python: Basic Starlark implementation of PyInfo

Not yet tested or exercised; to be done in a followup change.

Work towards #15897

PiperOrigin-RevId: 507902946
Change-Id: I0b203892d28e6b00f4ee6567e26abda84e6f71bc

src/main/starlark/builtins_bzl/common/python/providers.bzl

@@ -111,6 +111,60 @@ the same conventions as the standard CPython interpreter.
 
 PyRuntimeInfo = _builtins.toplevel.PyRuntimeInfo
 
+def _check_arg_type(name, required_type, value):
+    value_type = type(value)
+    if value_type != required_type:
+        fail("parameter '{}' got value of type '{}', want '{}'".format(
+            name,
+            value_type,
+            required_type,
+        ))
+
+def _PyInfo_init(
+        *,
+        transitive_sources,
+        uses_shared_libraries = False,
+        imports = depset(),
+        has_py2_only_sources = False,
+        has_py3_only_sources = False):
+    _check_arg_type("transitive_sources", "depset", transitive_sources)
+    _check_arg_type("uses_shared_libraries", "bool", uses_shared_libraries)
+    _check_arg_type("imports", "depset", imports)
+    _check_arg_type("has_py2_only_sources", "bool", has_py2_only_sources)
+    _check_arg_type("has_py2_only_sources", "bool", has_py3_only_sources)
+    return {
+        "transitive_sources": transitive_sources,
+        "imports": imports,
+        "uses_shared_libraries": uses_shared_libraries,
+        "has_py2_only_sources": has_py2_only_sources,
+        "has_py3_only_sources": has_py2_only_sources,
+    }
+
+StarlarkPyInfo, _unused_raw_ctor = provider(
+    "Encapsulates information provided by the Python rules.",
+    init = _PyInfo_init,
+    fields = {
+        "transitive_sources": """\
+A (`postorder`-compatible) depset of `.py` files appearing in the target's
+`srcs` and the `srcs` of the target's transitive `deps`.
+""",
+        "uses_shared_libraries": """
+Whether any of this target's transitive `deps` has a shared library file (such
+as a `.so` file).
+
+This field is currently unused in Bazel and may go away in the future.
+""",
+        "imports": """\
+A depset of import path strings to be added to the `PYTHONPATH` of executable
+Python targets. These are accumulated from the transitive `deps`.
+The order of the depset is not guaranteed and may be changed in the future. It
+is recommended to use `default` order (the default).
+""",
+        "has_py2_only_sources": "Whether any of this target's transitive sources requires a Python 2 runtime.",
+        "has_py3_only_sources": "Whether any of this target's transitive sources requires a Python 3 runtime.",
+    },
+)
+
 PyInfo = _builtins.toplevel.PyInfo
 
 # TODO(b/203567235): Re-implement in Starlark