instance_path for namespace packages uses path closest to submodule

CHANGES.rst

@@ -8,6 +8,8 @@ Unreleased
 -   Inline some optional imports that are only used for certain CLI
     commands. :pr:`4606`
 -   Relax type annotation for ``after_request`` functions. :issue:`4600`
+-   ``instance_path`` for namespace packages uses the path closest to
+    the imported submodule. :issue:`4600`
 
 
 Version 2.1.2

src/flask/scaffold.py

@@ -780,30 +780,46 @@ def _matching_loader_thinks_module_is_package(loader, mod_name):
     )
 
 
-def _find_package_path(root_mod_name):
+def _find_package_path(import_name):
     """Find the path that contains the package or module."""
-    try:
-        spec = importlib.util.find_spec(root_mod_name)
+    root_mod_name, _, _ = import_name.partition(".")
 
-        if spec is None:
+    try:
+        root_spec = importlib.util.find_spec(root_mod_name)
+
+        if root_spec is None:
             raise ValueError("not found")
     # ImportError: the machinery told us it does not exist
     # ValueError:
     #    - the module name was invalid
     #    - the module name is __main__
-    #    - *we* raised `ValueError` due to `spec` being `None`
+    #    - *we* raised `ValueError` due to `root_spec` being `None`
     except (ImportError, ValueError):
         pass  # handled below
     else:
         # namespace package
-        if spec.origin in {"namespace", None}:
-            return os.path.dirname(next(iter(spec.submodule_search_locations)))
+        if root_spec.origin in {"namespace", None}:
+            package_spec = importlib.util.find_spec(import_name)
+            if package_spec is not None and package_spec.submodule_search_locations:
+                # Pick the path in the namespace that contains the submodule.
+                package_path = os.path.commonpath(
+                    package_spec.submodule_search_locations
+                )
+                search_locations = (
+                    location
+                    for location in root_spec.submodule_search_locations
+                    if package_path.startswith(location)
+                )
+            else:
+                # Pick the first path.
+                search_locations = iter(root_spec.submodule_search_locations)
+            return os.path.dirname(next(search_locations))
         # a package (with __init__.py)
-        elif spec.submodule_search_locations:
-            return os.path.dirname(os.path.dirname(spec.origin))
+        elif root_spec.submodule_search_locations:
+            return os.path.dirname(os.path.dirname(root_spec.origin))
         # just a normal module
         else:
-            return os.path.dirname(spec.origin)
+            return os.path.dirname(root_spec.origin)
 
     # we were unable to find the `package_path` using PEP 451 loaders
     loader = pkgutil.get_loader(root_mod_name)
@@ -845,8 +861,7 @@ def find_package(import_name: str):
     for import. If the package is not installed, it's assumed that the
     package was imported from the current working directory.
     """
-    root_mod_name, _, _ = import_name.partition(".")
-    package_path = _find_package_path(root_mod_name)
+    package_path = _find_package_path(import_name)
     py_prefix = os.path.abspath(sys.prefix)
 
     # installed to the system

tests/test_instance_config.py

@@ -59,6 +59,25 @@ def test_uninstalled_package_paths(modules_tmpdir, purge_module):
     assert app.instance_path == str(modules_tmpdir.join("instance"))
 
 
+def test_uninstalled_namespace_paths(tmpdir, monkeypatch, purge_module):
+    def create_namespace(package):
+        project = tmpdir.join(f"project-{package}")
+        monkeypatch.syspath_prepend(str(project))
+        project.join("namespace").join(package).join("__init__.py").write(
+            "import flask\napp = flask.Flask(__name__)\n", ensure=True
+        )
+        return project
+
+    _ = create_namespace("package1")
+    project2 = create_namespace("package2")
+    purge_module("namespace.package2")
+    purge_module("namespace")
+
+    from namespace.package2 import app
+
+    assert app.instance_path == str(project2.join("instance"))
+
+
 def test_installed_module_paths(
     modules_tmpdir, modules_tmpdir_prefix, purge_module, site_packages, limit_loader
 ):