Fix Excel-specific border styles

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\", \"slandDashDot\", 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."
    ]
@@ -2026,7 +2027,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3 (ipykernel)",
+   "display_name": "Python 3.10.4 ('pandas-testing')",
    "language": "python",
    "name": "python3"
   },
@@ -2040,7 +2041,12 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.5"
+   "version": "3.10.4"
+  },
+  "vscode": {
+   "interpreter": {
+    "hash": "99ba7404bdbf380b6668594445346cc49dc5e126b0bc53947f71689e253b4235"
+   }
   }
  },
  "nbformat": 4,

doc/source/whatsnew/v1.5.1.rst

@@ -23,7 +23,8 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Bug in :class:`CSSToExcelConverter` leading to error when unrecognized ``border-style`` (including ``"hair"``) provided to Excel writers (:issue:`48649`)
+- Bug in :class:`CSSToExcelConverter` where border would not fall back on black when border style or width provided without a color (:issue:`48649`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/excel.py

@@ -243,17 +243,18 @@ def _get_is_wrap_text(self, props: Mapping[str, str]) -> bool | None:
     def build_border(
         self, props: Mapping[str, str]
     ) -> dict[str, dict[str, str | None]]:
-        return {
-            side: {
-                "style": self._border_style(
-                    props.get(f"border-{side}-style"),
-                    props.get(f"border-{side}-width"),
-                    self.color_to_excel(props.get(f"border-{side}-color")),
-                ),
-                "color": self.color_to_excel(props.get(f"border-{side}-color")),
-            }
-            for side in ["top", "right", "bottom", "left"]
-        }
+        border_dict = {}
+        for side in ["top", "right", "bottom", "left"]:
+            style = self._border_style(
+                props.get(f"border-{side}-style"),
+                props.get(f"border-{side}-width"),
+                self.color_to_excel(props.get(f"border-{side}-color")),
+            )
+            color = self.color_to_excel(props.get(f"border-{side}-color"))
+            if style and style != "none" and color is None:
+                color = self.color_to_excel("black")
+            border_dict[side] = {"style": style, "color": color}
+        return border_dict
 
     def _border_style(self, style: str | None, width: str | None, color: str | None):
         # convert styles and widths to openxml, one of:
@@ -299,6 +300,24 @@ 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 (
+            "hair",
+            "mediumDashDot",
+            "dashDotDot",
+            "mediumDashDotDot",
+            "dashDot",
+            "slandDashDot",
+            "mediumDashed",
+        ):
+            # Excel-specific styles
+            return style
+        else:
+            warnings.warn(
+                f"Unhandled border style format: {repr(style)}",
+                CSSWarning,
+                stacklevel=find_stack_level(inspect.currentframe()),
+            )
+            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 +115,19 @@ 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",
+    ),
+    ("border-left-style: hair;", ["border", "left", "style"], "hair"),
+    # CSS should default to black if style provided w/o color
+    (
+        "border-left-style: hair;",
+        ["border", "left", "color", "rgb"],
+        {"xlsxwriter": "FF000000", "openpyxl": "00000000"},
+    ),
 ]
 
 

pandas/tests/io/formats/test_to_excel.py

@@ -113,63 +113,72 @@
             "border-style: solid",
             {
                 "border": {
-                    "top": {"style": "medium"},
-                    "bottom": {"style": "medium"},
-                    "left": {"style": "medium"},
-                    "right": {"style": "medium"},
+                    "top": {"style": "medium", "color": "000000"},
+                    "bottom": {"style": "medium", "color": "000000"},
+                    "left": {"style": "medium", "color": "000000"},
+                    "right": {"style": "medium", "color": "000000"},
                 }
             },
         ),
         (
             "border-style: solid; border-width: thin",
             {
                 "border": {
-                    "top": {"style": "thin"},
-                    "bottom": {"style": "thin"},
-                    "left": {"style": "thin"},
-                    "right": {"style": "thin"},
+                    "top": {"style": "thin", "color": "000000"},
+                    "bottom": {"style": "thin", "color": "000000"},
+                    "left": {"style": "thin", "color": "000000"},
+                    "right": {"style": "thin", "color": "000000"},
                 }
             },
         ),
         (
             "border-top-style: solid; border-top-width: thin",
-            {"border": {"top": {"style": "thin"}}},
+            {"border": {"top": {"style": "thin", "color": "000000"}}},
         ),
         (
             "border-top-style: solid; border-top-width: 1pt",
-            {"border": {"top": {"style": "thin"}}},
+            {"border": {"top": {"style": "thin", "color": "000000"}}},
+        ),
+        (
+            "border-top-style: solid",
+            {"border": {"top": {"style": "medium", "color": "000000"}}},
         ),
-        ("border-top-style: solid", {"border": {"top": {"style": "medium"}}}),
         (
             "border-top-style: solid; border-top-width: medium",
-            {"border": {"top": {"style": "medium"}}},
+            {"border": {"top": {"style": "medium", "color": "000000"}}},
         ),
         (
             "border-top-style: solid; border-top-width: 2pt",
-            {"border": {"top": {"style": "medium"}}},
+            {"border": {"top": {"style": "medium", "color": "000000"}}},
         ),
         (
             "border-top-style: solid; border-top-width: thick",
-            {"border": {"top": {"style": "thick"}}},
+            {"border": {"top": {"style": "thick", "color": "000000"}}},
         ),
         (
             "border-top-style: solid; border-top-width: 4pt",
-            {"border": {"top": {"style": "thick"}}},
+            {"border": {"top": {"style": "thick", "color": "000000"}}},
         ),
         (
             "border-top-style: dotted",
-            {"border": {"top": {"style": "mediumDashDotDot"}}},
+            {"border": {"top": {"style": "mediumDashDotDot", "color": "000000"}}},
         ),
         (
             "border-top-style: dotted; border-top-width: thin",
-            {"border": {"top": {"style": "dotted"}}},
+            {"border": {"top": {"style": "dotted", "color": "000000"}}},
+        ),
+        (
+            "border-top-style: dashed",
+            {"border": {"top": {"style": "mediumDashed", "color": "000000"}}},
         ),
-        ("border-top-style: dashed", {"border": {"top": {"style": "mediumDashed"}}}),
         (
             "border-top-style: dashed; border-top-width: thin",
-            {"border": {"top": {"style": "dashed"}}},
+            {"border": {"top": {"style": "dashed", "color": "000000"}}},
+        ),
+        (
+            "border-top-style: double",
+            {"border": {"top": {"style": "double", "color": "000000"}}},
         ),
-        ("border-top-style: double", {"border": {"top": {"style": "double"}}}),
         # - color
         (
             "border-style: solid; border-color: #0000ff",
@@ -240,10 +249,10 @@ def test_css_to_excel_multiple():
     assert {
         "font": {"bold": True, "underline": "single", "color": "FF0000"},
         "border": {
-            "top": {"style": "thin"},
-            "right": {"style": "thin"},
-            "bottom": {"style": "thin"},
-            "left": {"style": "thin"},
+            "top": {"style": "thin", "color": "000000"},
+            "right": {"style": "thin", "color": "000000"},
+            "bottom": {"style": "thin", "color": "000000"},
+            "left": {"style": "thin", "color": "000000"},
         },
         "alignment": {"horizontal": "center", "vertical": "top"},
     } == actual