py/objexcept: Add __traceback__ attribute getter.

This makes it easier to provide custom formatting of exception stack
traces, e.g. to make them fit on a tiny display.

Signed-off-by: David (Pololu) <dev-david@pololu.com>

Author: DavidEGrayson

py/objexcept.c

@@ -280,7 +280,23 @@ void mp_obj_exception_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         }
         return;
     }
-    if (attr == MP_QSTR_args) {
+    if (attr == MP_QSTR___traceback__) {
+        // We expose traceback data via 'exc.__traceback__' to allow
+        // custom formatting of the traceback by user code.
+        size_t entry_count = self->traceback_len / TRACEBACK_ENTRY_LEN;
+        size_t *data = self->traceback_data;
+        mp_obj_t obj = mp_obj_new_list(entry_count, NULL);
+        mp_obj_list_t *list = MP_OBJ_TO_PTR(obj);
+        for (size_t i = 0; i < entry_count; i++) {
+            size_t *src = &data[i * TRACEBACK_ENTRY_LEN];
+            mp_obj_t entry[3];
+            entry[0] = MP_OBJ_NEW_QSTR(src[0]);       // filename
+            entry[1] = MP_OBJ_NEW_SMALL_INT(src[1]);  // line number
+            entry[2] = MP_OBJ_NEW_QSTR(src[2]);       // block
+            list->items[i] = mp_obj_new_tuple(3, entry);
+        }
+        dest[0] = obj;
+    } else if (attr == MP_QSTR_args) {
         decompress_error_text_maybe(self);
         dest[0] = MP_OBJ_FROM_PTR(self->args);
     } else if (attr == MP_QSTR_value || attr == MP_QSTR_errno) {

tests/cpydiff/types_exception_traceback.py

@@ -0,0 +1,16 @@
+"""
+categories: Types,Exception
+description: The ``__traceback__`` attribute of exceptions is a simple list of tuples.
+cause: MicroPython is optimised to reduce code size.
+workaround: Either treat ``__traceback__`` as an opaque value and use the `traceback` module to format tracebacks, or check the type of ``__traceback__`` before using it.
+"""
+
+
+def foo():
+    1 / 0
+
+
+try:
+    foo()
+except Exception as e:
+    print(e.__traceback__)

tests/micropython/traceback.py

@@ -0,0 +1,26 @@
+# This comment allows test runners like run_script_on_remote_target
+# to safely prepend a line of code without messing up line numbers.
+
+
+def foo():
+    1 / 0
+
+
+def normalize(filename):
+    if filename == "<stdin>":
+        return "traceback.py"
+    return filename.split("/")[-1]
+
+
+try:
+    foo()
+except Exception as e:
+    tb = e.__traceback__
+    for t in tb:
+        print("%s:%d" % (normalize(t[0]), t[1]), *t[2:])
+    e.__traceback__ = None
+    print(e.__traceback__)
+    try:
+        e.args = 77
+    except AttributeError:
+        print("setting args not allowed")

tests/micropython/traceback.py.exp

@@ -0,0 +1,4 @@
+traceback.py:6 foo
+traceback.py:16 <module>
+[]
+setting args not allowed

tests/run-tests.py

@@ -80,6 +80,7 @@
         "misc/sys_settrace_features.py",
         "misc/sys_settrace_generator.py",
         "misc/sys_settrace_loop.py",
+        "micropython/traceback.py",
     ),
 }
 
@@ -108,6 +109,8 @@
         "micropython/schedule.py",
         # These require sys.exc_info().
         "misc/sys_exc_info.py",
+        # These require exception tracebacks.
+        "micropython/traceback.py",
         # These require sys.settrace().
         "misc/sys_settrace_cov.py",
         "misc/sys_settrace_features.py",
@@ -125,9 +128,7 @@
         "stress/list_sort.py",  # watchdog kicks in because it takes too long
     ),
     "minimal": (
-        "basics/class_inplace_op.py",  # all special methods not supported
-        "basics/subclass_native_init.py",  # native subclassing corner cases not support
-        "micropython/opt_level.py",  # don't assume line numbers are stored
+        "micropython/traceback.py",  # no line numbers, no list[N:] syntax
     ),
     "nrf": (
         "basics/io_buffered_writer.py",
@@ -827,6 +828,8 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
 
     # Skip platform-specific tests.
     skip_tests.update(platform_tests_to_skip.get(args.platform, ()))
+    if args.build == "minimal":
+        skip_tests.update(platform_tests_to_skip.get(args.build, ()))
 
     # Some tests are known to fail on 64-bit machines
     if pyb is None and platform.architecture()[0] == "64bit":