From 90c089d4a41ff864dd508089dc14f00cb82aaf07 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Sat, 1 Nov 2025 13:01:58 +0100
Subject: [PATCH] FIX: Gracefully handle numpy arrays as input to
 check_in_list()

Closes #30706
---
 lib/matplotlib/_api/__init__.py  | 16 +++++++++++++++-
 lib/matplotlib/tests/test_api.py |  5 +++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/lib/matplotlib/_api/__init__.py b/lib/matplotlib/_api/__init__.py
index 39496cfb0e82..216cc885ea24 100644
--- a/lib/matplotlib/_api/__init__.py
+++ b/lib/matplotlib/_api/__init__.py
@@ -116,6 +116,9 @@ def check_in_list(values, /, *, _print_supported_values=True, **kwargs):
     ----------
     values : iterable
         Sequence of values to check on.
+
+        Note: All values must support == comparisons.
+        This means in particular the entries must not be numpy arrays.
     _print_supported_values : bool, default: True
         Whether to print *values* when raising ValueError.
     **kwargs : dict
@@ -133,7 +136,18 @@ def check_in_list(values, /, *, _print_supported_values=True, **kwargs):
     if not kwargs:
         raise TypeError("No argument to check!")
     for key, val in kwargs.items():
-        if val not in values:
+        try:
+            exists = val in values
+        except ValueError:
+            # `in` internally uses `val == values[i]`. There are some objects
+            # that do not support == to arbitrary other objects, in particular
+            # numpy arrays.
+            # Since such objects are not allowed in values, we can gracefully
+            # handle the case that val (typically provided by users) is of such
+            # type and directly state it's not in the list instead of letting
+            # the individual `val == values[i]` ValueError surface.
+            exists = False
+        if not exists:
             msg = f"{val!r} is not a valid value for {key}"
             if _print_supported_values:
                 msg += f"; supported values are {', '.join(map(repr, values))}"
diff --git a/lib/matplotlib/tests/test_api.py b/lib/matplotlib/tests/test_api.py
index 58e7986bfce6..4d0241264ddb 100644
--- a/lib/matplotlib/tests/test_api.py
+++ b/lib/matplotlib/tests/test_api.py
@@ -150,3 +150,8 @@ def f() -> None:
 def test_empty_check_in_list() -> None:
     with pytest.raises(TypeError, match="No argument to check!"):
         _api.check_in_list(["a"])
+
+
+def test_check_in_list_numpy() -> None:
+    with pytest.raises(ValueError, match=r"array\(5\) is not a valid value"):
+        _api.check_in_list(['a', 'b'], value=np.array(5))