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Fable.Python

F# to Python compiler extension for Fable.

Project Structure

  • src/stdlib/ - Python standard library bindings (Builtins, Json, Os, etc.)
  • src/flask/ - Flask web framework bindings
  • src/fastapi/ - FastAPI web framework bindings
  • src/pydantic/ - Pydantic model bindings
  • test/ - Test files
  • examples/ - Example applications (flask, fastapi, django, timeflies)
  • build/ - Generated Python output (gitignored)

Build Commands

just clean           # Clean all build artifacts (build/, obj/, bin/, .fable/)
just build           # Build the project
just test-python     # Run Python tests
just restore         # Restore .NET and paket dependencies
just example-flask   # Build and run Flask example
just example-fastapi # Build and run FastAPI example
just dev-fastapi     # Run FastAPI with hot-reload

Build Output

Generated Python code goes to build/ directories (gitignored):

  • build/ - Main library output
  • build/tests/ - Test output
  • examples/*/build/ - Example outputs

Key Concepts

Fable Type Serialization

F# types compile to non-native Python types:

  • int → Int32 (not Python's int)
  • int64 → Int64
  • F# array → FSharpArray (not Python's list)
  • ResizeArray<T> → Python list
  • nativeint → Python int

Use Fable.Python.Json.dumps with fableDefault for JSON serialization of Fable types. Use ResizeArray<T> for collections in web API responses. Use Pydantic BaseModel for FastAPI request/response types (handles Int32 correctly).

See JSON.md for detailed serialization documentation.

Decorator Attributes

Route decorators use Py.DecorateTemplate:

[<Erase; Py.DecorateTemplate("""app.get("{0}")""")>]
type GetAttribute(path: string) = inherit Attribute()

Class attributes use Py.ClassAttributesTemplate for Pydantic-style classes.

Releasing

When asked to create a release, read RELEASING.md for the release process. Use Release-As: X.Y.Z-alpha.N.P in commit messages or PR descriptions to set the version.