don't add another backslash if we already have one (PY-1960)

python/src/com/jetbrains/python/editor/PythonEnterHandler.java

@@ -47,6 +47,13 @@ public class PythonEnterHandler implements EnterHandlerDelegate {
     if (element == null) {
       return Result.Continue;
     }
+    if (offset > 0) {
+      final PsiElement beforeCaret = file.findElementAt(offset-1);
+      if (beforeCaret instanceof PsiWhiteSpace && beforeCaret.getText().indexOf('\\') > 0) {
+        // we've got a backslash at EOL already, don't need another one
+        return Result.Continue;
+      }
+    }
     PsiElement statementBefore = findStatementBeforeCaret(file, offset);
     PsiElement statementAfter = findStatementAfterCaret(file, offset);
     if (statementBefore != statementAfter) {  // Enter pressed at statement break

python/testSrc/com/jetbrains/python/PyEditingTest.java

@@ -120,14 +120,18 @@ public class PyEditingTest extends PyLightFixtureTestCase {
     doTestEnter("z=1 <caret>2", "z=1 \n2");
   }
 
-  public void testEnterAtStartOfComment() {
+  public void testEnterAtStartOfComment() {  // PY-1958
     doTestEnter("# bar\n<caret># foo", "# bar\n\n# foo");
   }
 
-  public void testEnterAtEndOfComment() {
+  public void testEnterAtEndOfComment() {  // PY-1958
     doTestEnter("# bar<caret>\n# foo", "# bar\n\n# foo");
   }
 
+  public void testEnterAfterBackslash() {  // PY-1960
+    doTestEnter("s = \\<caret>\n'some string'", "s = \\\n\n'some string'");
+  }
+
   private void doTestEnter(String before, final String after) {
     int pos = before.indexOf("<caret>");
     before = before.replace("<caret>", "");