pepeto/python-tsp

Library to solve Traveling Salesperson Problems with pure Python code

Python TSP Solver

python-tsp is a library written in pure Python for solving typical Traveling Salesperson Problems (TSP). It can work with symmetric and asymmetric versions.

Installation

pip install python-tsp
poetry add python-tsp  # if using Poetry in the project

Quickstart

Regular TSP problem

Suppose we wish to find a Hamiltonian path with least cost for the following problem:

We can find an optimal path using a Dynamic Programming method with:

import numpy as np
from python_tsp.exact import solve_tsp_dynamic_programming

distance_matrix = np.array([
    [0,  5, 4, 10],
    [5,  0, 8,  5],
    [4,  8, 0,  3],
    [10, 5, 3,  0]
])
permutation, distance = solve_tsp_dynamic_programming(distance_matrix)

The solution will be [0, 1, 3, 2], with total distance 17.

There are also heuristic-based approaches to solve the same problem. For instance, to use a local search method:

from python_tsp.heuristics import solve_tsp_local_search

permutation, distance = solve_tsp_local_search(distance_matrix)

In this case there is generally no guarantee of optimality, but in this small instance the answer is normally a permutation with total distance 17 as well (notice in this case there are many permutations with the same optimal distance).

You can check here to see the list of available solvers.

Keep in mind that, by definition, the route always starts and finishes at node 0 (a closed path). So, in the previous example, after node 2 we go back to 0.

Open TSP problem

If your problem is of the "open" type, i.e., it is not required to go back to the origin, you can "trick" the solvers by setting all elements of the first column of the distance matrix to zero:

distance_matrix[:, 0] = 0
permutation, distance = solve_tsp_dynamic_programming(distance_matrix)

and we obtain [0, 2, 3, 1], with distance 12. Keep in mind that we still start and end at 0, but since the distance from everyone to 0 is null, in practice it is the same as stopping at node 1.

Notice that in this case the distance matrix is actually asymmetric, but the methods here are applicable as well.

Computing a distance matrix

The previous examples assumed you already had a distance matrix. If that is not the case, the distances module has prepared some functions to compute an Euclidean distance matrix, a Great Circle Distance, street distances (via OSRM) and support for TSPLIB-type files. Check the docs folder for some examples.

A more intricate example

Let us attempt to solve the a280.tsp TSPLIB file. It has 280 nodes, so an exact approach may take too long. Hence, let us start with a Local Search (LS) solver:

from python_tsp.distances import tsplib_distance_matrix
from python_tsp.heuristics import solve_tsp_local_search, solve_tsp_simulated_annealing

# Get corresponding distance matrix
tsplib_file = "tests/tsplib_data/a280.tsp"
distance_matrix = tsplib_distance_matrix(tsplib_file)

# Solve with Local Search using default parameters
permutation, distance = solve_tsp_local_search(distance_matrix)
# distance: 3064

When calling solve_tsp_local_search like this, we are starting with a random permutation, using the 2-opt scheme as neighborhood, and running it until a local optimum is obtained. Check the solvers documentation for more information.

In my specific run, I obtained a permutation with total distance 3064. The actual best solution for this instance is 2579, so our solution has a 18.8% gap. Remember this solver is a heuristic, and thus it has no business in finding the actual optimum. Moreover, you can get different results trying distinct perturbation schemes and starting points.

Since the local search solver only obtains local minima, maybe we can get more lucky with a metaheuristic such as the Simulated Annealing (SA):

permutation2, distance2 = solve_tsp_simulated_annealing(distance_matrix)
# distance: 2830

In my execution, I got a 2830 as distance, representing a 9.7% gap, a great improvement over the local search. The SA input parameters are basically the same as the LS, but you can check the solvers documentation for more details as well.

If you are familiarized with metaheuristics, you would know that the SA does not guarantee a local minimum, despite its solution being better than the LS in this case. Thus, maybe we can squeeze some improvement by running a local search starting with its returned solution:

permutation3, distance3 = solve_tsp_local_search(distance_matrix, x0=permutation2)
# distance: 2825

So, that was o.k., nothing groundbreaking, but a nice combo to try in some situations. Nevertheless, if we change the perturbation scheme to, say, PS3:

permutation4, distance4 = solve_tsp_local_search(
    distance_matrix, x0=permutation2, perturbation_scheme="ps3"
)
# distance: 2746

and there we go, a distance of 2746 or a 6.5% gap of the optimum. Notice we set the x_0 to the permutation returned by the SA in the last run.

Check again the solvers documentation to get an idea of these perturbation schemes.

In this case, PS3 and PS6 have larger neighborhood sizes, so we may get a better chance of improvement by switching to them in the LS step. Test other schemes and see if you can get different results.

Finally, if you don't feel like fine-tunning the solvers for each problem, a rule of thumb that worked relatively well for me is to run the SA with a 2-opt and follow it by a LS with PS3 or PS6, like

permutation, distance = solve_tsp_simulated_annealing(distance_matrix)
permutation2, distance2 = solve_tsp_local_search(
    distance_matrix, x0=permutation, perturbation_scheme="ps3"
)

Methods available

See here for a list of available solvers and how to use them.

For developers

The project uses Python Poetry to manage dependencies. Check the website for installation instructions.

After that, clone the repo and install dependencies with poetry install.

Here are the detailed steps that should be followed before making a pull request:

# Black and flake8 to be conformant with PEP8
poetry run black .
poetry run flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics --ignore=E203,W503
poetry run flake8 . --count --exit-zero --max-complexity=10 --max-line-length=79 --statistics

# Mypy for proper type hints
poetry run mypy --ignore-missing-imports .

You can also run all of these steps at once with the check-up bash script:

./.scripts/checkup_scripts.sh
bash ./.scripts/checkup_scripts.sh  # if the previous one fails

Finally (and of course), make sure all tests pass and you get at least 95% of coverage:

poetry run pytest --cov=. --cov-report=term-missing --cov-fail-under=95 tests/

Python version support

To help keeping this library relatively up to date and maintainable but not to a point of becoming bleeding edge, it follows at least the supported version of Debian Stable. New features won't be backported to older versions, but this can be accomplished for bug fixes. Just open an issue in case you find a problem.

Release Notes and Changelog

Releases 0.4.X

Release 0.4.0

• Add Branch and Bound to the list of exact solvers. Thanks @luanleonardo for this contribution.

Python support: Python >= 3.8.1

Releases 0.3.X

Release 0.3.1

• Replace heuristic log messages with regular prints. The logs could be compromised with outer level configurations and start to pollute the stdout. Prints are easier to manipulate.

• Add a verbose parameter to heuristics to print execution messages in the stdout.

  Thanks for @FrickTobias for pointing this issue and providing a fix.

Python support: Python >= 3.7.1

Release 0.3.0

• Add support for street distance matrix calculation via an OSRM server.

Python support: Python >= 3.7.1

Releases 0.2.X

Release 0.2.1

• Improve TSLIB support by using the TSPLIB95 library .

Python support: Python >= 3.6

Release 0.2.0

• Add distance matrix support for TSPLIB files (symmetric and asymmetric instances);
• Add new neighborhood types for local search based methods: PS4, PS5, PS6 and 2-opt;
• Local Search and Simulated Annealing use 2-opt scheme as default;
• Both local search based methods now respect a maximum processing time if provided;
• The primitive print to display iterations information is replaced by a proper log.

Python support: Python >= 3.6

Releases 0.1.X

Release 0.1.2

• Local search and Simulated Annealing random solution now begins at root node 0 just like the exact methods.

Python support: Python >= 3.6

Release 0.1.1

• Improve Python versions support.

Python support: Python >= 3.6

Release 0.1.0

• Initial version. Support for the following solvers:
  • Exact (Brute force and Dynamic Programming);
  • Heuristics (Local Search and Simulated Annealing).
• The local search-based algorithms can be run with neighborhoods PS1, PS2 and PS3.

Python support: Python >= 3.8

Contributors

• @FrickTobias