wasi-graphviz

Run Graphviz from Python via WebAssembly — no native Graphviz binary, no Node.js, no browser required.

Why this exists

Rendering DOT graphs from Python sits in an awkward gap.

• Browser-based WASM renderers like @hpcc-js/wasm-graphviz ship Graphviz as a WebAssembly module — but it's an Emscripten build wrapped in JavaScript glue, designed to run in a browser or Node. Tools like easydot wrap that flow so you can call it from Python and display SVGs in a notebook, but the renderer itself still ultimately needs a JS runtime / browser surface to produce pixels — fine for interactive notebooks, awkward for headless static SVG generation in CI, batch jobs, or server-side pipelines.
• System-binary Python wrappers like graphviz and pygraphviz shell out to a system-installed dot or link against libcgraph. That means every deployment target — laptops, CI runners, Lambda functions, Docker base images — has to install Graphviz separately, and your code has to spawn subprocesses or manage shared-library loading.

wasi-graphviz plugs the gap: a wasm32-wasi build of upstream Graphviz plus a thin Python wrapper. pip install is the entire install story, the wheel is ~440 KB, and rendering happens entirely in-process — no subprocesses, no system packages, no JS, no browser. Works the same in a notebook, a CI job, a serverless function, or an offline air-gapped environment.

The wheel itself does not include a WASM runtime — that's an explicit choice (see Installation below) so you can pick the runtime that matches your deployment constraints.

Installation

wasi-graphviz needs a runtime to execute the bundled graphviz.wasm. Two are supported, picked via extras:

# Recommended for most users — fast native runtime
pip install wasi-graphviz[wasmtime]

# Pure-Python runtime — slow, but works anywhere CPython does (not Pyodide)
pip install wasi-graphviz[pywasm]

# Install both — `render(..., backend="auto")` will prefer wasmtime
pip install wasi-graphviz[all]

Choosing a backend

backend="auto" (the default) prefers wasmtime when available and silently falls back to pywasm, so most code can ignore the distinction. Force one explicitly when you have a reason — see Backend selection below.

Pyodide / marimo / WebAssembly-based Python environments

pywasm is pure-Python, but it imports fcntl (for stdin handling) which is not available in Pyodide because Pyodide itself runs inside a browser WebAssembly sandbox that lacks POSIX file-control APIs.

Neither backend works in Pyodide today. A future browser / pyodide backend (using the browser's native WebAssembly object + a WASI polyfill) is possible, but not yet implemented.

If you need Graphviz in a Pyodide or marimo notebook, use easydot instead — it wraps the browser's @hpcc-js/wasm renderer and works out of the box in those environments.

Benchmarks

Median wall time per render on Apple M-series, Python 3.11, measured via pytest-benchmark (run yourself with uv run pytest tests/test_benchmarks.py -m perf --benchmark-only):

pywasm is a pure-Python WASM interpreter, so the ratio is roughly "interpreted Python evaluating WASM bytecode" vs "native compiled code" — expect orders of magnitude, not factors. Use wasmtime unless your environment forbids native code.

Quick start

from wasi_graphviz import render

# Render a simple graph to SVG (uses wasmtime if available, falls back to pywasm)
svg = render("digraph G { a -> b; }")
print(svg.decode("utf-8"))

Usage

Basic rendering

from wasi_graphviz import render

# Render to SVG with default dot engine
svg = render("digraph G { a -> b; }")

# Use a different layout engine
svg = render("graph G { a -- b; }", engine="neato")

# Render to DOT format
output = render("digraph G { a -> b; }", format="dot")

Backend selection

See the trade-off table above for when to pick which.

from wasi_graphviz import render

# Auto-select (prefer wasmtime, fall back to pywasm)
svg = render("digraph G { a -> b; }", backend="auto")

# Force pywasm — pure Python, slow but maximally portable
svg = render("digraph G { a -> b; }", backend="pywasm")

# Force wasmtime — fast native runtime, requires compiled extension
svg = render("digraph G { a -> b; }", backend="wasmtime")

Error handling

from wasi_graphviz import render, RenderError

try:
    svg = render("not valid dot {")
except RenderError as e:
    print(f"Render failed: {e}")

Supported layout engines

All major Graphviz layout engines work:

• dot — hierarchical layouts (default)
• neato — spring model
• circo — circular layout
• fdp — force-directed placement
• sfdp — scalable FDP
• twopi — radial layouts
• osage — array-based layouts
• patchwork — treemaps

Supported output formats

The core plugin supports:

• svg (default)
• dot
• json
• ps
• map
• fig
• tk

Architecture

The project consists of three layers:

1. WASM artifact (graphviz.wasm)

   • Graphviz 15.0.0 compiled for wasm32-wasi
   • Exposes a plain C ABI: graphviz_render, graphviz_free, graphviz_last_error, graphviz_version
   • No Emscripten, no JS glue

2. Python backends

   • PywasmBackend — pure-Python interpreter with built-in WASI support
   • WasmtimeBackend — fast native runtime with full WASI support

3. Public API

   • render(dot, format="svg", engine="dot", backend="auto") -> bytes

Building from source

See BUILD.md for detailed build instructions.

Quick summary:

# Install build tools
pixi install

# Build the WASM artifact
python scripts/prepare_graphviz_wasi.py build/src/graphviz-15.0.0
pixi run cmake -S build/src/graphviz-15.0.0 -B build/graphviz-cmake \
  -DCMAKE_TOOLCHAIN_FILE=$(pwd)/native/wasm32-wasi-toolchain.cmake \
  ...
pixi run cmake --build build/graphviz-cmake --parallel
# ... compile wrapper, link, validate

Development

# Run tests (perf benchmarks are skipped by default)
uv run pytest

# Format and lint
uv run ruff check .
uv run ruff format .

# Run benchmarks comparing wasmtime vs pywasm across graph sizes
uv run pytest tests/test_benchmarks.py -m perf --benchmark-only

License & attribution

This package is licensed under the Eclipse Public License 2.0 (EPL-2.0). See LICENSE for the full text.

The wheel bundles a compiled build of Graphviz (also EPL-2.0). The full EPL-2.0 text is also shipped inside the wheel at wasi_graphviz/assets/GRAPHVIZ_LICENSE. Source for the bundled Graphviz version is available upstream: https://gitlab.com/graphviz/graphviz/-/tree/15.0.0.

Modifications applied to the Graphviz source before compilation are described in scripts/prepare_graphviz_wasi.py and are themselves licensed under EPL-2.0. See NOTICE for the full attribution.

wasi-graphviz is an unofficial repackaging and is not affiliated with or endorsed by the Graphviz project.

First functional v0.1.0 built with Kimi K2.6 in ~1h, single session (81% context used). Total cost: ~$1