The code structure

Generally speaking, the directories directly under open_spiel are C++ (except for integration_tests and python). A similar structure is available in open_spiel/python, containing the Python equivalent code.

Some top level directories are special:

  • open_spiel/integration_tests: Generic (python) tests for all the games.

  • open_spiel/tests: The C++ common test utilities.

  • open_spiel/scripts: The scripts useful for development (building, running tests, etc).

For example, we have for C++:

  • open_spiel/: Contains the game abstract C++ API.

  • open_spiel/games: Contains the games C++ implementations.

  • open_spiel/algorithms: The C++ algorithms implemented in OpenSpiel.

  • open_spiel/examples: The C++ examples.

  • open_spiel/tests: The C++ common test utilities.

For Python you have:

  • open_spiel/python/examples: The Python examples.

  • open_spiel/python/algorithms/: The Python algorithms.

C++ and Python implementations.

Some objects (e.g. Policy, CFRSolver, BestResponse) are available both in C++ and Python. The goal is to be able to use C++ objects in place of Python objects for most of the cases. In particular, for the objects that are well supported, expect to have in the test for the Python object, a test checking that both the C++ and the Python implementation behave the same.

Adding a game

We describe here only the simplest and fastest way to add a new game. It is ideal to first be aware of the general API (see spiel.h).

  1. Choose a game to copy from in games/ (or python/games/). Suggested games: Tic-Tac-Toe and Breakthrough for perfect information without chance events, Backgammon or Pig for perfect information games with chance events, Goofspiel and Oshi-Zumo for simultaneous move games, and Leduc poker and Liar’s dice for imperfect information games. For the rest of these steps, we assume Tic-Tac-Toe.

  2. Copy the header and source: tic_tac_toe.h,, and to new_game.h,, and (or and

  3. Configure CMake:

    • If you are working with C++: add the new game’s source files to games/CMakeLists.txt.

    • If you are working with C++: add the new game’s test target to games/CMakeLists.txt.

    • If you are working with Python: add the test to python/CMakeLists.txt and import it in python/games/

  4. Update boilerplate C++/Python code:

    • In new_game.h, rename the header guard at the the top and bottom of the file.

    • In the new files, rename the inner-most namespace from tic_tac_toe to new_game.

    • In the new files, rename TicTacToeGame and TicTacToeState to NewGameGame and NewGameState.

    • At the top of, change the short name to new_game and include the new game’s header.

  5. Update Python integration tests:

    • Add the short name to the list of expected games in python/tests/

  6. You should now have a duplicate game of Tic-Tac-Toe under a different name. It should build and the test should run, and can be verified by rebuilding and running the example examples/example --game=new_game.

  7. Now, change the implementations of the functions in NewGameGame and NewGameState to reflect your new game’s logic. Most API functions should be clear from the game you copied from. If not, each API function that is overridden will be fully documented in superclasses in spiel.h.

  8. Once done, rebuild and rerun the tests to ensure everything passes (including your new game’s test!).

  9. Add a playthrough file to catch regressions:

    • Run ./open_spiel/scripts/ new_game to generate a random game, to be used by integration tests to prevent any regression. open_spiel/integration_tests/ will automatically load the playthroughs and compare them to newly generated playthroughs.

    • If you have made a change that affects playthroughs, run ./scripts/ to update them.

Conditional dependencies

The goal is to make it possible to optionally include external dependencies and build against them. The setup was designed to met the following needs:

  • Single source of truth: We want a single action to be sufficient to manage the conditional install and build. Thus, we use bash environment variables, that are read both by the install script ( to know whether we should clone the dependency, and by CMake to know whether we should include the files in the target. Tests can also access the bash environment variable.

  • Light and safe defaults: By default, we exclude the dependencies to diminish install time and compilation time. If the bash variable is unset, we download the dependency and we do not build against it.

  • Respect the user-defined values: The script, which is included in all the scripts that needs to access the constant values, do not override the constants but set them if and only if they are undefined. This respects the user-defined values, e.g. on their .bashrc or on the command line.

When you add a new conditional dependency, you need to touch:

  • the root CMakeLists.txt to add the option, with an OFF default

  • add the option to scripts/

  • change to make sure the dependency is installed

  • use constructs like if (${OPEN_SPIEL_OPEN_SPIEL_BUILD_WITH_HANABI}) in CMake to optionally add the targets to build.

Debugging tools

For complex games it may be tricky to get all the details right. Reading through the playthrough You can visualize small game trees using open_spiel/python/examples/ or for large games there is an interactive viewer for OpenSpiel games called SpielViz.