py: Add module weak link support.

With this patch a port can enable module weak link support and provide
a dict of qstr->module mapping.  This mapping is looked up only if an
import fails to find the requested module in the filesystem.

This allows to have the builtin module named, eg, usocket, and provide
a weak link of "socket" to the same module, but this weak link can be
overridden if a file by the name "socket.py" is found in the import
path.

1 files changed, 21 insertions, 3 deletions

diff --git a/py/builtinimport.c b/py/builtinimport.c
index ffdadf022..1272d5941 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -44,6 +44,7 @@
 #include "runtime0.h"
 #include "runtime.h"
 #include "builtin.h"
+#include "builtintables.h"
 
 #if 0 // print debugging info
 #define DEBUG_PRINT (1)
@@ -251,12 +252,29 @@ mp_obj_t mp_builtin___import__(mp_uint_t n_args, mp_obj_t *args) {
             }
             DEBUG_printf("Current path: %s\n", vstr_str(&path));
 
-            // fail if we couldn't find the file
             if (stat == MP_IMPORT_STAT_NO_EXIST) {
-                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", qstr_str(mod_name)));
+                #if MICROPY_MODULE_WEAK_LINKS
+                // check if there is a weak link to this module
+                if (i == mod_len) {
+                    mp_map_elem_t *el = mp_map_lookup((mp_map_t*)&mp_builtin_module_weak_links_dict_obj.map, MP_OBJ_NEW_QSTR(mod_name), MP_MAP_LOOKUP);
+                    if (el == NULL) {
+                        goto no_exist;
+                    }
+                    // found weak linked module
+                    module_obj = el->value;
+                } else {
+                    no_exist:
+                #else
+                {
+                #endif
+                    // couldn't find the file, so fail
+                    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", qstr_str(mod_name)));
+                }
+            } else {
+                // found the file, so get the module
+                module_obj = mp_module_get(mod_name);
             }
 
-            module_obj = mp_module_get(mod_name);
             if (module_obj == MP_OBJ_NULL) {
                 // module not already loaded, so load it!