Fix Excel-specific border styles (#48660)

* Add tests

* Add support for "hair" style

* Support excel-specific border styles

* Use black border if styled but color unspecified

* Added documentation and whatsnew

* Black linter

* Fixed tests for Excel border color fallback

* Revert style.ipynb metadata

* Fix typo

* Revert border-color default value

* Revert "Fixed tests for Excel border color fallback"

This reverts commit d2680ef.

* Revert border color default tests

* Updated whatsnew entry

* Add method link to whatsnew

* Add tests and fix case sensitivity

* Apply black linter

* Update v1.5.2.rst

* Fix documentation typo

* Test that border shorthand works

* Append Excel styles to shorthand parsing

* Update to find_stack_level call

* Update v1.5.3.rst

Author: tehunter

doc/source/user_guide/style.ipynb

@@ -1594,8 +1594,9 @@
     "\n",
     "\n",
     "- Only CSS2 named colors and hex colors of the form `#rgb` or `#rrggbb` are currently supported.\n",
-    "- The following pseudo CSS properties are also available to set excel specific style properties:\n",
+    "- The following pseudo CSS properties are also available to set Excel specific style properties:\n",
     "    - `number-format`\n",
+    "    - `border-style` (for Excel-specific styles: \"hair\", \"mediumDashDot\", \"dashDotDot\", \"mediumDashDotDot\", \"dashDot\", \"slantDashDot\", or \"mediumDashed\")\n",
     "\n",
     "Table level styles, and data cell CSS-classes are not included in the export to Excel: individual cells must have their properties mapped by the `Styler.apply` and/or `Styler.applymap` methods."
    ]

doc/source/whatsnew/v1.5.3.rst

@@ -24,7 +24,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Bug in :meth:`.Styler.to_excel` leading to error when unrecognized ``border-style`` (e.g. ``"hair"``) provided to Excel writers (:issue:`48649`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/css.py

@@ -105,9 +105,9 @@ def expand(self, prop, value: str) -> Generator[tuple[str, str], None, None]:
             f"border{side}-width": "medium",
         }
         for token in tokens:
-            if token in self.BORDER_STYLES:
+            if token.lower() in self.BORDER_STYLES:
                 border_declarations[f"border{side}-style"] = token
-            elif any(ratio in token for ratio in self.BORDER_WIDTH_RATIOS):
+            elif any(ratio in token.lower() for ratio in self.BORDER_WIDTH_RATIOS):
                 border_declarations[f"border{side}-width"] = token
             else:
                 border_declarations[f"border{side}-color"] = token
@@ -181,6 +181,13 @@ class CSSResolver:
         "ridge",
         "inset",
         "outset",
+        "mediumdashdot",
+        "dashdotdot",
+        "hair",
+        "mediumdashdotdot",
+        "dashdot",
+        "slantdashdot",
+        "mediumdashed",
     ]
 
     SIDE_SHORTHANDS = {

pandas/io/formats/excel.py

@@ -159,6 +159,22 @@ class CSSToExcelConverter:
         "fantasy": 5,  # decorative
     }
 
+    BORDER_STYLE_MAP = {
+        style.lower(): style
+        for style in [
+            "dashed",
+            "mediumDashDot",
+            "dashDotDot",
+            "hair",
+            "dotted",
+            "mediumDashDotDot",
+            "double",
+            "dashDot",
+            "slantDashDot",
+            "mediumDashed",
+        ]
+    }
+
     # NB: Most of the methods here could be classmethods, as only __init__
     #     and __call__ make use of instance attributes.  We leave them as
     #     instancemethods so that users can easily experiment with extensions
@@ -306,6 +322,16 @@ def _border_style(self, style: str | None, width: str | None, color: str | None)
             if width_name in ("hair", "thin"):
                 return "dashed"
             return "mediumDashed"
+        elif style in self.BORDER_STYLE_MAP:
+            # Excel-specific styles
+            return self.BORDER_STYLE_MAP[style]
+        else:
+            warnings.warn(
+                f"Unhandled border style format: {repr(style)}",
+                CSSWarning,
+                stacklevel=find_stack_level(),
+            )
+            return "none"
 
     def _get_width_name(self, width_input: str | None) -> str | None:
         width = self._width_to_float(width_input)

pandas/tests/io/excel/test_style.py

@@ -115,6 +115,12 @@ def test_styler_to_excel_unstyled(engine):
         ["border", "left", "color", "rgb"],
         {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
     ),
+    # Border styles
+    (
+        "border-left-style: hair; border-left-color: black",
+        ["border", "left", "style"],
+        "hair",
+    ),
 ]
 
 
@@ -196,6 +202,62 @@ def test_styler_to_excel_basic_indexes(engine, css, attrs, expected):
             assert sc_cell == expected
 
 
+# From https://openpyxl.readthedocs.io/en/stable/api/openpyxl.styles.borders.html
+# Note: Leaving behavior of "width"-type styles undefined; user should use border-width
+# instead
+excel_border_styles = [
+    # "thin",
+    "dashed",
+    "mediumDashDot",
+    "dashDotDot",
+    "hair",
+    "dotted",
+    "mediumDashDotDot",
+    # "medium",
+    "double",
+    "dashDot",
+    "slantDashDot",
+    # "thick",
+    "mediumDashed",
+]
+
+
+@pytest.mark.parametrize(
+    "engine",
+    ["xlsxwriter", "openpyxl"],
+)
+@pytest.mark.parametrize("border_style", excel_border_styles)
+def test_styler_to_excel_border_style(engine, border_style):
+    css = f"border-left: {border_style} black thin"
+    attrs = ["border", "left", "style"]
+    expected = border_style
+
+    pytest.importorskip(engine)
+    df = DataFrame(np.random.randn(1, 1))
+    styler = df.style.applymap(lambda x: css)
+
+    with tm.ensure_clean(".xlsx") as path:
+        with ExcelWriter(path, engine=engine) as writer:
+            df.to_excel(writer, sheet_name="dataframe")
+            styler.to_excel(writer, sheet_name="styled")
+
+        openpyxl = pytest.importorskip("openpyxl")  # test loading only with openpyxl
+        with contextlib.closing(openpyxl.load_workbook(path)) as wb:
+
+            # test unstyled data cell does not have expected styles
+            # test styled cell has expected styles
+            u_cell, s_cell = wb["dataframe"].cell(2, 2), wb["styled"].cell(2, 2)
+        for attr in attrs:
+            u_cell, s_cell = getattr(u_cell, attr, None), getattr(s_cell, attr)
+
+        if isinstance(expected, dict):
+            assert u_cell is None or u_cell != expected[engine]
+            assert s_cell == expected[engine]
+        else:
+            assert u_cell is None or u_cell != expected
+            assert s_cell == expected
+
+
 def test_styler_custom_converter():
     openpyxl = pytest.importorskip("openpyxl")