Call prolog predicate from python


Question

I have some .pl file and I want to call predicate declared in it from python script. How can I do that?

For example, test.pl

rD( [], Ans, Ans ).
rD( [X|Xs], Ans, Acc ) :-
    member( X, Acc ),
    rD( Xs, Ans, Acc ), !.
rD( [X|Xs], Ans, Acc ) :-
    \+member( X, Acc ),
    append( Acc, [X], AccNew ),
    rD( Xs, Ans, AccNew ), !.

Working like

?- rD( [1,2,3,4,5,4], X ).
X = [1, 2, 3, 4, 5].

I want to call rD somehow from python script and get answer in result variable

result
[1, 2, 3, 4, 5]

ps: it is just an example, and I don't want to rewrite my current Prolog program.

1
4
5/20/2018 1:28:07 PM

Accepted Answer

Not that I have a direct experience with it, but there is a project called PySWIP that provides a bridge between Python and SWI-Prolog. The wiki hosted on Google Code's project pages holds installation instructions and some usage examples.

EDIT (5th of July 2019)

PySWIP now seems to be maintained on Github, with its own installation instructions. TLDR: installing SWI-Prolog and pip install pyswip should do the job, for both Python 2 and 3.

3
7/5/2019 1:28:46 PM

Since you don't want to "rewrite my current Prolog program," I think the natural approach is to make an external call from Python to SWI-Prolog, passing appropriate command line arguments.

Take a look at this SO discussion, How to call external command in Python, from Sept. 2008. Using the subprocess module allows the stdout from an external command to be piped to the Python process and read as a stream there.

This reduces the problem to choosing command line arguments for SWI-Prolog. It is possible to invoke SWI-Prolog indirectly, through a shell-script on Unix-like systems or a "DOS" batch/cmd file on Windows, but I'll omit further mention of such an indirect call.

See especially the discussion in Sec. 2.4.2 of the SWI-Prolog manual (linked above) of -g and -t options. For example:

swipl --quiet -t rD( [1,2,3,4,5,4], X ),halt

will likely do what you want. The --quiet option suppresses the banner/welcome message, which you probably want to simplify parsing the output as received by Python.


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