From 1fbbbb6bcc14f4ec986116544de5546c46ed5c35 Mon Sep 17 00:00:00 2001
From: kwy404 <thekaway404@gmail.com>
Date: Sat, 26 Sep 2026 02:46:30 -0300
Subject: [PATCH] Fix GameNode.set_eval() dropping mate 0 scores

set_eval() only wrote a mate annotation when score.white().mate() was
truthy, so Mate(0) and MateGiven were silently discarded, and an existing
[%eval ...] annotation was removed instead of being replaced by
[%eval #0]. GameNode.eval() already parses [%eval #0], so writing back the
parsed score lost it.
---
 chess/pgn.py | 2 +-
 test.py      | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/chess/pgn.py b/chess/pgn.py
index 3d4d7e5ff..627f3fd84 100644
--- a/chess/pgn.py
+++ b/chess/pgn.py
@@ -497,7 +497,7 @@ def set_eval(self, score: Optional[chess.engine.PovScore], depth: Optional[int]
             cp = score.white().score()
             if cp is not None:
                 eval = f"[%eval {float(cp) / 100:.2f}{depth_suffix}]"
-            elif score.white().mate():
+            elif score.white().mate() is not None:
                 eval = f"[%eval #{score.white().mate()}{depth_suffix}]"
 
         self._replace_or_add_annotation(eval, EVAL_REGEX)
diff --git a/test.py b/test.py
index a481c2adb..0ec62b8ae 100755
--- a/test.py
+++ b/test.py
@@ -2957,6 +2957,13 @@ def test_eval(self):
             game.set_eval(chess.engine.PovScore(chess.engine.Cp(cp), chess.WHITE))
             self.assertEqual(game.eval().white().cp, cp)
 
+    def test_eval_mate_zero(self):
+        game = chess.pgn.read_game(io.StringIO("1. f3 e5 2. g4 Qh4# { [%eval #0] } 0-1"))
+        node = game.end()
+        node.set_eval(node.eval())
+        self.assertEqual(node.comments, ["[%eval #0]"])
+        self.assertEqual(node.eval().relative, chess.engine.Mate(0))
+
     def test_float_emt(self):
         game = chess.pgn.Game()
         game.comments = ["[%emt 0:00:01.234]"]