don't add parentheses when completing single-arg decorator (PY-2210)

python/src/com/jetbrains/python/psi/impl/PyCallExpressionHelper.java

@@ -176,6 +176,10 @@ public class PyCallExpressionHelper {
   ) {
     //return getImplicitArgumentCount(functionBeingCalled, null, null, qualifierIsAnInstance(callReference, TypeEvalContext.fast()));
     if (typeContext == null) typeContext = TypeEvalContext.fast();
+    final PyDecorator decorator = PsiTreeUtil.getParentOfType(callReference, PyDecorator.class);
+    if (decorator != null && PsiTreeUtil.isAncestor(decorator.getCallee(), callReference, false)) {
+      return 1;
+    }
     QualifiedResolveResult followed = callReference.followAssignmentsChain(typeContext);
     return getImplicitArgumentCount(functionBeingCalled, null, null, isQualifiedByInstance(functionBeingCalled, followed.getLastQualifier(), typeContext));
   }

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

@@ -8,6 +8,7 @@ import com.intellij.psi.PsiElement;
 import com.intellij.psi.PsiNamedElement;
 import com.intellij.psi.ResolveState;
 import com.intellij.psi.scope.PsiScopeProcessor;
+import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.util.Icons;
 import com.jetbrains.python.codeInsight.PyClassInsertHandler;
 import com.jetbrains.python.codeInsight.PyFunctionInsertHandler;
@@ -56,7 +57,8 @@ public class VariantsProcessor implements PsiScopeProcessor {
 
   protected LookupElementBuilder setupItem(LookupElementBuilder item) {
     if (!myPlainNamesOnly) {
-      if (item.getObject() instanceof PyFunction && ((PyFunction) item.getObject()).getProperty() == null) {
+      if (item.getObject() instanceof PyFunction && ((PyFunction) item.getObject()).getProperty() == null &&
+          !isSingleArgDecoratorCall(myContext, (PyFunction)item.getObject())) {
         item = item.setInsertHandler(PyFunctionInsertHandler.INSTANCE);
       }
       else if (item.getObject() instanceof PyClass) {
@@ -69,6 +71,17 @@ public class VariantsProcessor implements PsiScopeProcessor {
     return item;
   }
 
+  private static boolean isSingleArgDecoratorCall(PsiElement elementInCall, PyFunction callee) {
+    if (callee.getParameterList().getParameters().length > 1) {
+      return false;
+    }
+    PyDecorator decorator = PsiTreeUtil.getParentOfType(elementInCall, PyDecorator.class);
+    if (decorator == null) {
+      return false;
+    }
+    return PsiTreeUtil.isAncestor(decorator.getCallee(), elementInCall, false);
+  }
+
   protected static LookupElementBuilder setItemNotice(final LookupElementBuilder item, String notice) {
     return item.setTypeText(notice);
   }

python/testData/completion/noParensForDecorator.after.py

@@ -0,0 +1,4 @@
+def my_decorator(f):
+    return f
+
+@my_decorator<caret>

python/testData/completion/noParensForDecorator.py

@@ -0,0 +1,4 @@
+def my_decorator(f):
+    return f
+
+@my_deco<caret>

python/testSrc/com/jetbrains/python/PythonCompletionTest.java

@@ -5,7 +5,6 @@
 package com.jetbrains.python;
 
 import com.intellij.codeInsight.lookup.LookupElement;
-import com.intellij.openapi.application.ApplicationManager;
 import com.jetbrains.python.fixtures.PyLightFixtureTestCase;
 
 import java.util.Arrays;
@@ -224,4 +223,8 @@ public class PythonCompletionTest extends PyLightFixtureTestCase {
     myFixture.completeBasic();
     myFixture.checkResultByFile(dirname + "importedModule.after.py");
   }
+
+  public void testNoParensForDecorator() {  // PY-2210
+    doTest();
+  }
 }