Let stub-defined __all__ override imports

This enables the use case where objects should be available for
lazy-loading while not advertising them. This might be useful for
deprecations.

lazy_loader/__init__.py

@@ -282,6 +282,7 @@ class _StubVisitor(ast.NodeVisitor):
     def __init__(self):
         self._submodules = set()
         self._submod_attrs = {}
+        self._all = None
 
     def visit_ImportFrom(self, node: ast.ImportFrom):
         if node.level != 1:
@@ -300,6 +301,38 @@ def visit_ImportFrom(self, node: ast.ImportFrom):
         else:
             self._submodules.update(alias.name for alias in node.names)
 
+    def visit_Assign(self, node: ast.Assign):
+        assigned_list = None
+        for name in node.targets:
+            if name.id == "__all__":
+                assigned_list = node.value
+
+        if assigned_list is None:
+            return  # early
+        elif not isinstance(assigned_list, ast.List):
+            msg = (
+                f"expected a list assigned to `__all__`, found {type(assigned_list)!r}"
+            )
+            raise ValueError(msg)
+
+        if self._all is not None:
+            msg = "expected only one definition of `__all__` in stub"
+            raise ValueError(msg)
+        self._all = set()
+
+        for constant in assigned_list.elts:
+            if (
+                not isinstance(constant, ast.Constant)
+                or not isinstance(constant.value, str)
+                or assigned_list == ""
+            ):
+                msg = (
+                    "expected `__all__` to contain only non-empty strings, "
+                    f"got {constant!r}"
+                )
+                raise ValueError(msg)
+            self._all.add(constant.value)
+
 
 def attach_stub(package_name: str, filename: str):
     """Attach lazily loaded submodules, functions from a type stub.
@@ -308,6 +341,10 @@ def attach_stub(package_name: str, filename: str):
     infer ``submodules`` and ``submod_attrs``. This allows static type checkers
     to find imports, while still providing lazy loading at runtime.
 
+    If the stub file defines `__all__`, it must contain a simple list of
+    non-empty strings. In this case, the content of `__dir__()` may be
+    intentionally different from `__all__`.
+
     Parameters
     ----------
     package_name : str
@@ -339,4 +376,10 @@ def attach_stub(package_name: str, filename: str):
 
     visitor = _StubVisitor()
     visitor.visit(stub_node)
-    return attach(package_name, visitor._submodules, visitor._submod_attrs)
+
+    __getattr__, __dir__, __all__ = attach(
+        package_name, visitor._submodules, visitor._submod_attrs
+    )
+    if visitor._all is not None:
+        __all__ = visitor._all
+    return __getattr__, __dir__, __all__

lazy_loader/tests/test_lazy_loader.py

@@ -143,6 +143,35 @@ def test_stub_loading_parity():
     assert stub_getter("some_func") == fake_pkg.some_func
 
 
+FAKE_STUB_OVERRIDE_ALL = """
+__all__ = [
+    "rank",
+    "gaussian",
+    "sobel",
+    "scharr",
+    "roberts",
+    # `prewitt` not included!
+    "__version__",  # included but not imported in stub
+]
+
+from . import rank
+from ._gaussian import gaussian
+from .edges import sobel, scharr, prewitt, roberts
+"""
+
+
+def test_stub_override_all(tmp_path):
+    stub = tmp_path / "stub.pyi"
+    stub.write_text(FAKE_STUB_OVERRIDE_ALL)
+    _get, _dir, _all = lazy.attach_stub("my_module", str(stub))
+
+    expect_dir = {"gaussian", "sobel", "scharr", "prewitt", "roberts", "rank"}
+    assert set(_dir()) == expect_dir
+
+    expect_all = {"rank", "gaussian", "sobel", "scharr", "roberts", "__version__"}
+    assert set(_all) == expect_all
+
+
 def test_stub_loading_errors(tmp_path):
     stub = tmp_path / "stub.pyi"
     stub.write_text("from ..mod import func\n")