py/runtime: Support importing a method from an instance.

This change follows CPython behaviour, allowing use of:

    from instance import method

to import a bound method from a class instance, eg registered via
setting `sys.modules["instance"] = instance`.

Admittedly this is probably a very rarely used pattern in Python, but it
resolves a long standing comment about whether or not this is actually
possible (it turns out it is possible!).  A test is added to show how it
works.

The main reason for this change is to fix a problem with imports in the
webassembly port: prior to this fix, it was not possible to do `from
js_module import function`, where `js_module` is a JavaScript object
registered to be visible to Python through the webassembly API function
`registerJsModule(js_module)`.  But now with this fix that is possible.

Signed-off-by: Damien George <[email protected]>

Author: dpgeorge

py/runtime.c

@@ -1562,14 +1562,11 @@ mp_obj_t mp_import_from(mp_obj_t module, qstr name) {
     mp_obj_t dest[2];
 
     mp_load_method_maybe(module, name, dest);
-
     if (dest[1] != MP_OBJ_NULL) {
-        // Hopefully we can't import bound method from an object
-    import_error:
-        mp_raise_msg_varg(&mp_type_ImportError, MP_ERROR_TEXT("can't import name %q"), name);
-    }
-
-    if (dest[0] != MP_OBJ_NULL) {
+        // Importing a bound method from a class instance.
+        return mp_obj_new_bound_meth(dest[0], dest[1]);
+    } else if (dest[0] != MP_OBJ_NULL) {
+        // Importing a function or attribute.
         return dest[0];
     }
 
@@ -1602,6 +1599,9 @@ mp_obj_t mp_import_from(mp_obj_t module, qstr name) {
     goto import_error;
 
     #endif
+
+import_error:
+    mp_raise_msg_varg(&mp_type_ImportError, MP_ERROR_TEXT("can't import name %q"), name);
 }
 
 void mp_import_all(mp_obj_t module) {

tests/basics/import_instance_method.py

@@ -0,0 +1,38 @@
+# Test importing a method from a class instance.
+# This is not a common thing to do, but ensures MicroPython has the same semantics as CPython.
+
+import sys
+
+if not hasattr(sys, "modules"):
+    print("SKIP")
+    raise SystemExit
+
+
+class A:
+    def __init__(self, value):
+        self.value = value
+
+    def meth(self):
+        return self.value
+
+    def meth_with_arg(self, a):
+        return [self.value, a]
+
+
+# Register a class instance as the module "mod".
+sys.modules["mod"] = A(1)
+
+# Try importing it as a module.
+import mod
+
+print(mod.meth())
+print(mod.meth_with_arg(2))
+
+# Change the module.
+sys.modules["mod"] = A(3)
+
+# Try importing it using "from ... import".
+from mod import meth, meth_with_arg
+
+print(meth())
+print(meth_with_arg(4))