Car = create_type("Car") at the top of the class definition.
• The positional arguments of @predicate and @action decorators are the types of the respective arguments.
• Methods decorated with @predicate should have empty bodies.
• Methods decorated with @action return a tuple of two lists.

3. Define the problem

At the bottom part of aircargo.py, there is another class called AirCargoProblem. Again, the structure of the class is similar to how a PDDL problem should be defined.

• Name of the domain is the name of the Python class (AirCargoProblem).
• Objects are defined as the instance attributes in the __init__ method.
• Initial states are defined as a methods decorated with @init.
• Goal is defined as an instance methods decorated with @goal.

Complete the class definition as follows:

from py2pddl import goal, init

class AirCargoProblem(AirCargoDomain):

    def __init__(self):
        super().__init__()
        self.cargos = AirCargoDomain.Cargo.create_objs([1, 2], prefix="c")
        self.planes = AirCargoDomain.Plane.create_objs([1, 2], prefix="p")
        self.airports = AirCargoDomain.Airport.create_objs(["sfo", "jfk"])

    @init
    def init(self):
        at = [self.cargo_at(self.cargos[1], self.airports["sfo"]),
              self.cargo_at(self.cargos[2], self.airports["jfk"]),
              self.plane_at(self.planes[1], self.airports["sfo"]),
              self.plane_at(self.planes[2], self.airports["jfk"]),]
        return at

    @goal
    def goal(self):
        return [self.cargo_at(self.cargos[1], self.airports["jfk"]),
                self.cargo_at(self.cargos[2], self.airports["sfo"])]

Note:

• The Python objects (cargos, planes and airports) are created using the respective types defined in the AirCargoDomain. For example, AirCargoDomain.Cargo.create_objs([1, 2], None, "c") will create a Python dictionary {1: AirCargoDomain.Plane("p1"), 2: AirCargoDomain.Plane("p2")}. This allows cleaner access to these objects while defining initial state and goal, which usually can get pretty messy.
• The PDDL objects defined in the __init__ are meant to be used across the 2 instance methods.
• Any method decorated with @init must return a list.
• Any method decorated with @goal must return a list.

4. Parse

• Generate the PDDL files from the command line by runnning

  python -m py2pddl.parse aircargo.py
  

• You can also import the parsing function from the module

  from py2pddl import parse
  parse("aircargo.py")
  

• The class itself also contains methods to generate the domain and problem PDDL files separately. These methods were inherited from Domain.

  from aircargo import AirCargoProblem
  
  problem = AirCargoProblem()
  problem.generate_domain_pddl()
  problem.generate_problem_pddl()
  

Here is the generated domain.pddl file.

(define
	(domain somedomain)
	(:requirements :strips :typing)
	(:types
		airport
		cargo
		plane
	)
	(:predicates
		(cargo_at ?c - cargo ?a - airport)
		(in_ ?c - cargo ?p - plane)
		(plane_at ?p - plane ?a - airport)
	)
	(:action fly
		:parameters (?p - plane ?orig - airport ?dest - airport)
		:precondition (plane_at ?p ?orig)
		:effect (and (not (plane_at ?p ?orig)) (plane_at ?p ?dest))
	)
	(:action load
		:parameters (?c - cargo ?p - plane ?a - airport)
		:precondition (and (cargo_at ?c ?a) (plane_at ?p ?a))
		:effect (and (not (cargo_at ?c ?a)) (in_ ?c ?p))
	)
	(:action unload
		:parameters (?c - cargo ?p - plane ?a - airport)
		:precondition (and (in_ ?c ?p) (plane_at ?p ?a))
		:effect (and (cargo_at ?c ?a) (not (in_ ?c ?p)))
	)
)

And here is the generated problem.pddl file.

(define
	(problem someproblem)
	(:domain somedomain)
	(:objects
		sfo jfk - airport
		c1 c2 - cargo
		p1 p2 - plane
	)
	(:init (cargo_at c1 sfo) (cargo_at c2 jfk) (plane_at p1 sfo) (plane_at p2 jfk))
	(:goal (and (cargo_at c1 jfk) (cargo_at c2 sfo)))
)

Then use your favourite planner like Fast Downward. To output a plan. Here's the plan generated from the above PDDL:

(load c1 p1 sfo)
(fly p1 sfo jfk)
(load c2 p1 jfk)
(unload c1 p1 jfk)
(fly p1 jfk sfo)
(unload c2 p1 sfo)
; cost = 6 (unit cost)

See more examples in the pddl/ folder.

5. Generate the problems dynamically

If you want the problem PDDL to be more dynamic if you have changing inits and goals, you could use dictionaries and specify in the init or goal keyword argument.

p.generate_problem_pddl(
    goal={"cargo": "C2"})

Examples

Below are several example domains. The respective Python files, PDDL files and sas_plan files (generated using Fast Downward) can be found in the pddl/ folder here.

• Logistics
• Blocks world
• Aircargo

Resources

• PDDL4J
• Fast Downward
• Planning Wiki
• IPC 2018 domains
• Classical domains
• Planning domains

If you use this software for your work, please cite us as follows:

@article{bin_Karim_py2pddl_2020,
author = {bin Karim, Raimi},
journal = {https://github.com/remykarem/py2pddl},
month = {11},
title = {{py2pddl}},
year = {2020}
}