fixed PY-12195 Changing signature of decorator removes '@'

python/src/com/jetbrains/python/refactoring/changeSignature/PyChangeSignatureUsageProcessor.java

@@ -110,8 +110,12 @@ public class PyChangeSignatureUsageProcessor implements ChangeSignatureUsageProc
         final PyElementGenerator elementGenerator = PyElementGenerator.getInstance(element.getProject());
         StringBuilder builder = buildSignature((PyChangeInfo)changeInfo, call);
 
-        final PyExpression newCall =
-          elementGenerator.createExpressionFromText(LanguageLevel.forElement(element), builder.toString());
+        final PyExpression newCall;
+        if (call instanceof PyDecorator) {
+          newCall = elementGenerator.createDecoratorList("@" + builder.toString()).getDecorators()[0];
+        }
+        else
+          newCall = elementGenerator.createExpressionFromText(LanguageLevel.forElement(element), builder.toString());
         call.replace(newCall);
 
         return true;

python/testData/refactoring/changeSignature/decorator.after.py

@@ -0,0 +1,10 @@
+
+def decorator(arg1):
+    print('Wrapped with', arg1, arg2)
+    def wraps(f):
+        return f
+    return wraps
+
+@decorator('arg1')
+def do_things():
+    print('Things done')

python/testData/refactoring/changeSignature/decorator.before.py

@@ -0,0 +1,10 @@
+
+def dec<caret>orator(arg1, arg2):
+    print('Wrapped with', arg1, arg2)
+    def wraps(f):
+        return f
+    return wraps
+
+@decorator('arg1', 'arg2')
+def do_things():
+    print('Things done')

python/testSrc/com/jetbrains/python/refactoring/changeSignature/PyChangeSignatureTest.java

@@ -31,7 +31,7 @@ import java.util.List;
 /**
  * User : ktisha
  */
-@TestDataPath("$CONTENT_ROOT/../testData/refactoring/changeSignature/")
+@TestDataPath("$CONTENT_ROOT/../testData/")
 public class PyChangeSignatureTest extends PyTestCase {
 
   public void testChooseSuperMethod() {
@@ -176,6 +176,10 @@ public class PyChangeSignatureTest extends PyTestCase {
                      PyBundle.message("refactoring.change.signature.dialog.validation.parameter.name"));
   }
 
+  public void testDecorator() {
+    doChangeSignatureTest("decorator", Arrays.asList(new PyParameterInfo(0, "arg1", null, false)));
+  }
+
   public void testNonDefaultAfterDefault() {
     doValidationTest(null, Arrays.asList(new PyParameterInfo(-1, "a", "2", false), new PyParameterInfo(1, "b", "2", false)), null);
   }