Add a sys.meta_path importer for .NET namespaces and types

src/core/IronPython/Hosting/PythonCommandLine.cs

@@ -176,7 +176,7 @@ protected override void Initialize() {
             // the first entry in sys.path. If running a script file, however, the CWD should not be added;
             // instead, the script's containg folder should be added.
 
-            string fullPath = "."; // this is a valid path resolving to current working dir. Pinky-swear.
+            string fullPath = ""; // the current working directory, importlib resolves it with os.getcwd()
 
             if (Options.Command == null && Options.FileName != null) {
                 if (Options.FileName == "-") {

src/core/IronPython/Runtime/Importer.cs

@@ -564,8 +564,9 @@ private static object ImportTopAbsolute(CodeContext/*!*/ context, string/*!*/ na
             }
 
             if (TryLoadMetaPathModule(context, name, null, out ret)) {
-                // importlib's BuiltinImporter loads clr before ImportBuiltin gets a chance to
-                if (ret == context.LanguageContext.ClrModule) {
+                // importlib's BuiltinImporter loads clr before ImportBuiltin gets a chance to,
+                // and ClrImporter loads .NET namespaces and types
+                if (ret is MemberTracker || ret is PythonType { IsSystemType: true } || ret == context.LanguageContext.ClrModule) {
                     context.ShowCls = true;
                 }
                 return ret;
@@ -842,7 +843,8 @@ private static object LoadFromDisk(CodeContext context, string name, string full
             // default behavior
             string pathname;
             try {
-                pathname = context.LanguageContext.DomainManager.Platform.CombinePaths(str, name);
+                // an empty path entry is the current working directory
+                pathname = context.LanguageContext.DomainManager.Platform.CombinePaths(str.Length == 0 ? "." : str, name);
             } catch (ArgumentException) {
                 return null; // ignore invalid paths
             }
@@ -934,5 +936,74 @@ internal static PythonModule LoadPackageFromSource(CodeContext/*!*/ context, str
             Assert.NotNull(sourceCode, name, path);
             return context.LanguageContext.CompileModule(path, name, sourceCode, ModuleOptions.Initialize | ModuleOptions.Optimized);
         }
+
+#nullable enable
+
+        /// <summary>
+        /// sys.meta_path finder for .NET namespaces and types. It is added after importlib's PathFinder
+        /// so Python modules take precedence over .NET namespaces and types.
+        /// </summary>
+        [PythonType]
+        public sealed class ClrImporter {
+            private readonly object _moduleSpec;
+
+            internal ClrImporter(object moduleSpec) {
+                _moduleSpec = moduleSpec;
+            }
+
+            public object? find_spec(CodeContext/*!*/ context, [NotNone] string fullname, object? path = null, object? target = null) {
+                object? member = FindMember(context, fullname);
+                if (member is null) return null;
+
+                // we return "this" as the loader for the module, which uses the legacy load_module
+                // rather than exec_module since types and namespaces don't support setting
+                // the module attributes (__name__, etc.)
+                return PythonCalls.CallWithKeywordArgs(context, _moduleSpec, [fullname, this, member is NamespaceTracker], ["is_package"]);
+            }
+
+            public object? find_module(CodeContext/*!*/ context, [NotNone] string fullname, object? path = null) {
+                return FindMember(context, fullname) is null ? null : this;
+            }
+
+            public object load_module(CodeContext/*!*/ context, [NotNone] string fullname) {
+                var modules = context.LanguageContext.SystemStateModules;
+                if (modules.TryGetValue(fullname, out object? existing) && existing is not null) {
+                    return existing;
+                }
+
+                object member = FindMember(context, fullname) ?? throw PythonOps.ImportError("No module named {0}", fullname);
+                modules[fullname] = member;
+                return member;
+            }
+
+            /// <summary>
+            /// Looks up a top-level namespace or type, or a child of a namespace that has been imported.
+            /// Returns the same object as attribute access on the namespace.
+            /// </summary>
+            private static object? FindMember(CodeContext/*!*/ context, string fullname) {
+                PythonContext pc = context.LanguageContext;
+                NamespaceTracker? parent;
+                string name;
+
+                int lastDot = fullname.LastIndexOf('.');
+                if (lastDot == -1) {
+                    // names in the runtime's globals (e.g. COM type libraries) take precedence over namespaces
+                    // and are left to the legacy importer
+                    if (PythonOps.ScopeTryGetMember(context, pc.DomainManager.Globals, fullname, out _)) return null;
+                    parent = pc.TopNamespace;
+                    name = fullname;
+                } else {
+                    parent = pc.SystemStateModules.TryGetValue(fullname.Substring(0, lastDot), out object? parentModule) ? parentModule as NamespaceTracker : null;
+                    name = fullname.Substring(lastDot + 1);
+                }
+
+                if (parent is null) return null;
+
+                object member = NamespaceTrackerOps.GetCustomMember(context, parent, name);
+                return member == OperationFailed.Value ? null : member;
+            }
+        }
+
+#nullable restore
     }
 }

src/core/IronPython/Runtime/PythonContext.cs

@@ -306,6 +306,8 @@ void BootstrapImportLib() {
 #if PYTHON_37_OR_GREATER
                     PythonOps.Invoke(SharedClsContext, _frozen_importlib, "_install_external_importers");
 #endif
+                    var moduleSpec = PythonOps.GetBoundAttr(SharedClsContext, _frozen_importlib, "ModuleSpec");
+                    (GetSystemStateValue("meta_path") as PythonList)?.append(new Importer.ClrImporter(moduleSpec));
                 } catch { }
 
                 PythonModule LoadModuleFromResource(string name, string resourceName) {

tests/suite/test_imp.py

@@ -1064,13 +1064,83 @@ def test_import_clr_show_cls(self):
         clr = sys.modules.pop('clr')
         try:
             g = {}
-            exec("r1 = hasattr('abc', 'Split')\nimport clr\nr2 = hasattr('abc', 'Split')", g)
+            exec(compile("r1 = hasattr('abc', 'Split')\nimport clr\nr2 = hasattr('abc', 'Split')", '<string>', 'exec', dont_inherit=True), g)
             self.assertFalse(g['r1'])
             self.assertTrue(g['r2'])
             self.assertIs(g['clr'], clr)
         finally:
             sys.modules['clr'] = clr
 
+    @skipUnlessIronPython()
+    def test_clr_importer(self):
+        import importlib
+        import System
+        import System.Collections.Generic
+
+        names = [type(finder).__name__ if not isinstance(finder, type) else finder.__name__ for finder in sys.meta_path]
+        self.assertLess(names.index('PathFinder'), names.index('ClrImporter'))
+
+        # namespaces and types are imported as the same objects as attribute access gives
+        self.assertIs(importlib.import_module('System.Collections.Generic'), System.Collections.Generic)
+        self.assertIs(importlib.import_module('System.Math'), System.Math)
+        self.assertIs(importlib.import_module('.Generic', 'System.Collections'), System.Collections.Generic)
+        self.assertRaises(ImportError, importlib.import_module, 'System.Nonexistent')
+
+        # the first import of a namespace goes through ClrImporter
+        index = names.index('ClrImporter')
+        finder = sys.meta_path[index]
+        found = []
+        class Spy:
+            def find_module(self, fullname, path=None):
+                found.append(fullname)
+                return finder.find_module(fullname, path)
+
+        system = sys.modules.pop('System')
+        sys.meta_path[index] = Spy()
+        try:
+            g = {}
+            exec(compile("r1 = hasattr('abc', 'Split')\nimport System\nr2 = hasattr('abc', 'Split')", '<string>', 'exec', dont_inherit=True), g)
+            self.assertEqual(found, ['System'])
+            self.assertFalse(g['r1'])
+            self.assertTrue(g['r2'])
+            self.assertIs(g['System'], system)
+        finally:
+            sys.meta_path[index] = finder
+            sys.modules['System'] = system
+
+    @skipUnlessIronPython()
+    def test_clr_importer_global_type(self):
+        # the first import of a type in the global namespace enables CLS members
+        import clr
+        from System.Reflection import AssemblyName, TypeAttributes
+        from System.Reflection.Emit import AssemblyBuilder, AssemblyBuilderAccess
+        ab = AssemblyBuilder.DefineDynamicAssembly(AssemblyName('ClrImporterGlobalType'), AssemblyBuilderAccess.Run)
+        ab.DefineDynamicModule('ClrImporterGlobalType').DefineType('ClrImporterGlobalType', TypeAttributes.Public).CreateType()
+        clr.AddReference(ab)
+
+        g = {}
+        exec(compile("r1 = hasattr('abc', 'Split')\nimport ClrImporterGlobalType\nr2 = hasattr('abc', 'Split')", '<string>', 'exec', dont_inherit=True), g)
+        self.assertFalse(g['r1'])
+        self.assertTrue(g['r2'])
+        self.assertEqual(g['ClrImporterGlobalType'].__name__, 'ClrImporterGlobalType')
+
+    @skipUnlessIronPython()
+    def test_clr_importer_globals(self):
+        # the runtime's globals take precedence over namespaces
+        import clr
+        import System
+        runtime_globals = clr.GetCurrentRuntime().Globals
+        value = object()
+        system = sys.modules.pop('System')
+        runtime_globals.System = value
+        try:
+            g = {}
+            exec("import System", g)
+            self.assertIs(g['System'], value)
+        finally:
+            del runtime_globals.System
+            sys.modules['System'] = system
+
     def test_get_frozen_object(self):
         # frozen objects not supported, this always fails
         self.assertRaises(ImportError, imp.get_frozen_object, 'foo')