@@ -258,7 +258,9 @@ def _should_auto_indent(buffer: list[str], pos: int) -> bool:
             if buffer[pos] == "\n":
                 break
             if buffer[pos] == "#":
-                last_char = None
+                last_char = "#"
+            elif last_char == "#" and buffer[pos] == ":":
+                last_char = ":"
     return last_char == ":"
 
 

@@ -186,6 +186,75 @@ def test_newline_within_block_trailing_whitespace(self):
         self.assert_screen_equals(reader, expected)
         self.assertTrue(reader.finished)
 
+    def test_single_comment_character_after_colon(self):
+        code = (
+            "def foo(): #"
+        )
+        events = itertools.chain(
+            code_to_events(code),
+            [
+                # go to the end of the first line
+                Event(evt="key", data="up", raw=bytearray(b"\x1bOA")),
+                Event(evt="key", data="\x05", raw=bytearray(b"\x1bO5")),
+                # new line after single comment character
+                Event(evt="key", data="\n", raw=bytearray(b"\n")),
+                # go to end of second line
+                Event(evt="key", data="down", raw=bytearray(b"\x1bOB")),
+                Event(evt="key", data="\x05", raw=bytearray(b"\x1bO5")),
+                # write an "a"
+                Event(evt="key", data="a", raw=bytearray(b"a"))
+            ],
+        )
+        no_paste_reader = functools.partial(prepare_reader, paste_mode=False)
+        reader, _ = handle_all_events(events, prepare_reader=no_paste_reader)
+        expected = (
+            "def foo(): #\n"
+            "    a"
+        )
+        self.assert_screen_equals(reader, expected)
+
+    def test_no_code_only_comment_on_single_line(self):
+        code = (
+            "def foo():"
+        )
+        events = itertools.chain(
+            code_to_events(code),
+            [
+                # go to the end of the first line
+                Event(evt="key", data="up", raw=bytearray(b"\x1bOA")),
+                Event(evt="key", data="\x05", raw=bytearray(b"\x1bO5")),
+                # new line at end of first line and write "# foo" on next line
+                Event(evt="key", data="\n", raw=bytearray(b"\n")),
+                Event(evt="key", data="#", raw=bytearray(b"#")),
+                Event(evt="key", data=" ", raw=bytearray(b" ")),
+                Event(evt="key", data="f", raw=bytearray(b"f")),
+                Event(evt="key", data="o", raw=bytearray(b"o")),
+                Event(evt="key", data="o", raw=bytearray(b"o")),
+                Event(evt="key", data="\n", raw=bytearray(b"\n")),
+                # go to end of second line
+                Event(evt="key", data="down", raw=bytearray(b"\x1bOB")),
+                Event(evt="key", data="\x05", raw=bytearray(b"\x1bO5")),
+                # write an "a"
+                Event(evt="key", data="a", raw=bytearray(b"a")),
+                Event(evt="key", data=" ", raw=bytearray(b" ")),
+                Event(evt="key", data="#", raw=bytearray(b"#")),
+                Event(evt="key", data=" ", raw=bytearray(b" ")),
+                Event(evt="key", data="f", raw=bytearray(b"f")),
+                Event(evt="key", data="o", raw=bytearray(b"o")),
+                Event(evt="key", data="o", raw=bytearray(b"o")),
+                Event(evt="key", data="\n", raw=bytearray(b"\n")),
+            ],
+        )
+        no_paste_reader = functools.partial(prepare_reader, paste_mode=False)
+        reader, _ = handle_all_events(events, prepare_reader=no_paste_reader)
+        expected = (
+            "def foo():\n"
+            "    # foo\n"
+            "    a # foo\n"
+            "    "
+        )
+        self.assert_screen_equals(reader, expected)
+
     def test_input_hook_is_called_if_set(self):
         input_hook = MagicMock()
         def _prepare_console(events):

diff --git a/Misc/NEWS.d/next/Library/2024-07-29-06-42-01.gh-issue-122237.7w1yfJ.rst b/Misc/NEWS.d/next/Library/2024-07-29-06-42-01.gh-issue-122237.7w1yfJ.rst
@@ -0,0 +1,27 @@
+#
+# Please enter the relevant GitHub issue number here:
+#
+.. gh-issue: 122237
+
+#
+# Uncomment one of these "section:" lines to specify which section
+# this entry should go in in Misc/NEWS.
+#
+#.. section: Security
+#.. section: Core and Builtins
+section: Library
+#.. section: Documentation
+#.. section: Tests
+#.. section: Build
+#.. section: Windows
+#.. section: macOS
+#.. section: IDLE
+#.. section: Tools/Demos
+#.. section: C API
+
+# Write your Misc/NEWS entry below.  It should be a simple ReST paragraph.
+# Don't start with "- Issue #<n>: " or "- gh-issue<n>: " or that sort of stuff.
+###########################################################################
+Fixed auto-indentation in the REPL when a single line comment character followed a colon on the
+same line or when a line containing no code and only a comment followed (directly or indirectly) a
+line containing a colon. Patch by Anson Trapani.