Support PEP420 (implicit namespace packages) as --pyargs target.

Previously, when running `--pyargs pkg`, if you didn't have `pkg/__init__.py`,
pytest would fail with `ERROR: module or package not found: pkg (missing __init__.py?)`.

Now it's discovering the package and tests inside it correctly.

If used in conjunction with `consider_namespace_packages` in config, test
modules get correct `__package__` and `__name__` attributes as well.

Fixes: #478

Other relevant issues:
- #2371
- #10569

In addition, remove `"namespace"` origin handling -- this value isn't used since python 3.8.
See:
- python/cpython#5481
- https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.submodule_search_locations

Author: karlicoss

AUTHORS

@@ -134,6 +134,7 @@ Deysha Rivera
 Dheeraj C K
 Dhiren Serai
 Diego Russo
+Dima Gerasimov
 Dmitry Dygalo
 Dmitry Pribysh
 Dominic Mortlock

changelog/478.feature.rst

@@ -0,0 +1 @@
+Support PEP420 (implicit namespace packages) as `--pyargs` target.

src/_pytest/main.py

@@ -991,11 +991,19 @@ def search_pypath(module_name: str) -> str | None:
     # ValueError: not a module name
     except (AttributeError, ImportError, ValueError):
         return None
-    if spec is None or spec.origin is None or spec.origin == "namespace":
+
+    if spec is None:
         return None
-    elif spec.submodule_search_locations:
-        return os.path.dirname(spec.origin)
+    elif (
+        spec.submodule_search_locations is not None
+        and len(spec.submodule_search_locations) > 0
+    ):
+        # If submodule_search_locations is set, it's a package (regular or namespace).
+        # Typically there is a single entry, but documentation claims it can be empty too
+        #  (e.g. if the package has no physical location).
+        return spec.submodule_search_locations[0]
     else:
+        # Must be a simple module.
         return spec.origin
 
 

testing/test_main.py

@@ -169,8 +169,13 @@ def test_dir(self, invocation_path: Path) -> None:
         ):
             resolve_collection_argument(invocation_path, "src/pkg::foo::bar")
 
-    def test_pypath(self, invocation_path: Path) -> None:
+    @pytest.mark.parametrize("namespace_package", [False, True])
+    def test_pypath(self, namespace_package: bool, invocation_path: Path) -> None:
         """Dotted name and parts."""
+        if namespace_package:
+            # Namespace package doesn't have to contain __init__py
+            (invocation_path / "src/pkg/__init__.py").unlink()
+
         assert resolve_collection_argument(
             invocation_path, "pkg.test", as_pypath=True
         ) == CollectionArgument(