bpo-30597: Show expected input in custom 'print' error message. (#2009)

1 files changed, 45 insertions, 3 deletions

diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index 858eff5..190ad06 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -2862,6 +2862,49 @@ _PyErr_TrySetFromCause(const char *format, ...)
  * or minus, using the stream redirection syntax).
  */
 
+
+// Static helper for setting legacy print error message
+static int
+_set_legacy_print_statement_msg(PySyntaxErrorObject *self, Py_ssize_t start)
+{
+    PyObject *strip_sep_obj = PyUnicode_FromString(" \t\r\n");
+    if (strip_sep_obj == NULL)
+        return -1;
+
+    // PRINT_OFFSET is to remove `print ` word from the data.
+    const int PRINT_OFFSET = 6;
+    Py_ssize_t text_len = PyUnicode_GET_LENGTH(self->text);
+    PyObject *data = PyUnicode_Substring(self->text, PRINT_OFFSET, text_len);
+
+    if (data == NULL) {
+        Py_DECREF(strip_sep_obj);
+        return -1;
+    }
+    PyObject *new_data = _PyUnicode_XStrip(data, 2, strip_sep_obj);
+    Py_DECREF(data);
+    Py_DECREF(strip_sep_obj);
+
+    if (new_data == NULL) {
+        return -1;
+    }
+    // gets the modified text_len after stripping `print `
+    text_len = PyUnicode_GET_LENGTH(new_data);
+    const char *maybe_end_arg = "";
+    if (text_len > 0 && PyUnicode_READ_CHAR(new_data, text_len-1) == ',') {
+        maybe_end_arg = " end=\" \"";
+    }
+    PyObject *error_msg = PyUnicode_FromFormat(
+        "Missing parentheses in call to 'print'. Did you mean print(%U%s)?",
+        new_data, maybe_end_arg
+    );
+    Py_DECREF(new_data);
+    if (error_msg == NULL)
+        return -1;
+
+    Py_XSETREF(self->msg, error_msg);
+    return 1;
+}
+
 static int
 _check_for_legacy_statements(PySyntaxErrorObject *self, Py_ssize_t start)
 {
@@ -2897,9 +2940,8 @@ _check_for_legacy_statements(PySyntaxErrorObject *self, Py_ssize_t start)
     }
     if (PyUnicode_Tailmatch(self->text, print_prefix,
                             start, text_len, -1)) {
-        Py_XSETREF(self->msg,
-                  PyUnicode_FromString("Missing parentheses in call to 'print'"));
-        return 1;
+
+        return _set_legacy_print_statement_msg(self, start);
     }
 
     /* Check for legacy exec statements */