Glossary

The terms these pages use, in alphabetical order. Each entry links to the page that explains it in full.

Artifact set. The files a release ships: python.wasm, python-snapshot.wasm, python-stdlib.zip, MANIFEST and SHA256SUMS. See Artifacts.

Door. The guest's one way to call the host: a single host function, bound to the guest's host module, that takes request bytes and returns response bytes. You pass it to run as host=, or let the tool protocol use it through tools=. See Tool protocol.

Fuel. A count of the guest's executed instructions. A fuel cap stops a run after that many instructions, and fuel_used reports how many ran. Because fuel counts operations and not time, a limit is the same on every machine. See Fuel.

Grant. A host folder you hand to the guest at a guest path, with Grant(host, guest). A grant is read-only unless you pass writable=True. See Grants.

Guest. The Python program running inside python.wasm, in the sandbox. The host is the program that runs it, such as your Python process through run.

Mount. A folder in the guest whose files your own code serves. The guest uses ordinary calls such as open and os.listdir, and your provider answers each one. See Mounts.

Preopen. A host directory handed to a WASI guest at a guest path when it starts. WASI gives a guest no file access except through preopens. The stdlib zip, every grant and every mount reach the guest as preopens.

Provider. The object you write to serve a mount. It answers calls such as stat, list and read for paths inside the mount. See Mounts.

Reactor. A WASI module that the host initializes once and then calls into, instead of one that runs a single main. python.wasm is a reactor: the host calls _initialize, then one run per instance. See How it works.

Root. The guest's /. By default the guest has no writable /. root=True gives a call a fresh private folder as its /, removed afterwards; root= a path gives it that host folder. See Grants.

Session. One instance of the runtime kept across several calls, each a turn. Variables, functions and imports made in one turn are there in the next. See Sessions.

Snapshot. python-snapshot.wasm: python.wasm after the interpreter has started and the stdlib's bootstrap modules are imported, saved with Wizer. It pre-imports no package. A call on the snapshot skips the interpreter's start-up. See How it works.

Stdlib zip. python-stdlib.zip: the standard library and the Python code of every package in the set, as bytecode in one zip. Every call sees it read-only at /usr/local/lib. See Guest filesystem.

The judge. The project's test suite, run with uv run pytest. It judges every check and prints a scoreboard of passes, known gaps, blocked checks and failures. See Your first change.

Turn. One call to a session's run. See Sessions.

WASI. The WebAssembly System Interface: the operating-system interface a WebAssembly module uses for files, the clock, arguments and standard output. See Why WASI.