From 10263d6de3ef2b6b7848e448169014c2ede32706 Mon Sep 17 00:00:00 2001
From: Gertjan van Zwieten <gertjanvanzwieten@evalf.nl>
Date: Mon, 23 Mar 2026 10:23:35 +0100
Subject: [PATCH 1/2] Avoid using equals comparison in annotation types

---
 ags/_mapping.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/ags/_mapping.py b/ags/_mapping.py
index 7cd9201..c094b47 100644
--- a/ags/_mapping.py
+++ b/ags/_mapping.py
@@ -150,12 +150,12 @@ def mapping_for(T) -> Mapping:
             with context(f".{param.name}"):
                 if param.kind not in (param.POSITIONAL_OR_KEYWORD, param.KEYWORD_ONLY):
                     raise TypeError("positional-only arguments are not supported")
-                if param.annotation != param.empty:
+                if param.annotation is not param.empty:
                     mapping = mapping_for(param.annotation)
-                    if param.default != param.empty:
+                    if param.default is not param.empty:
                         with context("(default)"):
                             mapping.lower(param.default, inject_none)
-                elif param.default != param.empty:
+                elif param.default is not param.empty:
                     mapping = mapping_for(type(param.default))
                 else:
                     raise TypeError(f"cannot establish type for parameter {param.name}")

From 3e2f1e83b4269d93166e3defb9e84a0a9ae6ed0c Mon Sep 17 00:00:00 2001
From: Gertjan van Zwieten <gertjanvanzwieten@evalf.nl>
Date: Wed, 25 Mar 2026 15:53:13 +0100
Subject: [PATCH 2/2] Replace ErrorContext by notes

---
 ags/_mapping.py | 32 +++++++++++---------------------
 test.py         | 34 +++++++++++++++++++++++++---------
 2 files changed, 36 insertions(+), 30 deletions(-)

diff --git a/ags/_mapping.py b/ags/_mapping.py
index c094b47..c4e226d 100644
--- a/ags/_mapping.py
+++ b/ags/_mapping.py
@@ -28,27 +28,17 @@ def __enter__(self):
         pass
 
     def __exit__(self, exc_type, exc_value, traceback):
-        if exc_value and len(exc_value.args) == 1:
-            (arg,) = exc_value.args
-            if not isinstance(arg, ErrorContext):
-                arg = ErrorContext(arg)
-                exc_value.args = (arg,)
-            arg.prepend(self.context)
-
-
-class ErrorContext:
-    def __init__(self, message):
-        self.message = message
-        self.context = ""
-
-    def prepend(self, context):
-        self.context = context + self.context
-
-    def __str__(self):
-        return f"in {self.context}: {self.message}"
-
-    def __repr__(self):
-        return f"in {self.context}: {self.message!r}"
+        if not exc_value:
+            return
+        if not hasattr(exc_value, '__notes__'):
+            notes = []
+            exc_value.__notes__ = notes
+        else:
+            notes = exc_value.__notes__
+        note = "In: " + self.context
+        if notes and notes[-1].startswith("In: "):
+            note += notes.pop()[4:]
+        notes.append(note)
 
 
 def mismatch(expect, got):
diff --git a/test.py b/test.py
index d14d392..74d2f88 100644
--- a/test.py
+++ b/test.py
@@ -7,6 +7,7 @@
 from datetime import date, time, datetime
 from doctest import DocFileSuite
 import sys
+import traceback
 
 from ags import _mapping
 
@@ -86,10 +87,15 @@ class A:
             i: int = 10
             s: str = 20
 
-        with self.assertRaisesRegex(
-            ValueError, r"in .s\(default\): expects str, got int"
-        ):
+        with self.assertRaises(ValueError) as cm:
             _mapping.mapping_for(A)
+        s = traceback.format_exception(cm.exception)
+        self.assertEqual(s, [
+            'ValueError: expects str, got int\n',
+            'In: .s(default)\n',
+        ] if sys.version_info >= (3, 11) else [
+            'ValueError: expects str, got int\n',
+        ])
 
     def test_boundargs(self):
         def f(i: int, s: str):
@@ -104,10 +110,15 @@ def f(i: int = 10, s: str = 20):
             pass
 
         sig = signature(f)
-        with self.assertRaisesRegex(
-            ValueError, r"in .s\(default\): expects str, got int"
-        ):
+        with self.assertRaises(ValueError) as cm:
             _mapping.mapping_for(sig)
+        s = traceback.format_exception(cm.exception)
+        self.assertEqual(s, [
+            'ValueError: expects str, got int\n',
+            'In: .s(default)\n',
+        ] if sys.version_info >= (3, 11) else [
+            'ValueError: expects str, got int\n',
+        ])
 
     def test_union(self):
         for modern in False, True:
@@ -191,10 +202,15 @@ def __eq__(self, other):
     def test_exception(self):
         T = dict[str, list[int]]
         m = _mapping.mapping_for(T)
-        with self.assertRaisesRegex(
-            AssertionError, r"in \[b\]\[1\]: <class 'str'> is not <class 'int'>"
-        ):
+        with self.assertRaises(AssertionError) as cm:
             m.unlower({"a": [10, 20], "b": [30, "40", 50]}, self.mysurject)
+        s = traceback.format_exception(cm.exception)
+        self.assertEqual(s, [
+            "AssertionError: <class 'str'> is not <class 'int'>\n",
+            'In: [b][1]\n',
+        ] if sys.version_info >= (3, 11) else [
+            "AssertionError: <class 'str'> is not <class 'int'>\n",
+        ])
 
 
 class Demo: