PY-20776 Report empty expression fragments inside f-strings

python/src/META-INF/python-core-common.xml

@@ -644,6 +644,7 @@
     <dumbAnnotator implementation="com.jetbrains.python.validation.GeneratorInArgumentListAnnotator"/>
     <dumbAnnotator implementation="com.jetbrains.python.validation.StarAnnotator"/>
     <dumbAnnotator implementation="com.jetbrains.python.validation.StringLiteralQuotesAnnotator"/>
+    <dumbAnnotator implementation="com.jetbrains.python.validation.FStringsAnnotator"/>
     <dumbAnnotator implementation="com.jetbrains.python.validation.DumbAwareHighlightingAnnotator"/>
 
     <customTargetExpressionStubType implementation="com.jetbrains.python.psi.impl.stubs.PropertyStubType"/>

python/src/com/jetbrains/python/validation/FStringsAnnotator.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2000-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.jetbrains.python.validation;
+
+import com.intellij.lang.ASTNode;
+import com.intellij.openapi.util.TextRange;
+import com.jetbrains.python.codeInsight.fstrings.FStringParser;
+import com.jetbrains.python.codeInsight.fstrings.FStringParser.FragmentOffsets;
+import com.jetbrains.python.psi.PyStringLiteralExpression;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.List;
+
+import static com.jetbrains.python.psi.PyUtil.StringNodeInfo;
+
+/**
+ * @author Mikhail Golubev
+ */
+public class FStringsAnnotator extends PyAnnotator {
+  @Override
+  public void visitPyStringLiteralExpression(PyStringLiteralExpression pyString) {
+    for (ASTNode node : pyString.getStringNodes()) {
+      if (new StringNodeInfo(node).isFormatted()) {
+        final int nodeOffset = node.getTextRange().getStartOffset();
+        final List<FragmentOffsets> fragments = FStringParser.parse(node.getText());
+        for (FragmentOffsets fragment : fragments) {
+          if (fragment.getLeftBraceOffset() + 1 >= fragment.getContentEndOffset()) {
+            report(fragment.getContentRange().shiftRight(nodeOffset), "Empty expressions are not allowed inside f-strings");
+          }
+        }
+      }
+    }
+  }
+
+  private void report(@NotNull TextRange range, @NotNull String message) {
+    getHolder().createErrorAnnotation(range, message);
+  }
+}

python/testData/highlighting/fStringEmptyExpressions.py

@@ -0,0 +1,6 @@
+f'{<error descr="Empty expressions are not allowed inside f-strings"></error>}'
+f'{<error descr="Empty expressions are not allowed inside f-strings"></error>'
+<error descr="Missing closing quote [']">f'{<error descr="Empty expressions are not allowed inside f-strings"></error></error>
+f'{<error descr="Empty expressions are not allowed inside f-strings"></error>!r}'
+f'{<error descr="Empty expressions are not allowed inside f-strings"></error>:2.3}'
+f'{42:2.{<error descr="Empty expressions are not allowed inside f-strings"></error>}}'

python/testSrc/com/jetbrains/python/PythonHighlightingTest.java

@@ -305,6 +305,11 @@ public class PythonHighlightingTest extends PyTestCase {
     doTest();
   }
 
+  // PY-20776
+  public void testFStringEmptyExpressions() {
+    runWithLanguageLevel(LanguageLevel.PYTHON36, () -> doTest(true, false));
+  }
+
   // ---
   private void doTest(final LanguageLevel languageLevel, final boolean checkWarnings, final boolean checkInfos) {
     PythonLanguageLevelPusher.setForcedLanguageLevel(myFixture.getProject(), languageLevel);