Don't materialize a dict in MappingProxy.TryGetValue

src/core/IronPython/Runtime/Types/MappingProxy.cs

@@ -13,13 +13,30 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 
+using Microsoft.Scripting.Runtime;
+
 using IronPython.Runtime.Operations;
 
 namespace IronPython.Runtime.Types {
     [PythonType("mappingproxy")]
     public sealed class MappingProxy : IDictionary<object?, object?>, IDictionary {
         internal PythonDictionary GetDictionary(CodeContext context) => dictionary ?? type!.GetMemberDictionary(context, false);
 
+        private bool TryGetValue(CodeContext context, object? key, out object? value) {
+            if (dictionary is not null) {
+                return dictionary.TryGetValue(key, out value);
+            }
+
+            string? name = key as string ?? (key as Extensible<string>)?.Value;
+            if (name is not null && type!.TryLookupSlot(context, name, out PythonTypeSlot slot)) {
+                value = slot is PythonTypeUserDescriptorSlot dts ? dts.Value : slot;
+                return true;
+            }
+
+            value = null;
+            return false;
+        }
+
         private readonly PythonDictionary? dictionary;
         private readonly PythonType? type;
 
@@ -36,7 +53,7 @@ public MappingProxy([NotNone] PythonDictionary dict) {
 
         public int __len__(CodeContext context) => GetDictionary(context).Count;
 
-        public bool __contains__(CodeContext/*!*/ context, object? value) => GetDictionary(context).TryGetValue(value, out _);
+        public bool __contains__(CodeContext/*!*/ context, object? value) => TryGetValue(context, value, out _);
 
         public string/*!*/ __str__(CodeContext/*!*/ context) => DictionaryOps.__repr__(context, this);
 
@@ -58,7 +75,7 @@ public string __repr__(CodeContext/*!*/ context) {
         }
 
         public object? get(CodeContext/*!*/ context, object? k, object? d = null) {
-            if (GetDictionary(context).TryGetValue(k, out object? res)) {
+            if (TryGetValue(context, k, out object? res)) {
                 return res;
             }
 
@@ -111,7 +128,7 @@ public object __eq__(CodeContext/*!*/ context, object? other) {
         #region IDictionary Members
 
         public object? this[object? key] {
-            get => GetDictionary(DefaultContext.Default)[key];
+            get => TryGetValue(DefaultContext.Default, key, out object? value) ? value : throw PythonOps.KeyError(key);
             [PythonHidden]
             set => throw PythonOps.TypeError("'mappingproxy' object does not support item assignment");
         }
@@ -187,7 +204,7 @@ ICollection IDictionary.Values {
 
         bool IDictionary<object?, object?>.Remove(object? key) => throw new InvalidOperationException("mappingproxy is read-only");
 
-        bool IDictionary<object?, object?>.TryGetValue(object? key, out object? value) => GetDictionary(DefaultContext.Default).TryGetValue(key, out value);
+        bool IDictionary<object?, object?>.TryGetValue(object? key, out object? value) => TryGetValue(DefaultContext.Default, key, out value);
 
         ICollection<object?> IDictionary<object?, object?>.Values => GetDictionary(DefaultContext.Default).Values;
 

tests/suite/test_dict.py

@@ -1033,4 +1033,21 @@ def test_cp34770(self):
         m[u64] = 'b'
         self.assertEqual(m[9223372036854775808], 'b')
 
+    def test_mappingproxy_str_subclass_key(self):
+        class S(str): pass
+        class C:
+            x = 1
+
+        for d, key, value in ((str.__dict__, '__len__', str.__dict__['__len__']), (C.__dict__, 'x', 1)):
+            self.assertIn(S(key), d)
+            self.assertIs(d.get(S(key)), value)
+            self.assertIs(d[S(key)], value)
+            self.assertNotIn(S('nope'), d)
+            with self.assertRaises(KeyError):
+                d[S('nope')]
+
+        for key in (1, None, b'x'):
+            self.assertNotIn(key, C.__dict__)
+            self.assertIsNone(C.__dict__.get(key))
+
 run_test(__name__)