BUG/ENH: Translate CSS border properties for Styler.to_excel

doc/source/user_guide/style.ipynb

@@ -1577,6 +1577,9 @@
     "Some support (*since version 0.20.0*) is available for exporting styled `DataFrames` to Excel worksheets using the `OpenPyXL` or `XlsxWriter` engines. CSS2.2 properties handled include:\n",
     "\n",
     "- `background-color`\n",
+    "- `border-style` properties\n",
+    "- `border-width` properties\n",
+    "- `border-color` properties\n",
     "- `color`\n",
     "- `font-family`\n",
     "- `font-style`\n",
@@ -1587,7 +1590,7 @@
     "- `white-space: nowrap`\n",
     "\n",
     "\n",
-    "- Currently broken: `border-style`, `border-width`, `border-color` and their {`top`, `right`, `bottom`, `left` variants}\n",
+    "- Shorthand and side-specific border properties are supported (e.g. `border-style` and `border-left-style`) as well as the `border` shorthands for all sides (`border: 1px solid green`) or specified sides (`border-left: 1px solid green`). Using a `border` shorthand will override any border properties set before it (See [CSS Working Group](https://drafts.csswg.org/css-backgrounds/#border-shorthands) for more details)\n",
     "\n",
     "\n",
     "- Only CSS2 named colors and hex colors of the form `#rgb` or `#rrggbb` are currently supported.\n",

doc/source/whatsnew/v1.5.0.rst

@@ -20,7 +20,7 @@ Styler
 ^^^^^^
 
   - New method :meth:`.Styler.to_string` for alternative customisable output methods (:issue:`44502`)
-  - Various bug fixes, see below.
+  - Added the ability to render ``border`` and ``border-{side}`` CSS properties in Excel (:issue:`42276`)
 
 .. _whatsnew_150.enhancements.enhancement2:
 
@@ -50,8 +50,10 @@ These are bug fixes that might have notable behavior changes.
 
 .. _whatsnew_150.notable_bug_fixes.notable_bug_fix1:
 
-notable_bug_fix1
-^^^^^^^^^^^^^^^^
+Styler
+^^^^^^
+
+- Fixed bug in :class:`CSSToExcelConverter` leading to ``TypeError`` when border color provided without border style for ``xlsxwriter`` engine (:issue:`42276`)
 
 .. _whatsnew_150.notable_bug_fixes.notable_bug_fix2:
 

pandas/io/formats/css.py

@@ -4,6 +4,10 @@
 from __future__ import annotations
 
 import re
+from typing import (
+    Callable,
+    Generator,
+)
 import warnings
 
 
@@ -13,8 +17,33 @@ class CSSWarning(UserWarning):
     """
 
 
-def _side_expander(prop_fmt: str):
-    def expand(self, prop, value: str):
+def _side_expander(prop_fmt: str) -> Callable:
+    """
+    Wrapper to expand shorthand property into top, right, bottom, left properties
+
+    Parameters
+    ----------
+    side : str
+        The border side to expand into properties
+
+    Returns
+    -------
+        function: Return to call when a 'border(-{side}): {value}' string is encountered
+    """
+
+    def expand(self, prop, value: str) -> Generator[tuple[str, str], None, None]:
+        """
+        Expand shorthand property into side-specific property (top, right, bottom, left)
+
+        Parameters
+        ----------
+            prop (str): CSS property name
+            value (str): String token for property
+
+        Yields
+        ------
+            Tuple (str, str): Expanded property, value
+        """
         tokens = value.split()
         try:
             mapping = self.SIDE_SHORTHANDS[len(tokens)]
@@ -27,12 +56,72 @@ def expand(self, prop, value: str):
     return expand
 
 
+def _border_expander(side: str = "") -> Callable:
+    """
+    Wrapper to expand 'border' property into border color, style, and width properties
+
+    Parameters
+    ----------
+    side : str
+        The border side to expand into properties
+
+    Returns
+    -------
+        function: Return to call when a 'border(-{side}): {value}' string is encountered
+    """
+    if side != "":
+        side = f"-{side}"
+
+    def expand(self, prop, value: str) -> Generator[tuple[str, str], None, None]:
+        """
+        Expand border into color, style, and width tuples
+
+        Parameters
+        ----------
+            prop : str
+                CSS property name passed to styler
+            value : str
+                Value passed to styler for property
+
+        Yields
+        ------
+            Tuple (str, str): Expanded property, value
+        """
+        tokens = value.split()
+        if len(tokens) == 0 or len(tokens) > 3:
+            warnings.warn(
+                f'Too many tokens provided to "{prop}" (expected 1-3)', CSSWarning
+            )
+
+        # TODO: Can we use current color as initial value to comply with CSS standards?
+        border_declarations = {
+            f"border{side}-color": "black",
+            f"border{side}-style": "none",
+            f"border{side}-width": "medium",
+        }
+        for token in tokens:
+            if token in self.BORDER_STYLES:
+                border_declarations[f"border{side}-style"] = token
+            elif any([ratio in token for ratio in self.BORDER_WIDTH_RATIOS]):
+                border_declarations[f"border{side}-width"] = token
+            else:
+                border_declarations[f"border{side}-color"] = token
+            # TODO: Warn user if item entered more than once (e.g. "border: red green")
+
+        # Per CSS, "border" will reset previous "border-*" definitions
+        yield from self.atomize(border_declarations.items())
+
+    return expand
+
+
 class CSSResolver:
     """
     A callable for parsing and resolving CSS to atomic properties.
     """
 
     UNIT_RATIOS = {
+        "pt": ("pt", 1),
+        "em": ("em", 1),
         "rem": ("pt", 12),
         "ex": ("em", 0.5),
         # 'ch':
@@ -76,6 +165,19 @@ class CSSResolver:
         }
     )
 
+    BORDER_STYLES = [
+        "none",
+        "hidden",
+        "dotted",
+        "dashed",
+        "solid",
+        "double",
+        "groove",
+        "ridge",
+        "inset",
+        "outset",
+    ]
+
     SIDE_SHORTHANDS = {
         1: [0, 0, 0, 0],
         2: [0, 1, 0, 1],
@@ -244,7 +346,7 @@ def _error():
             size_fmt = f"{val:f}pt"
         return size_fmt
 
-    def atomize(self, declarations):
+    def atomize(self, declarations) -> Generator[tuple[str, str], None, None]:
         for prop, value in declarations:
             attr = "expand_" + prop.replace("-", "_")
             try:
@@ -255,6 +357,12 @@ def atomize(self, declarations):
                 for prop, value in expand(prop, value):
                     yield prop, value
 
+    expand_border = _border_expander()
+    expand_border_top = _border_expander("top")
+    expand_border_right = _border_expander("right")
+    expand_border_bottom = _border_expander("bottom")
+    expand_border_left = _border_expander("left")
+
     expand_border_color = _side_expander("border-{:s}-color")
     expand_border_style = _side_expander("border-{:s}-style")
     expand_border_width = _side_expander("border-{:s}-width")

pandas/io/formats/excel.py

@@ -237,13 +237,14 @@ def build_border(
                 "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"]
         }
 
-    def _border_style(self, style: str | None, width: str | None):
+    def _border_style(self, style: str | None, width: str | None, color: str | None):
         # convert styles and widths to openxml, one of:
         #       'dashDot'
         #       'dashDotDot'
@@ -258,14 +259,20 @@ def _border_style(self, style: str | None, width: str | None):
         #       'slantDashDot'
         #       'thick'
         #       'thin'
-        if width is None and style is None:
+        if width is None and style is None and color is None:
+            # Return None will remove "border" from style dictionary
             return None
+
+        if width is None and style is None:
+            # Return "none" will keep "border" in style dictionary
+            return "none"
+
         if style == "none" or style == "hidden":
-            return None
+            return "none"
 
         width_name = self._get_width_name(width)
         if width_name is None:
-            return None
+            return "none"
 
         if style in (None, "groove", "ridge", "inset", "outset", "solid"):
             # not handled

pandas/tests/io/excel/test_style.py

@@ -70,6 +70,44 @@ def test_styler_to_excel_unstyled(engine):
         ["alignment", "vertical"],
         {"xlsxwriter": None, "openpyxl": "bottom"},  # xlsxwriter Fails
     ),
+    # Border widths
+    ("border-left: 2pt solid red", ["border", "left", "style"], "medium"),
+    ("border-left: 1pt dotted red", ["border", "left", "style"], "dotted"),
+    ("border-left: 2pt dotted red", ["border", "left", "style"], "mediumDashDotDot"),
+    ("border-left: 1pt dashed red", ["border", "left", "style"], "dashed"),
+    ("border-left: 2pt dashed red", ["border", "left", "style"], "mediumDashed"),
+    ("border-left: 1pt solid red", ["border", "left", "style"], "thin"),
+    ("border-left: 3pt solid red", ["border", "left", "style"], "thick"),
+    # Border expansion
+    (
+        "border-left: 2pt solid #111222",
+        ["border", "left", "color", "rgb"],
+        {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
+    ),
+    ("border: 1pt solid red", ["border", "top", "style"], "thin"),
+    (
+        "border: 1pt solid #111222",
+        ["border", "top", "color", "rgb"],
+        {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
+    ),
+    ("border: 1pt solid red", ["border", "right", "style"], "thin"),
+    (
+        "border: 1pt solid #111222",
+        ["border", "right", "color", "rgb"],
+        {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
+    ),
+    ("border: 1pt solid red", ["border", "bottom", "style"], "thin"),
+    (
+        "border: 1pt solid #111222",
+        ["border", "bottom", "color", "rgb"],
+        {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
+    ),
+    ("border: 1pt solid red", ["border", "left", "style"], "thin"),
+    (
+        "border: 1pt solid #111222",
+        ["border", "left", "color", "rgb"],
+        {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
+    ),
 ]
 
 
@@ -95,7 +133,7 @@ def test_styler_to_excel_basic(engine, css, attrs, expected):
             # 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), getattr(s_cell, attr)
+            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]
@@ -136,8 +174,8 @@ def test_styler_to_excel_basic_indexes(engine, css, attrs, expected):
             ui_cell, si_cell = wb["null_styled"].cell(2, 1), wb["styled"].cell(2, 1)
             uc_cell, sc_cell = wb["null_styled"].cell(1, 2), wb["styled"].cell(1, 2)
         for attr in attrs:
-            ui_cell, si_cell = getattr(ui_cell, attr), getattr(si_cell, attr)
-            uc_cell, sc_cell = getattr(uc_cell, attr), getattr(sc_cell, attr)
+            ui_cell, si_cell = getattr(ui_cell, attr, None), getattr(si_cell, attr)
+            uc_cell, sc_cell = getattr(uc_cell, attr, None), getattr(sc_cell, attr)
 
         if isinstance(expected, dict):
             assert ui_cell is None or ui_cell != expected[engine]

pandas/tests/io/formats/test_css.py

@@ -57,6 +57,8 @@ def test_css_parse_normalisation(name, norm, abnorm):
         ("font-size: 1unknownunit", "font-size: 1em"),
         ("font-size: 10", "font-size: 1em"),
         ("font-size: 10 pt", "font-size: 1em"),
+        # Too many args
+        ("border-top: 1pt solid red green", "border-top: 1pt solid green"),
     ],
 )
 def test_css_parse_invalid(invalid_css, remainder):
@@ -123,6 +125,65 @@ def test_css_side_shorthands(shorthand, expansions):
         assert_resolves(f"{shorthand}: 1pt 1pt 1pt 1pt 1pt", {})
 
 
+@pytest.mark.parametrize(
+    "shorthand,sides",
+    [
+        ("border-top", ["top"]),
+        ("border-right", ["right"]),
+        ("border-bottom", ["bottom"]),
+        ("border-left", ["left"]),
+        ("border", ["top", "right", "bottom", "left"]),
+    ],
+)
+def test_css_border_shorthand_sides(shorthand, sides):
+    def create_border_dict(sides, color=None, style=None, width=None):
+        resolved = {}
+        for side in sides:
+            if color:
+                resolved[f"border-{side}-color"] = color
+            if style:
+                resolved[f"border-{side}-style"] = style
+            if width:
+                resolved[f"border-{side}-width"] = width
+        return resolved
+
+    assert_resolves(
+        f"{shorthand}: 1pt red solid", create_border_dict(sides, "red", "solid", "1pt")
+    )
+
+
+@pytest.mark.parametrize(
+    "prop, expected",
+    [
+        ("1pt red solid", ("red", "solid", "1pt")),
+        ("red 1pt solid", ("red", "solid", "1pt")),
+        ("red solid 1pt", ("red", "solid", "1pt")),
+        ("solid 1pt red", ("red", "solid", "1pt")),
+        ("red solid", ("red", "solid", "1.500000pt")),
+        # Note: color=black is not CSS conforming
+        # (See https://drafts.csswg.org/css-backgrounds/#border-shorthands)
+        ("1pt solid", ("black", "solid", "1pt")),
+        ("1pt red", ("red", "none", "1pt")),
+        ("red", ("red", "none", "1.500000pt")),
+        ("1pt", ("black", "none", "1pt")),
+        ("solid", ("black", "solid", "1.500000pt")),
+        # Sizes
+        ("1em", ("black", "none", "12pt")),
+    ],
+)
+def test_css_border_shorthands(prop, expected):
+    color, style, width = expected
+
+    assert_resolves(
+        f"border-left: {prop}",
+        {
+            "border-left-color": color,
+            "border-left-style": style,
+            "border-left-width": width,
+        },
+    )
+
+
 @pytest.mark.parametrize(
     "style,inherited,equiv",
     [