don't insert parentheses when completing a function call if the function has any custom decorators (PY-4859)

python/src/com/jetbrains/python/psi/resolve/CompletionVariantsProcessor.java

@@ -47,6 +47,7 @@ public class CompletionVariantsProcessor extends VariantsProcessor {
     if (!myPlainNamesOnly) {
       if (!mySuppressParentheses &&
           object instanceof PyFunction && ((PyFunction)object).getProperty() == null &&
+          hasNoCustomDecorators((PyFunction)object) &&
           !isSingleArgDecoratorCall(myContext, (PyFunction)object)) {
         item = item.withInsertHandler(PyFunctionInsertHandler.INSTANCE);
         final PyParameterList parameterList = ((PyFunction)object).getParameterList();
@@ -102,6 +103,21 @@ public class CompletionVariantsProcessor extends VariantsProcessor {
     return item;
   }
 
+  private static boolean hasNoCustomDecorators(PyFunction function) {
+    PyDecoratorList decoratorList = function.getDecoratorList();
+    if (decoratorList == null) {
+      return true;
+    }
+    for (PyDecorator decorator : decoratorList.getDecorators()) {
+      PyQualifiedName name = decorator.getQualifiedName();
+      if (name == null || (!PyNames.CLASSMETHOD.equals(name.toString()) && !PyNames.STATICMETHOD.equals(name.toString()))) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
   private static boolean isSingleArgDecoratorCall(PsiElement elementInCall, PyFunction callee) {
     // special case hack to avoid the need of patching generator3.py
     PyClass containingClass = callee.getContainingClass();