From c6f04046e458c22bfc7d958a2e7616bcf3ed7cb4 Mon Sep 17 00:00:00 2001
From: Toluwalase Akinyemi <toluwalase.akinyemi1@gmail.com>
Date: Tue, 13 Oct 2026 12:49:40 +0100
Subject: [PATCH] Pretty-print builtin frozendict in assertion diffs

On Python 3.15+, register frozendict in PrettyPrinter._dispatch, reusing the OrderedDict printer since both repr as Name({...}). Previously it fell back to a single-line repr, making diffs hard to read.

Closes #15172
---
 AUTHORS                    |  1 +
 changelog/15172.bugfix.rst |  1 +
 src/_pytest/_io/pprint.py  |  5 +++++
 testing/io/test_pprint.py  | 27 +++++++++++++++++++++++++++
 4 files changed, 34 insertions(+)
 create mode 100644 changelog/15172.bugfix.rst

diff --git a/AUTHORS b/AUTHORS
index bd1e27cb37d..4627a7d2c18 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -497,6 +497,7 @@ Tim Strazny
 TJ Bruno
 Tobias Diez
 Tobias Petersen
+Toluwalase Akinyemi
 Tom Dalton
 Tom Most
 Tom Viner
diff --git a/changelog/15172.bugfix.rst b/changelog/15172.bugfix.rst
new file mode 100644
index 00000000000..2f97f57ffdb
--- /dev/null
+++ b/changelog/15172.bugfix.rst
@@ -0,0 +1 @@
+Assertion diffs now pretty-print the built-in :class:`frozendict` (Python 3.15+) across multiple lines, the same way :class:`dict` is displayed.
diff --git a/src/_pytest/_io/pprint.py b/src/_pytest/_io/pprint.py
index a550b65b0c8..14fd6906ed7 100644
--- a/src/_pytest/_io/pprint.py
+++ b/src/_pytest/_io/pprint.py
@@ -29,6 +29,7 @@
 from collections.abc import Iterator
 import dataclasses as _dataclasses
 import re
+import sys
 import types as _types
 from typing import Any
 
@@ -229,6 +230,10 @@ def _pprint_ordered_dict(
 
     _dispatch[_collections.OrderedDict.__repr__] = _pprint_ordered_dict
 
+    if sys.version_info >= (3, 15):
+        # frozendict's repr has the same shape as OrderedDict's: Name({...})
+        _dispatch[frozendict.__repr__] = _pprint_ordered_dict  # noqa: F821
+
     def _pprint_list(
         self,
         object: Any,
diff --git a/testing/io/test_pprint.py b/testing/io/test_pprint.py
index f7dbf748664..ca89368fbae 100644
--- a/testing/io/test_pprint.py
+++ b/testing/io/test_pprint.py
@@ -9,6 +9,7 @@
 from collections import UserList
 from collections import UserString
 from dataclasses import dataclass
+import sys
 import textwrap
 from types import MappingProxyType
 from types import SimpleNamespace
@@ -651,6 +652,32 @@ class MyNamespace(SimpleNamespace):
     assert pp.pformat(MyNamespace(one=1)) == "MyNamespace(\n    one=1,\n)"
 
 
+@pytest.mark.skipif(
+    sys.version_info < (3, 15), reason="frozendict is a builtin since Python 3.15"
+)
+def test_frozendict() -> None:
+    if sys.version_info >= (3, 15):  # for mypy, which checks against Python 3.10
+        pp = PrettyPrinter()
+        assert pp.pformat(frozendict()) == "frozendict()"  # noqa: F821
+        assert pp.pformat(frozendict({"one": 1, "two": 2})) == textwrap.dedent(  # noqa: F821
+            """\
+            frozendict({
+                'one': 1,
+                'two': 2,
+            })"""
+        )
+
+        class MyFrozenDict(frozendict):  # noqa: F821
+            pass
+
+        assert pp.pformat(MyFrozenDict({"one": 1})) == textwrap.dedent(
+            """\
+            MyFrozenDict({
+                'one': 1,
+            })"""
+        )
+
+
 def test_safe_tuple_sorts_unorderable_pairs() -> None:
     # ``_safe_tuple`` wraps each element of a 2-tuple in ``_safe_key`` so a
     # list of pairs with unorderable elements can be sorted without raising.