Flask backend for server-side Python execution#250
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milanofthe merged 27 commits intomainfrom Feb 11, 2026
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Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
- Migrate streaming from SSE to polling (start+poll pattern) - Fix stdin race condition with noop flush and leftover queue - Fix logging capture with permanent _ProtocolWriter on sys.stdout/stderr - Add recursion guard to _ProtocolWriter to prevent infinite loops - Fix ensureServerInit to retry on failure instead of caching rejection - Fix initBackendFromUrl to call init() for proper callback setup - Add resp.ok checks and poll response validation in FlaskBackend - Align stopStreaming to wait for server confirmation like Pyodide - Validate X-Session-ID header, return 400 when missing - Prevent orphan sessions in poll/exec/stop by using lookup instead of create - Move cleanup thread start into create_app factory - Fix requires-python to >=3.10 and add numpy dependency - Remove dead code: _capture_output, REQUEST_TIMEOUT, unused imports Co-Authored-By: KDW1 <KDW1@users.noreply.github.com>
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Adds a Flask/Waitress backend as an alternative to the Pyodide Web Worker for server-side Python execution. Useful for packages with native C extensions that aren't built for Emscripten.
Architecture
worker.py): REPL over stdin/stdout JSON lines. One per session. Handles exec, eval, and streaming with timeout watchdog (30s).app.py): Session management, routes translate HTTP into subprocess messages. Uses waitress in production, Flask dev server with--debug.venv.py): Workers run in a dedicated venv at~/.pathview/venv. Simulation packages install there, not in the user's environment.backend.ts): TypeScript client implementing the Backend interface. HTTP requests + long-polling for streaming.Key details
?backend=flask&host=http://localhost:5050PYTHON_PACKAGESindependencies.ts(same source of truth as Pyodide)pip install pathview→pathview servestream-execTesting