gh-140502: fix default REPL does not handle tab right

Lib/_pyrepl/utils.py

@@ -330,7 +330,11 @@ def disp_str(
         if colors and colors[0].span.start == i:  # new color starts now
             pre_color = theme[colors[0].tag]
 
-        if c == "\x1a":  # CTRL-Z on Windows
+        if c == "\t":  # gh-140502: handle tabs
+            width = 8 - (sum(char_widths) % 8)
+            chars.append(" " * width)
+            char_widths.append(width)
+        elif c == "\x1a":  # CTRL-Z on Windows
             chars.append(c)
             char_widths.append(2)
         elif ord(c) < 128:

Lib/test/test_pyrepl/test_pyrepl.py

@@ -37,6 +37,7 @@
     ReadlineConfig,
     _ReadlineWrapper,
 )
+from _pyrepl.utils import disp_str
 from _pyrepl.readline import multiline_input as readline_multiline_input
 
 try:
@@ -1911,3 +1912,19 @@ def test_ctrl_d_single_line_end_no_newline(self):
         )
         reader, _ = handle_all_events(events)
         self.assertEqual("hello", "".join(reader.buffer))
+
+
+class TestDispStr(TestCase):
+    def test_disp_str_width_calculation(self):
+        test_cases = [
+            ("\tX", [8, 1]),           # column 0 -> 8 spaces
+            ("X\t", [1, 7]),           # column 1 -> 7 spaces to column 8
+            ("ABC\tX", [1, 1, 1, 5, 1]),  # column 3 -> 5 spaces to column 8
+            ("XXXXXXX\t", [1, 1, 1, 1, 1, 1, 1, 1]),  # column 7 -> 1 space
+            ("XXXXXXXX\t", [1, 1, 1, 1, 1, 1, 1, 1, 8]),  # column 8 -> 8 spaces
+            ("中\tX", [2, 6, 1]),      # wide char + tab
+        ]
+        for buffer, expected_widths in test_cases:
+            with self.subTest(buffer=repr(buffer)):
+                _, widths = disp_str(buffer, 0)
+                self.assertEqual(widths, expected_widths)

Misc/NEWS.d/next/Library/2025-10-28-17-15-30.gh-issue-140502.ilUv1f.rst

@@ -0,0 +1 @@
+Fix: default REPL ``disp_str`` does not handle tab right.