From f9ec407bf0d6de0249737c4b1c2f7cdc7cd220a2 Mon Sep 17 00:00:00 2001
From: Ryan Chou <88779759+ryanchou1994@users.noreply.github.com>
Date: Mon, 21 Sep 2026 01:46:31 +0800
Subject: [PATCH] Clamp Flake8 diagnostic columns to nonnegative positions

---
 pylsp/plugins/flake8_lint.py     |  3 ++-
 test/plugins/test_flake8_lint.py | 29 +++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/pylsp/plugins/flake8_lint.py b/pylsp/plugins/flake8_lint.py
index b0a71b88..93161c3a 100644
--- a/pylsp/plugins/flake8_lint.py
+++ b/pylsp/plugins/flake8_lint.py
@@ -223,7 +223,8 @@ def parse_stdout(source, stdout):
 
         _, line, character, code, msg = parsed_line
         line = int(line) - 1
-        character = int(character) - 1
+        # Some plugins report column zero when no precise column is available.
+        character = max(0, int(character) - 1)
         # show also the code in message
         msg = code + " " + msg
         severity = lsp.DiagnosticSeverity.Warning
diff --git a/test/plugins/test_flake8_lint.py b/test/plugins/test_flake8_lint.py
index ad1dc4ff..b19c8ba8 100644
--- a/test/plugins/test_flake8_lint.py
+++ b/test/plugins/test_flake8_lint.py
@@ -6,6 +6,8 @@
 from textwrap import dedent
 from unittest.mock import patch
 
+import pytest
+
 from pylsp import lsp, uris
 from pylsp.plugins import flake8_lint
 from pylsp.workspace import Document
@@ -60,6 +62,33 @@ def test_flake8_lint(workspace) -> None:
         os.remove(name)
 
 
+@pytest.mark.parametrize(
+    "line, column, expected_character",
+    [
+        ("    # comment", 0, 0),
+        ("", 0, 0),
+        ("    # comment", 1, 0),
+        ("    # comment", 5, 4),
+    ],
+)
+def test_flake8_diagnostic_columns(workspace, line, column, expected_character) -> None:
+    source = f"def myfun():\n{line}\n    pass\n"
+    doc = Document("", workspace, source)
+    output = (
+        f"stdin:2:{column}: ANN201 Missing return type annotation for public function\n"
+    )
+
+    with patch("pylsp.plugins.flake8_lint.run_flake8", return_value=output):
+        diagnostics = flake8_lint.pylsp_lint(workspace, doc)
+
+    assert len(diagnostics) == 1
+    assert diagnostics[0]["range"] == {
+        "start": {"line": 1, "character": expected_character},
+        "end": {"line": 1, "character": len(line) + 1},
+    }
+    assert diagnostics[0]["code"] == "ANN201"
+
+
 def test_flake8_respecting_configuration(workspace) -> None:
     docs = [
         ("src/__init__.py", ""),