bpo-38530: Offer suggestions on AttributeError (#16856)
When printing AttributeError, PyErr_Display will offer suggestions of similar
attribute names in the object that the exception was raised from:
>>> collections.namedtoplo
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 99be629..321b04e 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -15,7 +15,7 @@
#include "pycore_interp.h" // PyInterpreterState.importlib
#include "pycore_object.h" // _PyDebug_PrintTotalRefs()
#include "pycore_parser.h" // _PyParser_ASTFromString()
-#include "pycore_pyerrors.h" // _PyErr_Fetch
+#include "pycore_pyerrors.h" // _PyErr_Fetch, _Py_Offer_Suggestions
#include "pycore_pylifecycle.h" // _Py_UnhandledKeyboardInterrupt
#include "pycore_pystate.h" // _PyInterpreterState_GET()
#include "pycore_sysmodule.h" // _PySys_Audit()
@@ -953,6 +953,16 @@ print_exception(PyObject *f, PyObject *value)
if (err < 0) {
PyErr_Clear();
}
+ PyObject* suggestions = _Py_Offer_Suggestions(value);
+ if (suggestions) {
+ // Add a trailer ". Did you mean: (...)?"
+ err = PyFile_WriteString(". Did you mean: ", f);
+ if (err == 0) {
+ err = PyFile_WriteObject(suggestions, f, Py_PRINT_RAW);
+ err += PyFile_WriteString("?", f);
+ }
+ Py_DECREF(suggestions);
+ }
err += PyFile_WriteString("\n", f);
Py_XDECREF(tb);
Py_DECREF(value);
diff --git a/Python/suggestions.c b/Python/suggestions.c
new file mode 100644
index 0000000..2c0858d
--- /dev/null
+++ b/Python/suggestions.c
@@ -0,0 +1,146 @@
+#include "Python.h"
+
+#include "pycore_pyerrors.h"
+
+#define MAX_DISTANCE 3
+#define MAX_CANDIDATE_ITEMS 100
+#define MAX_STRING_SIZE 20
+
+/* Calculate the Levenshtein distance between string1 and string2 */
+static size_t
+levenshtein_distance(const char *a, const char *b) {
+ if (a == NULL || b == NULL) {
+ return 0;
+ }
+
+ const size_t a_size = strlen(a);
+ const size_t b_size = strlen(b);
+
+ if (a_size > MAX_STRING_SIZE || b_size > MAX_STRING_SIZE) {
+ return 0;
+ }
+
+ // Both strings are the same (by identity)
+ if (a == b) {
+ return 0;
+ }
+
+ // The first string is empty
+ if (a_size == 0) {
+ return b_size;
+ }
+
+ // The second string is empty
+ if (b_size == 0) {
+ return a_size;
+ }
+
+ size_t *buffer = PyMem_Calloc(a_size, sizeof(size_t));
+ if (buffer == NULL) {
+ return 0;
+ }
+
+ // Initialize the buffer row
+ size_t index = 0;
+ while (index < a_size) {
+ buffer[index] = index + 1;
+ index++;
+ }
+
+ size_t b_index = 0;
+ size_t result = 0;
+ while (b_index < b_size) {
+ char code = b[b_index];
+ size_t distance = result = b_index++;
+ index = SIZE_MAX;
+ while (++index < a_size) {
+ size_t b_distance = code == a[index] ? distance : distance + 1;
+ distance = buffer[index];
+ if (distance > result) {
+ if (b_distance > result) {
+ result = result + 1;
+ } else {
+ result = b_distance;
+ }
+ } else {
+ if (b_distance > distance) {
+ result = distance + 1;
+ } else {
+ result = b_distance;
+ }
+ }
+ buffer[index] = result;
+ }
+ }
+ PyMem_Free(buffer);
+ return result;
+}
+
+static inline PyObject *
+calculate_suggestions(PyObject *dir,
+ PyObject *name) {
+ assert(!PyErr_Occurred());
+ assert(PyList_CheckExact(dir));
+
+ Py_ssize_t dir_size = PyList_GET_SIZE(dir);
+ if (dir_size >= MAX_CANDIDATE_ITEMS) {
+ return NULL;
+ }
+
+ Py_ssize_t suggestion_distance = PyUnicode_GetLength(name);
+ PyObject *suggestion = NULL;
+ for (int i = 0; i < dir_size; ++i) {
+ PyObject *item = PyList_GET_ITEM(dir, i);
+ const char *name_str = PyUnicode_AsUTF8(name);
+ if (name_str == NULL) {
+ PyErr_Clear();
+ continue;
+ }
+ Py_ssize_t current_distance = levenshtein_distance(PyUnicode_AsUTF8(name), PyUnicode_AsUTF8(item));
+ if (current_distance == 0 || current_distance > MAX_DISTANCE) {
+ continue;
+ }
+ if (!suggestion || current_distance < suggestion_distance) {
+ suggestion = item;
+ suggestion_distance = current_distance;
+ }
+ }
+ if (!suggestion) {
+ return NULL;
+ }
+ Py_INCREF(suggestion);
+ return suggestion;
+}
+
+static PyObject *
+offer_suggestions_for_attribute_error(PyAttributeErrorObject *exc) {
+ PyObject *name = exc->name; // borrowed reference
+ PyObject *obj = exc->obj; // borrowed reference
+
+ // Abort if we don't have an attribute name or we have an invalid one
+ if (name == NULL || obj == NULL || !PyUnicode_CheckExact(name)) {
+ return NULL;
+ }
+
+ PyObject *dir = PyObject_Dir(obj);
+ if (dir == NULL) {
+ return NULL;
+ }
+
+ PyObject *suggestions = calculate_suggestions(dir, name);
+ Py_DECREF(dir);
+ return suggestions;
+}
+
+// Offer suggestions for a given exception. Returns a python string object containing the
+// suggestions. This function does not raise exceptions and returns NULL if no suggestion was found.
+PyObject *_Py_Offer_Suggestions(PyObject *exception) {
+ PyObject *result = NULL;
+ assert(!PyErr_Occurred()); // Check that we are not going to clean any existing exception
+ if (PyErr_GivenExceptionMatches(exception, PyExc_AttributeError)) {
+ result = offer_suggestions_for_attribute_error((PyAttributeErrorObject *) exception);
+ }
+ assert(!PyErr_Occurred());
+ return result;
+}
+