Fix importing packages with missing __init__.py

src/_pytask/path.py

@@ -4,6 +4,7 @@
 
 import contextlib
 import functools
+import importlib.machinery
 import importlib.util
 import itertools
 import os
@@ -200,19 +201,31 @@ def _resolve_pkg_root_and_module_name(path: Path) -> tuple[Path, str]:
 
     Passing the full path to `models.py` will yield Path("src") and "app.core.models".
 
+    This function also handles namespace packages (directories without __init__.py)
+    by walking up the directory tree and checking if Python's import system can
+    resolve the computed module name to the given path. This prevents double-imports
+    when task files import each other using Python's standard import mechanism.
+
     Raises CouldNotResolvePathError if the given path does not belong to a package
-    (missing any __init__.py files).
+    (missing any __init__.py files) and no valid namespace package root is found.
 
     """
+    # First, try to find a regular package (with __init__.py files).
     pkg_path = _resolve_package_path(path)
     if pkg_path is not None:
         pkg_root = pkg_path.parent
-
-        names = list(path.with_suffix("").relative_to(pkg_root).parts)
-        if names[-1] == "__init__":
-            names.pop()
-        module_name = ".".join(names)
-        return pkg_root, module_name
+        module_name = _compute_module_name(pkg_root, path)
+        if module_name:
+            return pkg_root, module_name
+
+    # No regular package found. Check for namespace packages by walking up the
+    # directory tree and verifying that Python's import system would resolve
+    # the computed module name to this file.
+    for candidate in (path.parent, *path.parent.parents):
+        module_name = _compute_module_name(candidate, path)
+        if module_name and _is_importable(module_name, path):
+            # Found a root where Python's import system agrees with our module name.
+            return candidate, module_name
 
     msg = f"Could not resolve for {path}"
     raise CouldNotResolvePathError(msg)
@@ -222,6 +235,81 @@ class CouldNotResolvePathError(Exception):
     """Custom exception raised by _resolve_pkg_root_and_module_name."""
 
 
+def _spec_matches_module_path(
+    module_spec: importlib.machinery.ModuleSpec | None, module_path: Path
+) -> bool:
+    """Return true if the given ModuleSpec can be used to import the given module path.
+
+    Handles both regular modules (via origin) and namespace packages
+    (via submodule_search_locations).
+
+    """
+    if module_spec is None:
+        return False
+
+    if module_spec.origin:
+        return Path(module_spec.origin) == module_path
+
+    # For namespace packages, check submodule_search_locations.
+    # https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.submodule_search_locations
+    if module_spec.submodule_search_locations:
+        for location in module_spec.submodule_search_locations:
+            if Path(location) == module_path:
+                return True
+
+    return False
+
+
+def _is_importable(module_name: str, module_path: Path) -> bool:
+    """Check if a module name would resolve to the given path using Python's import.
+
+    This verifies that importing `module_name` via Python's standard import mechanism
+    (as if typed in the REPL) would load the file at `module_path`.
+
+    Note: find_spec() has a side effect of creating parent namespace packages in
+    sys.modules. We clean these up to avoid polluting the module namespace.
+    """
+    # Track modules before the call to clean up side effects
+    modules_before = set(sys.modules.keys())
+
+    try:
+        spec = importlib.util.find_spec(module_name)
+    except (ImportError, ValueError, ImportWarning):
+        return False
+    finally:
+        # Clean up any modules that were added as side effects.
+        # find_spec() can create parent namespace packages in sys.modules.
+        modules_added = set(sys.modules.keys()) - modules_before
+        for mod_name in modules_added:
+            sys.modules.pop(mod_name, None)
+
+    return _spec_matches_module_path(spec, module_path)
+
+
+def _compute_module_name(root: Path, module_path: Path) -> str | None:
+    """Compute a module name based on a path and a root anchor.
+
+    Returns None if the module name cannot be computed.
+
+    """
+    try:
+        path_without_suffix = module_path.with_suffix("")
+    except ValueError:
+        return None
+
+    try:
+        relative = path_without_suffix.relative_to(root)
+    except ValueError:
+        return None
+
+    names = list(relative.parts)
+    if not names:
+        return None
+    if names[-1] == "__init__":
+        names.pop()
+    return ".".join(names) if names else None
+
+
 def _import_module_using_spec(
     module_name: str, module_path: Path, module_location: Path
 ) -> ModuleType | None:
@@ -234,7 +322,11 @@ def _import_module_using_spec(
     # Checking with sys.meta_path first in case one of its hooks can import this module,
     # such as our own assertion-rewrite hook.
     for meta_importer in sys.meta_path:
-        spec = meta_importer.find_spec(module_name, [str(module_location)])
+        try:
+            spec = meta_importer.find_spec(module_name, [str(module_location)])
+        except (ImportError, KeyError, ValueError):
+            # Some meta_path finders raise exceptions when parent modules don't exist.
+            continue
         if spec is not None:
             break
     else:
@@ -243,6 +335,7 @@ def _import_module_using_spec(
         mod = importlib.util.module_from_spec(spec)
         sys.modules[module_name] = mod
         spec.loader.exec_module(mod)  # type: ignore[union-attr]
+        _insert_missing_modules(sys.modules, module_name)
         return mod
 
     return None

tests/test_path.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import importlib
+import importlib.util
 import sys
 import textwrap
 from contextlib import ExitStack as does_not_raise  # noqa: N813
@@ -389,3 +391,93 @@ def __init__(self) -> None:
         # Ensure we do not import the same module again (#11475).
         mod2 = import_path(init, root=tmp_path)
         assert mod is mod2
+
+    def test_import_path_namespace_package_consistent_with_python_import(
+        self, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+    ) -> None:
+        """
+        Ensure import_path uses module names consistent with Python's import
+        system for namespace packages (directories without __init__.py).
+
+        This prevents double-imports when task files import each other.
+        See: https://github.com/pytask-dev/pytask/issues/XXX
+
+        Structure:
+            src/
+                myproject/
+                    __init__.py          <- regular package
+                    tasks/               <- NO __init__.py (namespace package)
+                        task_a.py
+
+        With PYTHONPATH=src, Python imports task_a.py as "myproject.tasks.task_a".
+        pytask's import_path() should use the same name, not
+        "src.myproject.tasks.task_a".
+        """
+        # Create the directory structure
+        src_dir = tmp_path / "src"
+        pkg_dir = src_dir / "myproject"
+        tasks_dir = pkg_dir / "tasks"
+        tasks_dir.mkdir(parents=True)
+
+        # myproject is a regular package (has __init__.py)
+        (pkg_dir / "__init__.py").write_text("")
+
+        # tasks is a namespace package (NO __init__.py)
+        task_file = tasks_dir / "task_a.py"
+        task_file.write_text(
+            textwrap.dedent(
+                """
+                EXECUTION_COUNT = 0
+
+                def on_import():
+                    global EXECUTION_COUNT
+                    EXECUTION_COUNT += 1
+
+                on_import()
+                """
+            )
+        )
+
+        # Add src/ to sys.path (simulating PYTHONPATH=src)
+        monkeypatch.syspath_prepend(str(src_dir))
+
+        # Verify Python's import system would use "myproject.tasks.task_a"
+        expected_module_name = "myproject.tasks.task_a"
+        spec = importlib.util.find_spec(expected_module_name)
+        assert spec is not None, (
+            f"Python cannot find {expected_module_name!r} - test setup is wrong"
+        )
+        assert spec.origin == str(task_file)
+
+        # Now call pytask's import_path
+        mod = import_path(task_file, root=tmp_path)
+
+        # The module name should match what Python's import system uses
+        assert mod.__name__ == expected_module_name, (
+            f"import_path() used module name {mod.__name__!r} but Python's import "
+            f"system would use {expected_module_name!r}. This mismatch causes "
+            f"double-imports when task files import each other."
+        )
+
+        # The module should be registered in sys.modules under the correct name
+        assert expected_module_name in sys.modules
+        assert sys.modules[expected_module_name] is mod
+
+        # Importing via Python's standard mechanism should return the SAME module
+        # (not re-execute the file)
+        standard_import = importlib.import_module(expected_module_name)
+        assert standard_import is mod, (
+            "Python's import returned a different module object than import_path(). "
+            "This means the file would be executed twice."
+        )
+
+        # Verify the file was only executed once
+        assert mod.EXECUTION_COUNT == 1, (
+            f"Module was executed {mod.EXECUTION_COUNT} times instead of once. "
+            f"Double-import detected!"
+        )
+
+        # Cleanup
+        sys.modules.pop(expected_module_name, None)
+        sys.modules.pop("myproject.tasks", None)
+        sys.modules.pop("myproject", None)