gh-157757: Fix to make lazy import a.b as c import the module a.b (#158092)

* Import the module a lazy `import a.b as c` names

`import a.b as c` compiles to `IMPORT_NAME a.b` followed by `IMPORT_FROM b`.
Lazily, IMPORT_NAME leaves a placeholder holding "a.b", and IMPORT_FROM
rewrote it into the placeholder `lazy from a import b` produces.  Reification
then imported `a` alone and read `b` off it, so the module `a.b` was never
imported under its own name: an attribute of the package shadowing it answered
instead, and `math.pi`, which no module backs, bound the float where the eager
statement raises ModuleNotFoundError.

Mark the dotted import on the placeholder and keep the whole name on it.
Reification imports that name and then walks its components with IMPORT_FROM,
which is what the eager statement does.

The test pinning `lazy import math.pi as pi` as working is inverted, since the
eager statement raises.

* Stop excluding test_trace from the lazy-imports-all run

It passes now that a lazy `import a.b as c` imports the module: the
KeyError on 'test.tracedmodules.testmod' came from the submodule never
being imported under its own name.

* Rename lz_submodule to lz_dotted_as and trim the comments

The flag means "bind the whole dotted name, not the root", which the old
name did not say, and import.c already has unrelated lazy_pending_submodules
machinery to be confused with.

* Chain lazy IMPORT_FROM placeholders instead of flagging dotted imports

Each deferred IMPORT_FROM off a placeholder without a fromlist now keeps
the previous placeholder in lz_from and the attribute name in lz_attr.
Reification walks back to the placeholder IMPORT_NAME left, runs that
import, and replays the lookups in order with _PyEval_ImportFrom, which
is what the eager bytecode does.  This drops the lz_dotted_as flag and
also follows a custom __lazy_import__ that returns a placeholder for a
different module name.

* Chain every deferred IMPORT_FROM onto the previous placeholder

`lazy from a import b` now records its lookup the same way as `import
a.b as c`, so a placeholder holds either the module name and fromlist or
the previous placeholder and an attribute name, and reification has a
single path.  The import passes only the name being resolved as the
fromlist, so accessing b still does not import the other names'
submodules.

* Treat an empty fromlist on a lazy import placeholder as no fromlist

__import__("a.b", fromlist=()) returns the top-level package `a`, the same
as fromlist=None, but the placeholder kept the empty tuple.  Every consumer
of lz_attr then read it as a real fromlist: reification narrowed it to the
chained attribute and replayed the lookups on `a.b`, _PyEval_LazyImportFrom
took the attribute off sys.modules["a.b"], and the repr named `a.b.attr`.

_PyLazyImport_New already collapses None to NULL for exactly this reason,
so collapse an empty tuple there too and every site follows.

* gh-157757: Preserve empty fromlists for custom import hooks

---------

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: brittanyrey

Lib/test/lazy_imports_all_exclude.txt

@@ -35,6 +35,5 @@ test_pyrepl
 test_subprocess
 test_symtable
 test_tools
-test_trace
 test_type_annotations
 test_unittest

Lib/test/test_lazy_import/__init__.py

@@ -724,10 +724,17 @@ def test_non_package_lazily_imported(self):
         assert_python_ok("-c", code)
 
     def test_non_package_lazily_imported_as(self):
-        """Doing a dotted lazy import as still works"""
+        """A dotted lazy import as raises when the name is not a module."""
+        # gh-157757: the eager statement raises, so the lazy one raises too.
         code = textwrap.dedent("""
             lazy import math.pi as pi
-            pi
+
+            try:
+                pi
+            except ModuleNotFoundError:
+                pass
+            else:
+                raise AssertionError("ModuleNotFoundError was not raised")
         """)
         assert_python_ok("-c", code)
 
@@ -1170,6 +1177,22 @@ def test_accessing_one_name_leaves_others_as_proxies(self):
         self.assertEqual(result.returncode, 0, f"stdout: {result.stdout}, stderr: {result.stderr}")
         self.assertIn("OK", result.stdout)
 
+    def test_accessing_one_name_imports_only_its_submodule(self):
+        """Accessing one name should not import the other names' submodules."""
+        code = textwrap.dedent("""
+            import sys
+
+            lazy from test.test_lazy_import.data.pkg import b, bar, broken
+
+            # Importing bar prints, and importing broken raises.
+            b.foo()
+
+            assert "test.test_lazy_import.data.pkg.bar" not in sys.modules
+            assert "test.test_lazy_import.data.pkg.broken" not in sys.modules
+        """)
+        rc, out, err = assert_python_ok("-c", code)
+        self.assertEqual(out, b"")
+
     def test_all_names_reified_after_all_accessed(self):
         """All names should be reified after each is accessed."""
         code = textwrap.dedent("""
@@ -2209,6 +2232,122 @@ def test_import_after_variable_wins(self):
         ]
         self.assertIs(module_same_name_var_order2.bar, bar_mod)
 
+    def test_lazy_import_as_wins_over_variable(self):
+        """A dotted lazy import as imports the submodule the variable hides."""
+        # gh-157757: importing pkg.b rebinds pkg.b from the variable to the
+        # module, eagerly and lazily alike.
+        code = textwrap.dedent("""
+            import sys
+            import test.test_lazy_import.data.pkg as pkg
+            pkg.b = "hides the b submodule"
+
+            lazy import test.test_lazy_import.data.pkg.b as b
+            lazy import test.test_lazy_import.data.metasyntactic.foo.bar as bar
+
+            assert b is sys.modules["test.test_lazy_import.data.pkg.b"], b
+            assert bar is sys.modules[
+                "test.test_lazy_import.data.metasyntactic.foo.bar"], bar
+        """)
+        assert_python_ok("-c", code)
+
+    def test_dotted_as_of_loaded_module(self):
+        """A dotted lazy import as binds the module, not a same-named attribute."""
+        # importlib.metadata is already loaded and has a `metadata` attribute.
+        code = textwrap.dedent("""
+            import importlib.metadata
+            import importlib.metadata as eager
+
+            lazy import importlib.metadata as lazily
+
+            assert lazily is eager, lazily
+        """)
+        assert_python_ok("-c", code)
+
+    def test_dotted_as_replays_lookups_on_custom_placeholder(self):
+        """A dotted lazy import as looks up its names on what the hook returned."""
+        code = textwrap.dedent("""
+            import builtins
+            import xml.dom
+
+            # In a list, so the hook reading it does not resolve it.
+            placeholder = [__lazy_import__("xml")]
+            default = builtins.__lazy_import__
+            builtins.__lazy_import__ = lambda *args: placeholder[0]
+            lazy import fake.dom as dom
+            builtins.__lazy_import__ = default
+
+            assert dom is xml.dom, dom
+        """)
+        assert_python_ok("-c", code)
+
+    def test_empty_fromlist_placeholder_matches_no_fromlist(self):
+        """An empty fromlist behaves like None."""
+        code = textwrap.dedent("""
+            expected = "<lazy_import 'xml.dom'>"
+            # In lists, so reading them does not resolve them.
+            for fromlist in (None, ()):
+                same = [__lazy_import__("xml.dom", fromlist=fromlist)]
+                assert repr(same[0]) == expected, (fromlist, repr(same[0]))
+            bare = [__lazy_import__("xml.dom")]
+            assert repr(bare[0]) == expected, repr(bare[0])
+        """)
+        assert_python_ok("-c", code)
+
+    def test_empty_fromlist_preserved_for_custom_import(self):
+        code = textwrap.dedent("""
+            import builtins
+            import types
+
+            value = object()
+            module = types.SimpleNamespace(dom=value)
+            placeholder = [__lazy_import__("xml.dom", fromlist=())]
+            default_import = builtins.__import__
+            default_lazy_import = builtins.__lazy_import__
+            calls = []
+
+            def import_hook(name, globals, locals, fromlist, level):
+                assert name == "xml.dom", name
+                assert fromlist == (), fromlist
+                calls.append(fromlist)
+                return module
+
+            builtins.__import__ = import_hook
+            assert placeholder[0].resolve() is module
+            builtins.__lazy_import__ = lambda *args: placeholder[0]
+            lazy import fake.dom as dom
+            assert dom is value
+            builtins.__import__ = default_import
+            builtins.__lazy_import__ = default_lazy_import
+
+            assert calls == [(), ()], calls
+        """)
+        assert_python_ok("-c", code)
+
+    def test_dotted_as_replays_lookups_on_dotted_placeholder(self):
+        """A dotted lazy import as replays its names on the hook's package."""
+        # importlib.metadata has a `metadata` attribute of its own, which the
+        # placeholder for importlib must not answer with.
+        for target in ("xml.dom", "importlib.metadata"):
+            with self.subTest(target=target):
+                leaf = target.rpartition(".")[2]
+                code = textwrap.dedent(f"""
+                    import builtins
+                    import sys
+                    import {target}
+
+                    # In a list, so the hook reading it does not resolve it.
+                    placeholder = [__lazy_import__("{target}", fromlist=())]
+                    default = builtins.__lazy_import__
+                    builtins.__lazy_import__ = lambda *args: placeholder[0]
+                    lazy import fake.{leaf} as {leaf}
+                    builtins.__lazy_import__ = default
+
+                    name = repr(globals()["{leaf}"])
+                    assert name == "<lazy_import '{target}'>", name
+                    assert {leaf} is sys.modules["{target}"], {leaf}
+                """)
+                assert_python_ok("-c", code)
+
 
 class DeletedModuleReimportTests(unittest.TestCase):
     """Tests for reimporting after module deletion from sys.modules."""

Lib/test/test_lazy_import/data/pkg/broken.py

@@ -0,0 +1,2 @@
+# Submodule that raises an error during import
+raise ValueError("This module always fails to import")

Misc/NEWS.d/next/Core_and_Builtins/2026-09-18-15-20-00.gh-issue-157757.Kq3Lm7.rst

@@ -0,0 +1,3 @@
+Fix a lazy ``import a.b as c`` reading ``b`` off ``a`` instead of importing
+the module ``a.b``.  It now binds the submodule, and raises
+:exc:`ModuleNotFoundError` when no module backs the name.

Objects/lazyimportobject.c

@@ -14,8 +14,8 @@ PyObject *
 _PyLazyImport_New(_PyInterpreterFrame *frame, PyObject *builtins, PyObject *name, PyObject *fromlist)
 {
     PyLazyImportObject *m;
-    if (!name || !PyUnicode_Check(name)) {
-        PyErr_SetString(PyExc_TypeError, "expected str for name");
+    if (!name || !(PyUnicode_Check(name) || PyLazyImport_CheckExact(name))) {
+        PyErr_SetString(PyExc_TypeError, "expected str or lazy_import for name");
         return NULL;
     }
     if (fromlist == Py_None || fromlist == NULL) {
@@ -104,16 +104,45 @@ lazy_import_getattro(PyObject *op, PyObject *name)
     return value;
 }
 
+// The dotted name of the object that resolving the placeholder returns.
 static PyObject *
-lazy_import_name(PyLazyImportObject *m)
+lazy_import_path(PyLazyImportObject *m)
 {
-    if (m->lz_attr != NULL) {
-        if (PyUnicode_Check(m->lz_attr)) {
-            return PyUnicode_FromFormat("%U.%U", m->lz_from, m->lz_attr);
-        }
-        else {
-            return PyUnicode_FromFormat("%U...", m->lz_from);
+    if (PyLazyImport_CheckExact(m->lz_from)) {
+        PyObject *base = lazy_import_path((PyLazyImportObject *)m->lz_from);
+        if (base == NULL) {
+            return NULL;
         }
+        PyObject *res = PyUnicode_FromFormat("%U.%U", base, m->lz_attr);
+        Py_DECREF(base);
+        return res;
+    }
+    if (m->lz_attr != NULL &&
+        (!PyTuple_Check(m->lz_attr) || PyTuple_GET_SIZE(m->lz_attr) > 0)) {
+        return Py_NewRef(m->lz_from);
+    }
+    // __import__("a.b") returns the top-level package `a`.
+    Py_ssize_t dot = PyUnicode_FindChar(
+        m->lz_from, '.', 0, PyUnicode_GET_LENGTH(m->lz_from), 1
+    );
+    if (dot == -2) {
+        return NULL;
+    }
+    if (dot < 0) {
+        return Py_NewRef(m->lz_from);
+    }
+    return PyUnicode_Substring(m->lz_from, 0, dot);
+}
+
+static PyObject *
+lazy_import_name(PyLazyImportObject *m)
+{
+    if (PyLazyImport_CheckExact(m->lz_from)) {
+        return lazy_import_path(m);
+    }
+    if (m->lz_attr != NULL &&
+        (!PyTuple_Check(m->lz_attr) || PyTuple_GET_SIZE(m->lz_attr) > 0)) {
+        return PyUnicode_FromFormat("%U...", m->lz_from);
     }
     return Py_NewRef(m->lz_from);
 }

Python/ceval.c

@@ -3333,7 +3333,13 @@ _PyEval_LazyImportFrom(PyThreadState *tstate, _PyInterpreterFrame *frame, PyObje
     assert(PyUnicode_Check(name));
     PyObject *ret;
     PyLazyImportObject *d = (PyLazyImportObject *)v;
-    PyObject *mod = PyImport_GetModule(d->lz_from);
+    PyObject *mod = NULL;
+    // Only `from a import b` can take b off an already imported a;
+    // `import a.b as c` has to import a.b first.
+    if (d->lz_attr != NULL && PyTuple_Check(d->lz_attr) &&
+        PyTuple_GET_SIZE(d->lz_attr) > 0) {
+        mod = PyImport_GetModule(d->lz_from);
+    }
     if (mod != NULL) {
         // Check if the module already has the attribute, if so, resolve it
         // eagerly.
@@ -3353,34 +3359,7 @@ _PyEval_LazyImportFrom(PyThreadState *tstate, _PyInterpreterFrame *frame, PyObje
         Py_DECREF(mod);
     }
 
-    if (d->lz_attr != NULL) {
-        if (PyUnicode_Check(d->lz_attr)) {
-            PyObject *from = PyUnicode_FromFormat(
-                "%U.%U", d->lz_from, d->lz_attr);
-            if (from == NULL) {
-                return NULL;
-            }
-            ret = _PyLazyImport_New(frame, d->lz_builtins, from, name);
-            Py_DECREF(from);
-            return ret;
-        }
-    }
-    else {
-        Py_ssize_t dot = PyUnicode_FindChar(
-            d->lz_from, '.', 0, PyUnicode_GET_LENGTH(d->lz_from), 1
-        );
-        if (dot >= 0) {
-            PyObject *from = PyUnicode_Substring(d->lz_from, 0, dot);
-            if (from == NULL) {
-                return NULL;
-            }
-            ret = _PyLazyImport_New(frame, d->lz_builtins, from, name);
-            Py_DECREF(from);
-            return ret;
-        }
-    }
-    ret = _PyLazyImport_New(frame, d->lz_builtins, d->lz_from, name);
-    return ret;
+    return _PyLazyImport_New(frame, d->lz_builtins, v, name);
 }
 
 #define CANNOT_CATCH_MSG "catching classes that do not inherit from "\