Add __lazy_import__, check sys.modules before import

Author: DinoV

Include/internal/pycore_ceval.h

@@ -298,7 +298,7 @@ PyAPI_FUNC(void) _PyEval_FormatExcUnbound(PyThreadState *tstate, PyCodeObject *c
 PyAPI_FUNC(void) _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs);
 PyAPI_FUNC(PyObject *) _PyEval_ImportFrom(PyThreadState *, PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) _PyEval_LazyImportName(PyThreadState *tstate, PyObject *builtins, PyObject *globals, 
-            PyObject *name, PyObject *fromlist, PyObject *level);
+            PyObject *locals, PyObject *name, PyObject *fromlist, PyObject *level);
 PyAPI_FUNC(PyObject *) _PyEval_LazyImportFrom(PyThreadState *tstate, PyObject *v, PyObject *name);
 PyAPI_FUNC(PyObject *) _PyEval_ImportName(PyThreadState *tstate, PyObject *builtins, PyObject *globals, PyObject *locals,
             PyObject *name, PyObject *fromlist, PyObject *level);

Include/internal/pycore_global_objects_fini_generated.h

@@ -155,6 +155,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(__iter__)
         STRUCT_FOR_ID(__itruediv__)
         STRUCT_FOR_ID(__ixor__)
+        STRUCT_FOR_ID(__lazy_import__)
         STRUCT_FOR_ID(__le__)
         STRUCT_FOR_ID(__len__)
         STRUCT_FOR_ID(__length_hint__)

Include/internal/pycore_global_strings.h

@@ -35,6 +35,10 @@ extern PyObject *
 _PyImport_ResolveName(PyThreadState *tstate, PyObject *name, PyObject *globals, int level);
 extern PyObject *
 _PyImport_LoadLazyImportTstate(PyThreadState *tstate, PyObject *lazy_import);
+extern PyObject *
+_PyImport_LazyImportModuleLevelObject(PyThreadState *tstate, PyObject *name, PyObject *builtins, PyObject *globals,
+                                      PyObject *locals, PyObject *fromlist,
+                                      int level);
 
 
 #ifdef HAVE_DLOPEN
@@ -79,6 +83,10 @@ extern int _PyImport_IsDefaultImportFunc(
         PyInterpreterState *interp,
         PyObject *func);
 
+extern int _PyImport_IsDefaultLazyImportFunc(
+        PyInterpreterState *interp,
+        PyObject *func);
+
 extern PyObject * _PyImport_GetImportlibLoader(
         PyInterpreterState *interp,
         const char *loader_name);

Include/internal/pycore_import.h

@@ -313,6 +313,7 @@ struct _import_state {
     int dlopenflags;
 #endif
     PyObject *import_func;
+    PyObject *lazy_import_func;
     /* The global import lock. */
     _PyRecursiveMutex lock;
     /* diagnostic info in PyImport_ImportModuleLevelObject() */

Include/internal/pycore_interp_structs.h

@@ -2540,6 +2540,31 @@ def test_disallowed_reimport(self):
         self.assertIsNot(excsnap, None)
 
 
+class LazyImportTests(unittest.TestCase):
+    def tearDown(self):
+        """Make sure no modules pre-exist in sys.modules which are being used to
+        test."""
+        for key in list(sys.modules.keys()):
+            if key.startswith('test.test_import.data.lazy_imports'):
+                del sys.modules[key]
+
+    def test_basic_unused(self):
+        try:
+            import test.test_import.data.lazy_imports.basic_unused
+        except ImportError as e:
+            self.fail('lazy import failed')
+
+        self.assertFalse("test.test_import.data.lazy_imports.basic2" in sys.modules)
+
+    def test_basic_used(self):
+        try:
+            import test.test_import.data.lazy_imports.basic_used
+        except ImportError as e:
+            self.fail('lazy import failed')
+
+        self.assertTrue("test.test_import.data.lazy_imports.basic2" in sys.modules)
+
+
 class TestSinglePhaseSnapshot(ModuleSnapshot):
     """A representation of a single-phase init module for testing.
 

Include/internal/pycore_runtime_init_generated.h

@@ -0,0 +1,4 @@
+def f():
+   pass
+
+x = 42

Include/internal/pycore_unicodeobject_generated.h

@@ -0,0 +1 @@
+lazy import test.test_import.data.lazy_imports.basic2