Add keepTrailingNewline option to LegacyOverrides to match Python Jinja2 default behaviour

src/main/java/com/hubspot/jinjava/LegacyOverrides.java

@@ -21,6 +21,7 @@ public interface LegacyOverrides extends WithLegacyOverrides {
     .withAllowAdjacentTextNodes(true)
     .withUseTrimmingForNotesAndExpressions(true)
     .withKeepNullableLoopValues(true)
+    .withKeepTrailingNewline(true)
     .build();
   LegacyOverrides ALL = new Builder()
     .withEvaluateMapKeys(true)
@@ -32,6 +33,7 @@ public interface LegacyOverrides extends WithLegacyOverrides {
     .withAllowAdjacentTextNodes(true)
     .withUseTrimmingForNotesAndExpressions(true)
     .withKeepNullableLoopValues(true)
+    .withKeepTrailingNewline(false)
     .build();
 
   @Value.Default
@@ -79,6 +81,17 @@ default boolean isKeepNullableLoopValues() {
     return false;
   }
 
+  /**
+   * When {@code false} (default, legacy behaviour), the trailing newline of
+   * the rendered output is preserved — matching Jinjava's historical behaviour.
+   * When {@code true}, a single trailing newline is stripped from the rendered
+   * output, matching Python Jinja2's default ({@code keep_trailing_newline=False}).
+   */
+  @Value.Default
+  default boolean isKeepTrailingNewline() {
+    return true;
+  }
+
   class Builder extends ImmutableLegacyOverrides.Builder {}
 
   static Builder newBuilder() {

src/main/java/com/hubspot/jinjava/interpret/JinjavaInterpreter.java

@@ -510,7 +510,7 @@ private String render(Node root, boolean processExtendRoots, long renderLimit) {
       }
 
       if (ignoredOutput.length() > 0) {
-        return (
+        return stripTrailingNewlineIfNeeded(
           EagerReconstructionUtils.labelWithNotes(
             EagerReconstructionUtils.wrapInTag(
               ignoredOutput.toString(),
@@ -524,14 +524,26 @@ private String render(Node root, boolean processExtendRoots, long renderLimit) {
           output.getValue()
         );
       }
-      return output.getValue();
+      return stripTrailingNewlineIfNeeded(output.getValue());
     } finally {
       if (pushed) {
         JinjavaInterpreter.popCurrent();
       }
     }
   }
 
+  /**
+   * Strips a single trailing newline from the rendered output when
+   * {@code keepTrailingNewline} is {@code false} in {@link LegacyOverrides},
+   * matching Python Jinja2's default {@code keep_trailing_newline=False} behaviour.
+   */
+  private String stripTrailingNewlineIfNeeded(String output) {
+    if (!config.getLegacyOverrides().isKeepTrailingNewline() && output.endsWith("\n")) {
+      return output.substring(0, output.length() - 1);
+    }
+    return output;
+  }
+
   private void resolveBlockStubs(OutputList output) {
     resolveBlockStubs(output, new Stack<>());
   }

src/test/java/com/hubspot/jinjava/TrailingNewlineTest.java

@@ -0,0 +1,123 @@
+package com.hubspot.jinjava;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.google.common.collect.ImmutableMap;
+import java.util.HashMap;
+import org.junit.Test;
+
+public class TrailingNewlineTest {
+
+  private static final String TEMPLATE_WITH_TRAILING_NEWLINE = "hello\n";
+  private static final String TEMPLATE_WITHOUT_TRAILING_NEWLINE = "hello";
+  private static final String TEMPLATE_MULTIPLE_TRAILING_NEWLINES = "hello\n\n";
+
+  // ── keepTrailingNewline=true (legacy default: preserve \n) ─────────────────
+
+  @Test
+  public void itKeepsTrailingNewlineWhenLegacyOverrideIsTrue() {
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(
+          LegacyOverrides.newBuilder().withKeepTrailingNewline(true).build()
+        )
+        .build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITH_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello\n");
+  }
+
+  @Test
+  public void itKeepsTrailingNewlineWithNoneLegacyOverrides() {
+    // LegacyOverrides.NONE defaults keepTrailingNewline=true (legacy behaviour)
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig.newBuilder().withLegacyOverrides(LegacyOverrides.NONE).build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITH_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello\n");
+  }
+
+  // ── keepTrailingNewline=false (Python-compatible: strip trailing \n) ────────
+
+  @Test
+  public void itStripsTrailingNewlineWhenLegacyOverrideIsFalse() {
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(
+          LegacyOverrides.newBuilder().withKeepTrailingNewline(false).build()
+        )
+        .build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITH_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello");
+  }
+
+  @Test
+  public void itKeepsTrailingNewlineWithThreePointOLegacyOverrides() {
+    // LegacyOverrides.THREE_POINT_0 does not opt into Python-compatible stripping
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(LegacyOverrides.THREE_POINT_0)
+        .build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITH_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello\n");
+  }
+
+  @Test
+  public void itStripsTrailingNewlineWithAllLegacyOverrides() {
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig.newBuilder().withLegacyOverrides(LegacyOverrides.ALL).build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITH_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello");
+  }
+
+  // ── Edge cases ──────────────────────────────────────────────────────────────
+
+  @Test
+  public void itDoesNotAffectOutputWithNoTrailingNewline() {
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(
+          LegacyOverrides.newBuilder().withKeepTrailingNewline(false).build()
+        )
+        .build()
+    );
+    assertThat(jinjava.render(TEMPLATE_WITHOUT_TRAILING_NEWLINE, new HashMap<>()))
+      .isEqualTo("hello");
+  }
+
+  @Test
+  public void itStripsOnlyOneTrailingNewlineNotMultiple() {
+    // Python only strips a single trailing newline, not all of them.
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(
+          LegacyOverrides.newBuilder().withKeepTrailingNewline(false).build()
+        )
+        .build()
+    );
+    assertThat(jinjava.render(TEMPLATE_MULTIPLE_TRAILING_NEWLINES, new HashMap<>()))
+      .isEqualTo("hello\n");
+  }
+
+  @Test
+  public void itStripsTrailingNewlineFromRenderedExpressions() {
+    Jinjava jinjava = new Jinjava(
+      JinjavaConfig
+        .newBuilder()
+        .withLegacyOverrides(
+          LegacyOverrides.newBuilder().withKeepTrailingNewline(false).build()
+        )
+        .build()
+    );
+    assertThat(jinjava.render("{{ greeting }}\n", ImmutableMap.of("greeting", "hello")))
+      .isEqualTo("hello");
+  }
+}