py/builtinimport: Allow overriding of mp_builtin___import__.

This allows ports to override mp_builtin___import__.

This can be useful in MicroPython applications where
MICROPY_ENABLE_EXTERNAL_IMPORT has to be disabled due to its impact on
build size (2% to 2.5% of the minimal port). By overriding the otherwise
very minimal mp_builtin___import__, ports can still allow limited forms
of application-specific imports.

Signed-off-by: Laurens Valk <laurens@pybricks.com>

py/builtin.h

@@ -64,7 +64,13 @@ MP_DECLARE_CONST_FUN_OBJ_KW(mp_builtin_open_obj);
 
 #endif
 
+// A port can provide its own import handler by defining mp_builtin___import__.
+#ifndef mp_builtin___import__
+#define mp_builtin___import__ mp_builtin___import___default
+#endif
 mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args);
+mp_obj_t mp_builtin___import___default(size_t n_args, const mp_obj_t *args);
+
 mp_obj_t mp_micropython_mem_info(size_t n_args, const mp_obj_t *args);
 
 MP_DECLARE_CONST_FUN_OBJ_VAR(mp_builtin___build_class___obj);

py/builtinimport.c

@@ -467,7 +467,7 @@ STATIC mp_obj_t process_import_at_level(qstr full_mod_name, qstr level_mod_name,
     return module_obj;
 }
 
-mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
+mp_obj_t mp_builtin___import___default(size_t n_args, const mp_obj_t *args) {
     #if DEBUG_PRINT
     DEBUG_printf("__import__:\n");
     for (size_t i = 0; i < n_args; i++) {
@@ -566,7 +566,7 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
 
 #else // MICROPY_ENABLE_EXTERNAL_IMPORT
 
-mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
+mp_obj_t mp_builtin___import___default(size_t n_args, const mp_obj_t *args) {
     // Check that it's not a relative import
     if (n_args >= 5 && MP_OBJ_SMALL_INT_VALUE(args[4]) != 0) {
         mp_raise_NotImplementedError(MP_ERROR_TEXT("relative import"));