[python] PY-36230 fix duplicate "at" in live decorator template

GitOrigin-RevId: f69aae360eac73da13c313d84be8890c3f758cf5

python/pluginResources/intellij.python.community.impl.xml

@@ -278,6 +278,7 @@
                          implementation="com.jetbrains.python.codeInsight.liveTemplates.PythonTemplateContextType$Class"/>
     <liveTemplateContext contextId="Python_Top_Level" baseContextId="Python"
                          implementation="com.jetbrains.python.codeInsight.liveTemplates.PythonTemplateContextType$TopLevel"/>
+    <liveTemplatePreprocessor implementation="com.jetbrains.python.codeInsight.liveTemplates.PyDecoratorTemplatePreprocessor"/>
     <liveTemplateMacro implementation="com.jetbrains.python.codeInsight.liveTemplates.CollectionElementNameMacro"/>
     <liveTemplateMacro implementation="com.jetbrains.python.codeInsight.liveTemplates.PyClassNameMacro"/>
     <liveTemplateMacro implementation="com.jetbrains.python.codeInsight.liveTemplates.PyFunctionNameMacro"/>

python/src/com/jetbrains/python/codeInsight/liveTemplates/PyDecoratorTemplatePreprocessor.kt

@@ -0,0 +1,31 @@
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.jetbrains.python.codeInsight.liveTemplates
+
+import com.intellij.codeInsight.template.impl.TemplatePreprocessor
+import com.intellij.openapi.application.WriteAction
+import com.intellij.openapi.editor.Document
+import com.intellij.openapi.editor.Editor
+import com.intellij.psi.PsiFile
+import com.intellij.util.text.CharArrayUtil
+
+class PyDecoratorTemplatePreprocessor : TemplatePreprocessor {
+  override fun preprocessTemplate(editor: Editor, file: PsiFile, caretOffset: Int, textToInsert: String, templateText: String) {
+    if (!templateText.startsWith('@')) {
+      return
+    }
+
+    val document: Document = editor.getDocument()
+    if (caretOffset <= 0) {
+      return
+    }
+
+    val text: CharSequence = document.charsSequence
+    val checkPos = CharArrayUtil.shiftBackward(text, caretOffset - 1, " \t")
+
+    if (checkPos >= 0 && text[checkPos] == '@') {
+      WriteAction.run<RuntimeException> {
+        document.deleteString(checkPos, checkPos + 1)
+      }
+    }
+  }
+}

python/testSrc/com/jetbrains/python/codeInsight/liveTemplates/PyLiveTemplatesExpandingTest.java

@@ -85,6 +85,20 @@ public class PyLiveTemplatesExpandingTest extends PyTestCase {
     });
   }
 
+  // PY-36230
+  public void testPropertyDecoratorNoDuplicate() {
+    myFixture.configureByText("a.py", "class MyClass:\n    @prop<caret>");
+    myFixture.type("\t");
+    myFixture.checkResult("class MyClass:\n    @property\n    def <caret>(self):\n        return ");
+  }
+
+  // PY-36230
+  public void testPropertyDecoratorNoAtSymbol() {
+    myFixture.configureByText("a.py", "class MyClass:\n    prop<caret>");
+    myFixture.type("\t");
+    myFixture.checkResult("class MyClass:\n    @property\n    def <caret>(self):\n        return ");
+  }
+
   // PY-41231
   public void testIterableVariableWithTypeAnnotation() {
     doMultiFileTest();