Getting Prolog running inside Python

So that you can follow the code snippets, I will show you how to embed Prolog in Python in the least painful way.

Prolog is not hard to implement by hand, at least not the most common parts: logic resolution and arithmetic constraints. But that won’t get you the full power of Prolog, and will probably have a few bugs :)

So, there are a lot of different Prolog interpreters and compilers out there. There are GNU Prolog, CIAO Prolog, SICStus and SWI Prolog. Out of those, SWI Prolog has a good website with great documentation and a working Python bridge.

To start, install the library PySwip. Since I use uv to manage my dependencies, I run it like this:

uv add pyswip

And you must also install some version of SWI Prolog. There are installers for different operating systems and you will find them at the SWI Prolog download page.

Personally, I did it with homebrew, via:

brew install swipl

Now, in order to PySwip to find your SWI Prolog installation, it looks at two environment variables: LIBSWIPL_PATH and SWI_HOME_DIR.

Find them via:

swipl --dump-runtime-variables    

It will dump the environment variables from SWI Prolog, and you will look at the following lines:

PLBASE="/opt/homebrew/Cellar/swi-prolog/9.2.9/lib/swipl";
SWIPL_PACK_PATH="";
PLARCH="arm64-darwin";
PLBITS="64";
PLVERSION="90209";
PLSOEXT="so";
PLSOPATH="DYLD_LIBRARY_PATH";
PLLIBDIR="/opt/homebrew/Cellar/swi-prolog/9.2.9/lib/swipl/lib/arm64-darwin";
PLLIB="-lswipl";
PLLIBSWIPL="/opt/homebrew/Cellar/swi-prolog/9.2.9/lib/swipl/lib/arm64-darwin/libswipl.9.2.9.dylib";
PLSHARED="yes";
PLTHREADS="yes";

Now, create a new file, and:

• LIBSWIPL_PATH maps to PLLIBSWIPL

• SWI_HOME_DIR maps to PLBASE

Add those to a .env file. Then, before importing Prolog, ensure you have loaded it, (assuming you installed python-dotenv):

import dotenv
dotenv.load_env()
from pyswip import Prolog
prolog = Prolog()

And in order to communicate with Prolog, use consult to load a file, assert and query:

prolog.assertz("edge(a,b)")
prolog.assertz("edge(a,c)")
prolog.assertz("edge(c,d)")
prolog.assertz("path(X,Y) :- edge(X,Y)")
prolog.assertz("path(X,Y) :- edge(X,Z), path(Z, Y)")

# Get all points reachable from a.
print(prolog.query("path(a, X)"))

So this can start you playing with Prolog from inside Python, where you can glue it to other tech you like.