bpo-43950: support some multi-line expressions for PEP 657 (GH-27339)

This is basically something that I noticed up while fixing test runs for another issue. It is really common to have multiline calls, and when they fail the display is kind of weird since we omit the annotations. E.g;

```
 $ ./python t.py
Traceback (most recent call last):
  File "/home/isidentical/cpython/cpython/t.py", line 11, in <module>
    frame_1()
    ^^^^^^^^^
  File "/home/isidentical/cpython/cpython/t.py", line 5, in frame_1
    frame_2(              
  File "/home/isidentical/cpython/cpython/t.py", line 2, in frame_2
    return a / 0 / b / c
           ~~^~~
ZeroDivisionError: division by zero
```

This patch basically adds support for annotating the rest of the line, if the instruction covers multiple lines (start_line != end_line).

Automerge-Triggered-By: GH:isidentical

Author: isidentical

Lib/test/test_doctest.py

@@ -2835,6 +2835,7 @@ def test_unicode(): """
         Traceback (most recent call last):
           File ...
             exec(compile(example.source, filename, "single",
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           File "<doctest foo-bär@baz[0]>", line 1, in <module>
             raise Exception('clé')
             ^^^^^^^^^^^^^^^^^^^^^^

Lib/test/test_traceback.py

@@ -429,6 +429,30 @@ def f_with_multiline():
             '    ^^^^^^^^^^\n'
             f'  File "{__file__}", line {lineno_f+1}, in f_with_multiline\n'
             '    raise ValueError(\n'
+            '    ^^^^^^^^^^^^^^^^^'
+        )
+        result_lines = self.get_exception(f_with_multiline)
+        self.assertEqual(result_lines, expected_f.splitlines())
+
+    def test_caret_multiline_expression_bin_op(self):
+        # Make sure no carets are printed for expressions spanning multiple
+        # lines.
+        def f_with_multiline():
+            return (
+                1 /
+                0 +
+                2
+            )
+
+        lineno_f = f_with_multiline.__code__.co_firstlineno
+        expected_f = (
+            'Traceback (most recent call last):\n'
+            f'  File "{__file__}", line {self.callable_line}, in get_exception\n'
+            '    callable()\n'
+            '    ^^^^^^^^^^\n'
+            f'  File "{__file__}", line {lineno_f+2}, in f_with_multiline\n'
+            '    1 /\n'
+            '    ^^^'
         )
         result_lines = self.get_exception(f_with_multiline)
         self.assertEqual(result_lines, expected_f.splitlines())

Lib/traceback.py

@@ -4,6 +4,7 @@
 import itertools
 import linecache
 import sys
+from contextlib import suppress
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
            'format_exception_only', 'format_list', 'format_stack',
@@ -463,19 +464,20 @@ def format_frame(self, frame):
 
             stripped_characters = len(frame._original_line) - len(frame.line.lstrip())
             if (
-                frame.end_lineno == frame.lineno
-                and frame.colno is not None
+                frame.colno is not None
                 and frame.end_colno is not None
             ):
                 colno = _byte_offset_to_character_offset(frame._original_line, frame.colno)
                 end_colno = _byte_offset_to_character_offset(frame._original_line, frame.end_colno)
 
-                try:
-                    anchors = _extract_caret_anchors_from_line_segment(
-                        frame._original_line[colno - 1:end_colno - 1]
-                    )
-                except Exception:
-                    anchors = None
+                anchors = None
+                if frame.lineno == frame.end_lineno:
+                    with suppress(Exception):
+                        anchors = _extract_caret_anchors_from_line_segment(
+                            frame._original_line[colno - 1:end_colno - 1]
+                        )
+                else:
+                    end_colno = stripped_characters + len(frame.line.strip())
 
                 row.append('    ')
                 row.append(' ' * (colno - stripped_characters))

Python/traceback.c

@@ -720,11 +720,11 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
                               &end_line, &end_col_byte_offset)) {
         goto done;
     }
-    if (start_line != end_line) {
-        goto done;
-    }
 
-    if (start_col_byte_offset < 0 || end_col_byte_offset < 0) {
+    if (start_line < 0 || end_line < 0
+        || start_col_byte_offset < 0
+        || end_col_byte_offset < 0)
+    {
         goto done;
     }
 
@@ -762,11 +762,30 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
     char *primary_error_char = "^";
     char *secondary_error_char = primary_error_char;
 
-    int res = extract_anchors_from_line(filename, source_line, start_offset, end_offset,
-                                        &left_end_offset, &right_start_offset,
-                                        &primary_error_char, &secondary_error_char);
-    if (res < 0 && ignore_source_errors() < 0) {
-        goto done;
+    if (start_line == end_line) {
+        int res = extract_anchors_from_line(filename, source_line, start_offset, end_offset,
+                                            &left_end_offset, &right_start_offset,
+                                            &primary_error_char, &secondary_error_char);
+        if (res < 0 && ignore_source_errors() < 0) {
+            goto done;
+        }
+    }
+    else {
+        // If this is a multi-line expression, then we will highlight until
+        // the last non-whitespace character.
+        const char *source_line_str = PyUnicode_AsUTF8(source_line);
+        if (!source_line_str) {
+            goto done;
+        }
+
+        Py_ssize_t i = PyUnicode_GET_LENGTH(source_line);
+        while (--i >= 0) {
+            if (!IS_WHITESPACE(source_line_str[i])) {
+                break;
+            }
+        }
+
+        end_offset = i + 1;
     }
 
     err = print_error_location_carets(f, truncation, start_offset, end_offset,