Mikhail Arkhipov (MikhailArkhipov) · May 1, 2019 · Mar 1, 2019 · Mar 1, 2019 · Mar 5, 2019 · Mar 5, 2019
diff --git a/src/Analysis/Ast/Impl/Specializations/BuiltinsSpecializations.cs b/src/Analysis/Ast/Impl/Specializations/BuiltinsSpecializations.cs
@@ -92,15 +92,29 @@ public static IMember CollectionItem(IPythonModule module, IPythonFunctionOverlo
         public static IMember Open(IPythonModule declaringModule, IPythonFunctionOverload overload, IArgumentSet argSet) {
             var mode = argSet.GetArgumentValue<IPythonConstant>("mode");
 
-            var bytes = false;
-            if (mode != null) {
-                var modeString = mode.GetString();
-                bytes = modeString != null && modeString.Contains("b");
+            var binary = false;
+            var writable = false;
+            var readWrite = false;
+
+            var modeString = mode?.GetString();
+            if (modeString != null) {
+                binary = modeString.Contains("b");
+                writable = modeString.Contains("w") || modeString.Contains("a") || modeString.Contains("x");
+                readWrite = writable && modeString.Contains("r");
             }
 
+            string returnTypeName;
             var io = declaringModule.Interpreter.ModuleResolution.GetImportedModule("io");
-            var ioBase = io?.GetMember(bytes ? "BufferedIOBase" : "TextIOWrapper")?.GetPythonType();
-            return ioBase != null ? new PythonInstance(ioBase) : null;
+            if (binary) {
+                returnTypeName = writable ?
+                    readWrite ? "BufferedRandom" : "BufferedWriter"
+                    : "BufferedReader";
+            } else {
+                returnTypeName = "TextIOWrapper";
+            }
+
+            var returnType = io?.GetMember(returnTypeName)?.GetPythonType();
+            return returnType != null ? new PythonInstance(returnType) : null;
         }
     }
 }
diff --git a/src/Analysis/Ast/Impl/Types/PythonFunctionOverload.cs b/src/Analysis/Ast/Impl/Types/PythonFunctionOverload.cs
@@ -201,7 +201,10 @@ public IMember Call(IArgumentSet args, IPythonType self, Node callLocation = nul
                         var name = StaticReturnValue.GetPythonType()?.Name;
                         var typeDefVar = DeclaringModule.Analysis.GlobalScope.Variables[name];
                         if (typeDefVar?.Value is IGenericTypeDefinition gtp2) {
-                            return gtp2.Constraints.FirstOrDefault();
+                            // See if the instance (self) type satisfies one of the constraints.
+                            return selfClassType.Mro.Any(b => gtp2.Constraints.Any(c => c.Equals(b)))
+                                ? selfClassType
+                                : gtp2.Constraints.FirstOrDefault();
                         }
 
                         break;

diff --git a/src/Analysis/Ast/Test/ValuesTests.cs b/src/Analysis/Ast/Test/ValuesTests.cs
@@ -159,6 +159,38 @@ with X():
                     .Which.Should().HaveMember("x_method");
         }
 
+        [TestMethod, Priority(0)]
+        public async Task WithIOStatement() {
+            const string code = @"
+with open('a', 'r') as r:
+    pass
+with open('a', 'rb') as rb:
+    pass
+with open('a', 'w') as w:
+    pass
+with open('a', 'wb') as wb:
+    pass
+with open('a', 'rw') as rw:
+    pass
+with open('a', 'rwb') as rwb:
+    pass
+with open('a', 'x') as x:
+    pass
+with open('a', 'xb') as xb:
+    pass
+";
+            var analysis = await GetAnalysisAsync(code);
+
+            analysis.Should().HaveVariable("r").OfType("TextIOWrapper")
+                .And.HaveVariable("rb").OfType("BufferedReader")
+                .And.HaveVariable("w").OfType("TextIOWrapper")
+                .And.HaveVariable("wb").OfType("BufferedWriter")
+                .And.HaveVariable("rw").OfType("TextIOWrapper")
+                .And.HaveVariable("rwb").OfType("BufferedRandom")
+                .And.HaveVariable("x").OfType("TextIOWrapper")
+                .And.HaveVariable("xb").OfType("BufferedWriter");
+        }
+
         [TestMethod, Priority(0)]
         public async Task Global() {
             const string code = @"