test: add HTML formatter tests for shared styles functionality

Author: kosiew

python/tests/test_dataframe.py

@@ -1566,3 +1566,99 @@ def test_dataframe_repr_html_values(df):
         print(f"HTML output snippet: {html[:500]}...")
 
     assert len(matches) > 0, "Expected pattern of values not found in HTML output"
+
+
+def test_html_formatter_shared_styles(df, clean_formatter_state):
+    """Test that shared styles work correctly across multiple tables."""
+    from datafusion.html_formatter import (
+        get_formatter,
+        configure_formatter,
+        reset_styles_loaded_state,
+    )
+
+    # First, ensure we're using shared styles
+    configure_formatter(use_shared_styles=True)
+    formatter = get_formatter()
+
+    # Get HTML output for first table - should include styles
+    html_first = df._repr_html_()
+
+    # Verify styles are included in first render
+    assert "<style>" in html_first
+    assert ".expandable-container" in html_first
+
+    # Get HTML output for second table - should NOT include styles
+    html_second = df._repr_html_()
+
+    # Verify styles are NOT included in second render
+    assert "<style>" not in html_second
+    assert ".expandable-container" not in html_second
+
+    # Reset the styles loaded state and verify styles are included again
+    reset_styles_loaded_state()
+    html_after_reset = df._repr_html_()
+
+    # Verify styles are included after reset
+    assert "<style>" in html_after_reset
+    assert ".expandable-container" in html_after_reset
+
+
+def test_html_formatter_no_shared_styles(df, clean_formatter_state):
+    """Test that styles are always included when shared styles are disabled."""
+    from datafusion.html_formatter import configure_formatter
+
+    # Configure formatter to NOT use shared styles
+    configure_formatter(use_shared_styles=False)
+
+    # Generate HTML multiple times
+    html_first = df._repr_html_()
+    html_second = df._repr_html_()
+
+    # Verify styles are included in both renders
+    assert "<style>" in html_first
+    assert "<style>" in html_second
+    assert ".expandable-container" in html_first
+    assert ".expandable-container" in html_second
+
+
+def test_html_formatter_manual_format_html(clean_formatter_state):
+    """Test direct usage of format_html method with shared styles."""
+    from datafusion.html_formatter import (
+        get_formatter,
+        DataFrameHtmlFormatter,
+        reset_styles_loaded_state,
+    )
+    import pyarrow as pa
+
+    # Create sample data
+    batch = pa.RecordBatch.from_arrays(
+        [pa.array([1, 2, 3]), pa.array([4, 5, 6])],
+        names=["a", "b"],
+    )
+
+    formatter = get_formatter()
+
+    # First call should include styles
+    html_first = formatter.format_html([batch], batch.schema)
+    assert "<style>" in html_first
+
+    # Second call should not include styles (using shared styles by default)
+    html_second = formatter.format_html([batch], batch.schema)
+    assert "<style>" not in html_second
+
+    # Reset loaded state
+    reset_styles_loaded_state()
+
+    # After reset, styles should be included again
+    html_reset = formatter.format_html([batch], batch.schema)
+    assert "<style>" in html_reset
+
+    # Create a new formatter with shared_styles=False
+    local_formatter = DataFrameHtmlFormatter(use_shared_styles=False)
+
+    # Both calls should include styles
+    local_html_1 = local_formatter.format_html([batch], batch.schema)
+    local_html_2 = local_formatter.format_html([batch], batch.schema)
+
+    assert "<style>" in local_html_1
+    assert "<style>" in local_html_2