Issue #22557: Now importing already imported modules is up to 2.5 times faster.

Include/pystate.h

@@ -41,6 +41,7 @@ typedef struct _is {
 #endif
 
     PyObject *builtins_copy;
+    PyObject *import_func;
 } PyInterpreterState;
 #endif
 

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.6.0 alpha 4
 Core and Builtins
 -----------------
 
+- Issue #22557: Now importing already imported modules is up to 2.5 times faster.
+
 - Issue #17596: Include <wincrypt.h> to help with Min GW building.
 
 - Issue #27507: Add integer overflow check in bytearray.extend().  Patch by

Python/ceval.c

@@ -139,6 +139,7 @@ static int maybe_call_line_trace(Py_tracefunc, PyObject *,
                                  PyThreadState *, PyFrameObject *, int *, int *, int *);
 
 static PyObject * cmp_outcome(int, PyObject *, PyObject *);
+static PyObject * import_name(PyFrameObject *, PyObject *, PyObject *, PyObject *);
 static PyObject * import_from(PyObject *, PyObject *);
 static int import_all_from(PyObject *, PyObject *);
 static void format_exc_check_arg(PyObject *, const char *, PyObject *);
@@ -2808,37 +2809,15 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
         }
 
         TARGET(IMPORT_NAME) {
-            _Py_IDENTIFIER(__import__);
             PyObject *name = GETITEM(names, oparg);
-            PyObject *func = _PyDict_GetItemId(f->f_builtins, &PyId___import__);
-            PyObject *from, *level, *args, *res;
-            if (func == NULL) {
-                PyErr_SetString(PyExc_ImportError,
-                                "__import__ not found");
-                goto error;
-            }
-            Py_INCREF(func);
-            from = POP();
-            level = TOP();
-            args = PyTuple_Pack(5,
-                            name,
-                            f->f_globals,
-                            f->f_locals == NULL ?
-                                  Py_None : f->f_locals,
-                            from,
-                            level);
-            Py_DECREF(level);
-            Py_DECREF(from);
-            if (args == NULL) {
-                Py_DECREF(func);
-                STACKADJ(-1);
-                goto error;
-            }
+            PyObject *fromlist = POP();
+            PyObject *level = TOP();
+            PyObject *res;
             READ_TIMESTAMP(intr0);
-            res = PyEval_CallObject(func, args);
+            res = import_name(f, name, fromlist, level);
+            Py_DECREF(level);
+            Py_DECREF(fromlist);
             READ_TIMESTAMP(intr1);
-            Py_DECREF(args);
-            Py_DECREF(func);
             SET_TOP(res);
             if (res == NULL)
                 goto error;
@@ -5158,6 +5137,50 @@ cmp_outcome(int op, PyObject *v, PyObject *w)
     return v;
 }
 
+static PyObject *
+import_name(PyFrameObject *f, PyObject *name, PyObject *fromlist, PyObject *level)
+{
+    _Py_IDENTIFIER(__import__);
+    PyObject *import_func, *args, *res;
+
+    import_func = _PyDict_GetItemId(f->f_builtins, &PyId___import__);
+    if (import_func == NULL) {
+        PyErr_SetString(PyExc_ImportError, "__import__ not found");
+        return NULL;
+    }
+
+    /* Fast path for not overloaded __import__. */
+    if (import_func == PyThreadState_GET()->interp->import_func) {
+        int ilevel = _PyLong_AsInt(level);
+        if (ilevel == -1 && PyErr_Occurred()) {
+            return NULL;
+        }
+        res = PyImport_ImportModuleLevelObject(
+                        name,
+                        f->f_globals,
+                        f->f_locals == NULL ? Py_None : f->f_locals,
+                        fromlist,
+                        ilevel);
+        return res;
+    }
+
+    Py_INCREF(import_func);
+    args = PyTuple_Pack(5,
+                        name,
+                        f->f_globals,
+                        f->f_locals == NULL ? Py_None : f->f_locals,
+                        fromlist,
+                        level);
+    if (args == NULL) {
+        Py_DECREF(import_func);
+        return NULL;
+    }
+    res = PyEval_CallObject(import_func, args);
+    Py_DECREF(args);
+    Py_DECREF(import_func);
+    return res;
+}
+
 static PyObject *
 import_from(PyObject *v, PyObject *name)
 {

Python/pylifecycle.c

@@ -254,6 +254,11 @@ import_init(PyInterpreterState *interp, PyObject *sysmod)
     interp->importlib = importlib;
     Py_INCREF(interp->importlib);
 
+    interp->import_func = PyDict_GetItemString(interp->builtins, "__import__");
+    if (interp->import_func == NULL)
+        Py_FatalError("Py_Initialize: __import__ not found");
+    Py_INCREF(interp->import_func);
+
     /* Import the _imp module */
     impmod = PyInit_imp();
     if (impmod == NULL) {

Python/pystate.c

@@ -90,6 +90,7 @@ PyInterpreterState_New(void)
         interp->codecs_initialized = 0;
         interp->fscodec_initialized = 0;
         interp->importlib = NULL;
+        interp->import_func = NULL;
 #ifdef HAVE_DLOPEN
 #if HAVE_DECL_RTLD_NOW
         interp->dlopenflags = RTLD_NOW;
@@ -128,6 +129,7 @@ PyInterpreterState_Clear(PyInterpreterState *interp)
     Py_CLEAR(interp->builtins);
     Py_CLEAR(interp->builtins_copy);
     Py_CLEAR(interp->importlib);
+    Py_CLEAR(interp->import_func);
 }