fix(python): stop shipping build artifacts and ship the PEP 561 marker

.github/workflows/build.yml

@@ -179,6 +179,15 @@ jobs:
         if: matrix.platform == 'ubuntu-latest'
         run: ./build.sh fable-library --python
 
+      # PEP 561 requires the marker inside the package directory. CopyStage filters
+      # by extension, so it is easy to drop again; without it mypy refuses to read
+      # the core/*.pyi stubs at all.
+      - name: PEP 561 marker survives staging
+        if: matrix.platform == 'ubuntu-latest'
+        run: |
+          test -f temp/fable-library-py/fable_library/py.typed \
+            || { echo "::error::fable_library/py.typed missing from staged package; stubs would be inert for mypy"; exit 1; }
+
       - name: Fable Library - Python (Windows)
         if: matrix.platform == 'windows-latest'
         run: .\build.bat fable-library --python

src/Fable.Build/FableLibrary/Python.fs

@@ -31,6 +31,12 @@ type BuildFableLibraryPython(?skipCore: bool, ?postFableBuildStage: unit -> unit
         Directory.GetFiles(this.LibraryDir, "*") |> Shell.copyFiles this.BuildDir
         Directory.GetFiles(this.SourceDir, "*.py") |> Shell.copyFiles this.OutDir
 
+        // PEP 561 marker. It has to sit *inside* the package directory for type
+        // checkers to read the stubs at all, and the `*.py` filter above drops it --
+        // which is why every published wheel so far has shipped `core/*.pyi` that no
+        // consumer could see.
+        [ Path.Combine(this.SourceDir, "py.typed") ] |> Shell.copyFiles this.OutDir
+
         // Python extension modules
         Directory.GetFiles(Path.Combine(this.SourceDir, "core"), "*")
         |> Shell.copyFiles (Path.Combine(this.OutDir, "core"))

src/fable-library-py/pyproject.toml

@@ -23,20 +23,32 @@ dev = [
     "pydantic>=2.13.4",
 ]
 
-[tool.hatch.build.targets.sdist]
-include = ["./fable_library"]
-
-[tool.hatch.build.targets.wheel]
-include = ["./fable_library"]
-
-[tool.hatch.build.targets.wheel.sources]
-"./fable_library" = "fable_library"
+# Note: there is deliberately no [tool.hatch.*] config here. `build-backend` below
+# is maturin, so hatch settings would be dead config that merely *looks* like it
+# governs what ships -- which is a good way for a packaging bug to go unnoticed.
 
 [tool.maturin]
 python-source = "."                        # Look at project root
 module-name = "fable_library.core._core"   # Full module path
 features = ["pyo3/extension-module"]
 
+# `Fable.Library.Python.fsproj` lives inside the package directory, so `dotnet build`
+# drops MSBuild output into `fable_library/obj/`. With `python-source = "."` maturin
+# would package that wholesale, and one of those paths contains a comma
+# (`.NETCoreApp,Version=v10.0.AssemblyAttributes.fs`) -- RECORD is CSV, so the
+# resulting wheel is rejected by `uv pip install` as malformed. `pip` is lenient
+# enough to survive it, which makes the failure look intermittent.
+#
+# None of this shows up in a clean CI checkout, only for contributors who have run
+# `dotnet build`. Excluding here fixes it regardless of build order; deleting `obj/`
+# in the build script would not, since it regenerates on the next build.
+exclude = [
+    "fable_library/obj/**/*",
+    "**/__pycache__/**/*",
+    "fable_library/*.fs",
+    "fable_library/*.fsproj",
+]
+
 [tool.ty.environment]
 python-version = "3.12"
 
@@ -56,5 +68,5 @@ testpaths = ["tests", "tests-benchmark"]
 addopts = "--benchmark-columns=min,max,mean,stddev,iqr"
 
 [build-system]
-requires = ["hatchling", "maturin>=1.5,<2.0"]
+requires = ["maturin>=1.5,<2.0"]
 build-backend = "maturin"