used proper type check

fix a PY-6756 PyCharm erroneously reports "too many arguments" for certain string formatting lines.

python/src/com/jetbrains/python/inspections/PyStringFormatInspection.java

@@ -183,7 +183,11 @@ public class PyStringFormatInspection extends PyInspection {
         }
         else if (rightExpression instanceof PySliceExpression && s != null) {
           final PyType type = myTypeEvalContext.getType(((PySliceExpression)rightExpression).getOperand());
-          if (type == null || "list".equals(type.getName()) || "str".equals(type.getName())) {
+          final PyType stringType = PyBuiltinCache.getInstance(rightExpression).getStringType(LanguageLevel.forElement(rightExpression));
+          final PyType listType = PyBuiltinCache.getInstance(rightExpression).getListType();
+
+          if (type == null || PyTypeChecker.match(listType, type, myTypeEvalContext)
+              || PyTypeChecker.match(stringType, type, myTypeEvalContext)) {
             checkTypeCompatible(problemTarget, builtinCache.getStrType(),
                                 PyTypeParser.getTypeByName(problemTarget, s));
             return 1;

python/testData/inspections/PyStringFormatInspectionSlice/test.py

@@ -0,0 +1,9 @@
+def foo(x):
+	return x
+
+artist = foo(1)
+print('%s' % (artist.lower()[0:10]))
+
+unicode_content = foo(u"test")
+assert isinstance(unicode_content, unicode)
+print '%s...' % unicode_content[:200]

python/testSrc/com/jetbrains/python/PythonInspectionsTest.java

@@ -266,6 +266,10 @@ public class PythonInspectionsTest extends PyTestCase {
     doHighlightingTest(PyStringFormatInspection.class);
   }
 
+  public void testPyStringFormatInspectionSlice() {    //PY-6756
+    doHighlightingTest(PyStringFormatInspection.class);
+  }
+
   public void testPyUnnecessaryBackslashInspection() {    //PY-2952
     setLanguageLevel(LanguageLevel.PYTHON27);
     doHighlightingTest(PyUnnecessaryBackslashInspection.class);