pychoco

Current choco-solver version: 6.0.2

Python bindings for the Choco Constraint programming solver (https://choco-solver.org/).

Choco-solver is an open-source Java library for Constraint Programming (see https://choco-solver.org/). It comes with many features such as various types of variables, various state-of-the-art constraint, various search strategies, etc.

The pychoco library uses a native-build of the original Java Choco-solver library, in the form of a shared library, which means that it can be used without any JVM. This native-build is created with GraalVM native-image tool.

We heavily relied on JGraphT Python bindings source code to understand how such a thing could be achieved, so many thanks to JGraphT authors!

Installation

We automatically build 64-bit wheels for Python versions >= 3.10 on Linux (x86_64), Windows (AMD64) and MacOSX (arm64). They can be directly downloaded from PyPI (https://pypi.org/project/pychoco/) or using pip:

pip install pychoco

Documentation

If you do not have any knowledge about Constraint Programming (CP) and Choco-solver, you can have a look at https://choco-solver.org/tutos/ for a quick introduction to CP and to Choco-solver features. The tutorial in this website includes both Java and Python examples. For this Python API, we also provide an API documentation which is available online at https://pychoco.readthedocs.io/.

You can also have a look at the pychoco Cheat Sheet : pychoco cheat sheet

Finally, we designed a few notebooks examples that you can find in the examples directory.

Quickstart

pychoco's API is quite close to Choco's Java API. The first thing to do is to import the library and create a model object:

from pychoco import Model

model = Model("My Choco Model")

Then, you can use this model object to create variables:

intvars = model.intvars(10, 0, 10)
sum_var = model.intvar(0, 100)

You can also create views from this Model object:

b6 = model.int_ge_view(intvars[6], 6)

Create and post (or reify) constraints:

model.all_different(intvars).post()
model.sum(intvars, "=", sum_var).post()
b7 = model.arithm(intvars[7], ">=", 7).reify()

Solve your problem:

model.get_solver().solve()

And retrieve the solution:

print("intvars = {}".format([i.get_value() for i in intvars]))
print("sum = {}".format(sum_var.get_value()))
print("intvar[6] >= 6 ? {}".format(b6.get_value()))
print("intvar[7] >= 7 ? {}".format(b7.get_value()))

> intvars = [3, 5, 9, 6, 7, 2, 0, 1, 4, 8]
> sum = 45
> intvar[6] >= 6 ? False
> intvar[7] >= 7 ? False

Configuring search

Generic search strategies

Currently, the main limitation of pychoco is the customization of search strategies, which is not as advanced as the Java version. This is mainly due to the fact that pychoco's need to rely on a compiled C entrypoint to Choco-solver, which does not allow Python routines to be injected into the solving procedure. One possible solution would be to implement a parsing system to define custom search strategy in Choco-solver, and rely on this system in pychoco. However, this represents a considerable amount of work that we cannot commit to in the short term. Note: please do not hesitate to let us know, or open a pull request if you want to implement this feature, or suggest an alternative solution.

However, it is possible to rely on the generic search heuristics available in Choco-solver, through the Solver object. Currently available search strategies are: default_search, dom_over_w_deg_search, dom_over_w_deg_ref_search, activity_based_search, min_dom_lb_search, min_dom_ub_search, random_search, conflict_history_search, input_order_lb_search, input_order_ub_search, failure_length_based_search, failure_rate_based_search, pick_on_dom_search, pick_on_fil_search.

Example:

solver.set_dom_over_w_deg_search(decision_variables)

Hints

Hints can improve the search procedure by defining a partial solution and drive the search toward a solution. Hints apply on integer variables, and consist of couples of (variable, value).

Example:

solver.add_hint(cost, min_cost)

Parallel portfolio

The parallel portfolio is a powerful feature of Choco-solver which allows to solve a problem in parallel with different search strategies. Each solving thread can inform other when he finds a solution, leading them to update their bounds in case of an optimization process. To set up a parallel portfolio, it is necessary to construct as many identical models as the number of threads. This feature can very efficient to boost the optimization procedure.

Example:

from pychoco.model import Model
from pychoco.parallel_portfolio import ParallelPortfolio

pf = ParallelPortfolio()
pf.steal_nogoods_on_restarts()
for i in range(0, 5):
    m = Model()
    vars = m.intvars(10, 0, 100)
    nv = m.intvar(3, 4)
    m.n_values(vars, nv).post()
    s = m.intvar(0, 1000)
    m.sum(vars, "=", s).post()
    m.set_objective(s, True)
    pf.add_model(m)
sol = pf.find_best_solution()

Build from source

The following system dependencies are required to build pychoco from sources:

• GraalVM >= 22 (see https://www.graalvm.org/)
• Native Image component for GraalVM (see https://www.graalvm.org/22.1/reference-manual/native-image/)
• Apache Maven (see https://maven.apache.org/)
• Python >= 3.10 (see https://www.python.org/)
• SWIG >= 3 (see https://www.swig.org/)

Once these dependencies are satisfied, clone the current repository:

git clone --recurse-submodules https://github.com/chocoteam/pychoco.git

The --recurse-submodules is necessary as the choco-solver-capi is a separate git project included as a submodule (see https://github.com/chocoteam/choco-solver-capi). It contains all the necessary to compile Choco-solver as a shared native library using GraalVM native-image.

Ensure that the $JAVA_HOME environment variable is pointing to GraalVM, and from the cloned repository execute the following command:

sh build.sh

This command will compile Choco-solver into a shared native library and compile the Python bindings to this native API using SWIG.

Finally, run:

pip install .

And voilà !

Citation

Justeau-Allaire D, Prud’homme C (2025). pychoco: all-inclusive Python bindings for the Choco-solver constraint programming library. Journal of Open Source Software, 10(113), 8847, https://doi.org/10.21105/joss.08847

Getting help or contribute

We do our best to maintain pychoco and keep it up-to-date with choco-solver. However, if you see missing features, if you have any questions about using the library, suggestions for improvements, or if you detect a bug, please open an issue.