Specialize getattr() (#1146)

* specialize getattr

* fix args length check

* test non-string argument for getattr name

Author: jakebailey

src/Analysis/Ast/Impl/Modules/BuiltinsPythonModule.cs

@@ -147,6 +147,7 @@ private void SpecializeFunctions() {
             Analysis.SpecializeFunction("cmp", Interpreter.GetBuiltinType(BuiltinTypeId.Int));
             Analysis.SpecializeFunction("dir", BuiltinsSpecializations.ListOfStrings);
             Analysis.SpecializeFunction("eval", Interpreter.GetBuiltinType(BuiltinTypeId.Object));
+            Analysis.SpecializeFunction("getattr", BuiltinsSpecializations.GetAttr);
             Analysis.SpecializeFunction("globals", BuiltinsSpecializations.DictStringToObject);
             Analysis.SpecializeFunction(@"isinstance", _boolType);
             Analysis.SpecializeFunction(@"issubclass", _boolType);

src/Analysis/Ast/Impl/Specializations/BuiltinsSpecializations.cs

@@ -116,5 +116,30 @@ public static IMember Open(IPythonModule declaringModule, IPythonFunctionOverloa
             var returnType = io?.GetMember(returnTypeName)?.GetPythonType();
             return returnType != null ? new PythonInstance(returnType) : null;
         }
+
+        public static IMember GetAttr(IPythonModule module, IPythonFunctionOverload overload, IArgumentSet argSet) {
+            // TODO: Try __getattr__ first; this may not be as reliable in practice
+            // given we could be assuming that __getattr__ always returns the same type,
+            // which is incorrect more often than not.
+
+            var args = argSet.Values<IMember>();
+            if (args.Count < 2) {
+                return null;
+            }
+
+            var o = args[0];
+            var name = (args[1] as IPythonConstant)?.GetString();
+
+            IMember def = null;
+            if (args.Count >= 3) {
+                def = args[2];
+            }
+
+            if (name == null) {
+                return def;
+            }
+
+            return o?.GetPythonType().GetMember(name) ?? def;
+        }
     }
 }

src/Analysis/Ast/Test/ClassesTests.cs

@@ -572,5 +572,28 @@ class C(B):
             analysis.Should().HaveClass("B").Which.Should().HaveDocumentation("class B doc");
             analysis.Should().HaveClass("C").Which.Should().HaveDocumentation("class C doc");
         }
+
+        [TestMethod, Priority(0)]
+        public async Task GetAttr() {
+            const string code = @"
+class A:
+    def __init__(self):
+        self.x = 123
+
+a = A()
+b = getattr(a, 'x')
+c = getattr(a, 'y', 3.141)
+d = getattr(a)
+e = getattr()
+f = getattr(a, 3.141)
+";
+            var analysis = await GetAnalysisAsync(code);
+            analysis.Should().HaveVariable("a").OfType("A")
+                .And.HaveVariable("b").OfType(BuiltinTypeId.Int)
+                .And.HaveVariable("c").OfType(BuiltinTypeId.Float)
+                .And.HaveVariable("d").OfType(BuiltinTypeId.Unknown)
+                .And.HaveVariable("e").OfType(BuiltinTypeId.Unknown)
+                .And.HaveVariable("f").OfType(BuiltinTypeId.Unknown);
+        }
     }
 }