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🗃️ A Python ORM-like interface for the Clingo Answer Set Programming (ASP) reasoner

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Clingo ORM (Clorm)

Clorm is a Python library that provides an Object Relational Mapping (ORM) interface to the Clingo Answer Set Programming (ASP) solver.

For background, ASP is a declarative language for describing, and solving, hard search problems. Clingo is a feature rich ASP solver with an extensive, but relatively low-level, Python API.

The goal of this library is to make it easier to integrate Clingo within a Python application. It is implemented on top of the official Clingo API so is designed to supplement and not replace the Clingo API.

When integrating an ASP program into an application you typically want to model the domain as a statically written ASP program, but then to generate problem instances and process the results dynamically. Clorm makes this integration cleaner, both in terms of code readability but also by providing a framework that makes it easier to refactor the python code as the ASP program evolves.

The documentation is available online here.

Note: Clorm works with Python 3.8+ and Clingo 5.6+

Installation

Clorm requires Python 3.8+ and Clingo 5.6+. It can be installed using either the pip or conda package managers.

pip packages can be downloaded from PyPI:

$ pip install clorm

The alternative to install Clorm is with Anaconda. Assuming you have already installed some variant of Anaconda, first you need to install Clingo:

$ conda install -c potassco clingo

Then install Clorm:

$ conda install -c potassco clorm

Quick Start

The following example highlights the basic features of Clorm. The ASP and Python parts of this example are located in the examples sub-directory in the git repository. The ASP program is quickstart.lp and the Python program is quickstart.py. A clingo callable version with embedded Python is also provided and can be run with:

$ clingo embedded_quickstart.lp

Imagine you are running a courier company and you have drivers and items that need to be delivered on a daily basis. An item is delivered during one of four time slots, and you want to assign a driver to deliver each item, while also ensuring that all items are assigned and drivers aren't double-booked for a time slot.

You also want to apply some optimisation criteria. Firstly, you want to minimise the number of drivers that you use (for example, because bringing a driver on for a day has some fixed cost). Secondly, you want to deliver items as early in the day as possible.

The above crieria can be encoded with the following simple ASP program:

time(1..4).

1 { assignment(I, D, T) : driver(D), time(T) } 1 :- item(I).
:- assignment(I1, D, T), assignment(I2, D, T), I1 != I2.

working_driver(D) :- assignment(_,D,_).

#minimize { 1@2,D : working_driver(D) }.
#minimize { T@1,D : assignment(_,D,T) }.

This above ASP program encodes the problem domain and can be used to solve the problem for arbitrary instances by combining it with a problem instance (i.e., some combination of drivers and items).

We now use a Python program to dynamically generate the problem instance and to process the generated solutions. Each solution will be an assignment of drivers to items for a time slot.

First the relevant libraries need to be imported.

from clorm import Predicate, ConstantStr
from clorm.clingo import Control

Note: Importing from clorm.clingo instead of clingo.

While it is possible to use Clorm with the raw clingo library, a wrapper library is provided to make the integration seemless. This wrapper (should) behave identically to the original module, except that it extends the functionality to offer integration with Clorm objects. It is also possible to monkey patch Clingo if this is your preferred approach (see the documentation).

The next step is to define a data model that maps the Clingo predicates to Python objects. A Clingo predicate is mapped to Python by subclassing from a Predicate class. Similarly, to a standard Python dataclass the predicate class contains fields. In this case, each field maps to an ASP term and the type specification of the field determines the translation between Clingo and Python.

ASP's logic programming syntax allows for three primitive types: integer, string, and constant. From the Python side this corresponds to the standard types int and str, as well as a special Clorm defined type ConstantStr.

class Driver(Predicate):
    name: ConstantStr

class Item(Predicate):
    name: ConstantStr

class Assignment(Predicate):
    item: ConstantStr
    driver: ConstantStr
    time: int

The above code defines three classes to match the ASP program's input and output predicates. Driver maps to the driver/1 predicate, Item maps to item/1, and Assignment maps to assignment/3 (note: the /n is a common logic programming notation for specifying the arity of a predicate or function). A predicate can contain zero or more fields.

The number of fields in the Predicate declaration must match the predicate arity and the order in which they are declared must also match the position of each term in the ASP predicate.

Having defined the data model we now show how to dynamically add a problem instance, solve the resulting ASP program, and print the solution.

First the Clingo Control object needs to be created and initialised, and the static problem domain encoding must be loaded.

ctrl = Control(unifier=[Driver, Item, Assignment])
ctrl.load("quickstart.lp")

The clorm.clingo.Control object controls how the ASP solver is run. When the solver runs it generates models. These models constitute the solutions to the problem. Facts within a model are encoded as clingo.Symbol objects. The unifier argument defines how these symbols are turned into Predicate instances.

For every symbol fact in the model, Clorm will successively attempt to unify (or match) the symbol against the Predicates in the unifier list. When a match is found the symbol is used to define an instance of the matching predicate. Any symbol that does not unify against any of the predicates is ignored.

Once the control object is created and the unifiers specified the static ASP program is loaded.

Next we generate a problem instance by generating a lists of Driver and Item objects. These items are added to a clorm.FactBase object.

The clorm.FactBase class provides a specialised set-like container for storing facts (i.e., predicate instances). It provides the standard set operations but also implements a querying mechanism for a more database-like interface.

from clorm import FactBase

drivers = [ Driver(name=n) for n in ["dave", "morri", "michael" ] ]
items = [ Item(name="item{}".format(i)) for i in range(1,6) ]
instance = FactBase(drivers + items)

The Driver and Item constructors use named parameters that match the declared field names. Note: while you can use positional arguments to initialise instances, doing so will potentially make the code harder to refactor. So in general you should avoid using positional arguments except for a few cases (eg., simple tuples where the order is unlikely to change).

These facts can now be added to the control object and the combined ASP program grounded.

ctrl.add_facts(instance)
ctrl.ground([("base",[])])

At this point the control object is ready to be run and generate solutions. There are a number of ways in which the ASP solver can be run (see the Clingo API documentation). For this example, we use a mode where a callback function is specified. This function will then be called each time a model is found.

solution=None
def on_model(model):
    nonlocal solution        # Note: use `nonlocal` keyword depending on scope
    solution = model.facts(atoms=True)

ctrl.solve(on_model=on_model)
if not solution:
    raise ValueError("No solution found")

The on_model() callback is triggered for every new model. Because of the ASP optimisation statements this callback can potentially be triggered multiple times before an optimal model is found. Also, note that if the problem is unsatisfiable then it will never be called and you should always check for this case.

The line solution = model.facts(atoms=True) extracts only instances of the predicates that were registered with the unifier parameter. As mentioned earlier, any facts that fail to unify are ignored. In this case it ignores the working_driver/1 instances. The unified facts are stored and returned in a clorm.FactBase object.

The final step in this Python program involves querying the solution to print out the relevant parts. To do this we call the FactBase.select() member function that returns a suitable Select object.

from clorm import ph1_

query=solution.query(Assignment)\
              .where(Assignment.driver == ph1_)\
              .order_by(Assignment.time)

A Clorm query can be viewed as a simplified version of a traditional database query, and the function call syntax will be familiar to users of Python ORM's such as SQLAlchemy or Peewee.

Here we want to find Assignment instances that match the driver field to a special placeholder object ph1_ and to return the results sorted by the assignment time. The value of the ph1_ placeholder will be provided when the query is actually executed; separating specification from execution allows the query to be re-run multiple times with different values.

In particular, we now iterate over the list of drivers and execute the query for each driver and print the result.

for d in drivers:
    assignments = list(query.bind(d.name).all())
    if not assignments:
        print("Driver {} is not working today".format(d.name))
    else:
        print("Driver {} must deliver: ".format(d.name))
        for a in assignments:
            print("\t Item {} at time {}".format(a.item, a.time))

Calling query.bind(d.name) first creates a new query with the placeholder values assigned. Because d.name is the first parameter it matches against the placeholder ph1_ in the query definition. Clorm has four predefined placeholders but more can be created using the ph_ function.

Running this example produces the following results:

$ cd examples
$ python quickstart.py
Driver dave must deliver:
         Item item5 at time 1
         Item item4 at time 2
Driver morri must deliver:
         Item item1 at time 1
         Item item2 at time 2
         Item item3 at time 3
Driver michael is not working today

The above example shows some of the main features of Clorm and how to match the Python data model to the defined ASP predicates. For more details about how to use Clorm see the documentation.

Other Clorm Features

Beyond the basic features outlined above there are many other features of the Clorm library. These include:

  • Predicate definitions with complex-terms; by specifying an existing Predicate class, or Python tuples, as the field of a new Predicate sub-class.
class Event(Predicate):
    date: str
    name: str

class Log(Predicate):
    event: Event
    level: int

l1=Log(event=Event(date="2019-4-5",name="goto shops"),level=0)
% Corresponding ASP code
log(event("2019-04-05", "goto shops"), 0).
  • Extending the mapping to specialised types. In the above example the event date is specified as a string. This puts a burden on the Python developer to ensure that only strings of the appropriate format are used when instantiating an Event object. Instead a specialised translation can be specified by subclassing a BaseField or one of it's subclasses. A BaseField class is a a special type of class that contains the functions to map between Python objects and the underlying Clingo API Symbol objects.
from clorm import StringField          # StringField is a sub-class of BaseField
from clorm import field
import datetime

class DateField(StringField):
    pytocl = lambda dt: dt.strftime("%Y-%m-%d")
    cltopy = lambda s: datetime.datetime.strptime(s,"%Y-%m-%d").date()

class Event(Predicate):
    date: datetime.date = field
    name: str

l2=Log(event=Event(date=datetime.date(2019,3,15),name="travel"),level=0)
% Corresponding ASP code
log(event("2019-03-15", "travel"), 0).
  • Function definitions can be decorated with a data conversion signature to perform automatic type conversion for writing Python functions that can be called from an ASP program using the @-syntax.

    For example a function add can be decorated with a data conversion signature that accepts two input integers and expects an output integer.

@make_function_asp_callable
def add(a: int, b: int)-> int:
    a+b
% Calling the add function from ASP
f(@add(5,6)).    % grounds to f(11).
  • Note, the Clingo API does already perform some automatic data conversions. However these conversions are somewhat ad-hoc. Numbers and strings are automatically converted, but there is no mechanism to deal with constants or more complex terms.

    The Clorm mechanism of a data conversion signatures provide a more complete and transparent approach; it can deal with arbitrary conversions and all data conversions are clear since they are specified as part of the signature.

Development

  • Python version: Clorm is tested with Python versions 3.7 - 3.12
  • Clingo version: Clorm is typically tested with Clingo versions 5.5 - 5.7

Alternatives

I think an ORM interface provides a natural fit for getting data into and out of the Clingo solver. However, there will be other opinions on this. Also, data IO is only one aspect of how you might want to interact with the ASP solver.

So, here are some other projects for using Python and Clingo:

License

This project is licensed under the terms of the MIT license.