py: harden __import__ argument handling

py/import.go

@@ -108,7 +108,9 @@ func ImportModuleLevelObject(ctx Context, name string, globals, locals StringDic
 	}
 
 	if fromFile, ok := globals["__file__"]; ok {
-		opts.CurDir = filepath.Dir(string(fromFile.(String)))
+		if fromFileStr, ok := fromFile.(String); ok {
+			opts.CurDir = filepath.Dir(string(fromFileStr))
+		}
 	}
 
 	module, err := RunFile(ctx, srcPathname, opts, name)
@@ -344,14 +346,42 @@ func BuiltinImport(ctx Context, self Object, args Tuple, kwargs StringDict, curr
 	var globals Object = currentGlobal
 	var locals Object = NewStringDict()
 	var fromlist Object = Tuple{}
+	var fromlistTuple Tuple
 	var level Object = Int(0)
 
 	err := ParseTupleAndKeywords(args, kwargs, "U|OOOi:__import__", kwlist, &name, &globals, &locals, &fromlist, &level)
 	if err != nil {
 		return nil, err
 	}
-	if fromlist == None {
-		fromlist = Tuple{}
+	levelObj, ok := level.(Int)
+	if !ok {
+		return nil, ExceptionNewf(TypeError, "__import__() argument 5 must be int, not %s", level.Type().Name)
+	}
+	levelInt, err := levelObj.GoInt()
+	if err != nil {
+	    return nil, err
+	}
+
+	globalsDict, ok := globals.(StringDict)
+	if !ok {
+		if levelInt > 0 {
+			return nil, ExceptionNewf(TypeError, "globals must be a dict")
+		}
+		globalsDict = StringDict{}
 	}
-	return ImportModuleLevelObject(ctx, string(name.(String)), globals.(StringDict), locals.(StringDict), fromlist.(Tuple), int(level.(Int)))
+
+	localsDict, ok := locals.(StringDict)
+	if !ok {
+		localsDict = StringDict{}
+	}
+
+	fromlistTuple = Tuple{}
+	if fromlist != None {
+		fromlistTuple, err = SequenceTuple(fromlist)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return ImportModuleLevelObject(ctx, string(name.(String)), globalsDict, localsDict, fromlistTuple, levelInt)
 }

stdlib/builtin/tests/builtin.py

@@ -501,6 +501,26 @@ class C: pass
 assert lib.libfn() == 42
 assert lib.libvar == 43
 assert lib.libclass().method() == 44
+lib = __import__("lib", {}, {}, [""])
+assert lib.libfn() == 42
+ok = False
+try:
+    __import__("lib", {}, {}, 1)
+except TypeError:
+    ok = True
+assert ok, "TypeError not raised"
+lib = __import__("lib", 1, {}, [""])
+assert lib.libfn() == 42
+ok = False
+try:
+    __import__("lib", 1, {}, [""], 1)
+except TypeError as e:
+    if e.args[0] != "globals must be a dict":
+        raise
+    ok = True
+assert ok, "TypeError not raised"
+lib = __import__("lib", {"__file__": 1}, {}, [""])
+assert lib.libfn() == 42
 
 doc="input"
 import sys