ENH: Allow to assign/create custom body_style and header_style property to an instance of ExcelFormatter used in method df.to_excel()

doc/source/user_guide/io.rst

@@ -3880,6 +3880,19 @@ The look and feel of Excel worksheets created from pandas can be modified using
 * ``float_format`` : Format string for floating point numbers (default ``None``).
 * ``freeze_panes`` : A tuple of two integers representing the bottommost row and rightmost column to freeze. Each of these parameters is one-based, so (1, 1) will freeze the first row and first column (default ``None``).
 
+The styling and formatting of a worksheet's header and body can be easily customized using CSS by passing a dictionary object to
+``pd.io.formats.excel.ExcelFormatter.header_style`` and ``pd.io.formats.excel.ExcelFormatter.body_style``.
+
+.. code-block:: python
+
+    # Set Excel worksheet header and body style
+    >>> df = pd.DataFrame([{"A": 1, "B": 2, "C": 3}, {"A": 1, "B": 2, "C": 3}])
+    >>> pd.io.formats.excel.ExcelFormatter.header_style = {"font": {"bold": True},}
+    >>> pd.io.formats.excel.ExcelFormatter.body_style = {
+    ... "font": {"bold": True},"alignment": {"horizontal": "center", "vertical": "top"},
+    ... }
+    >>> df.to_excel("path_to_file.xlsx", "Sheet1", index=False)
+
 Using the `Xlsxwriter`_ engine provides many options for controlling the
 format of an Excel worksheet created with the ``to_excel`` method.  Excellent examples can be found in the
 `Xlsxwriter`_ documentation here: https://xlsxwriter.readthedocs.io/working_with_pandas.html

doc/source/whatsnew/v2.1.0.rst

@@ -124,6 +124,7 @@ Other enhancements
 - Added a new parameter ``by_row`` to :meth:`Series.apply` and :meth:`DataFrame.apply`. When set to ``False`` the supplied callables will always operate on the whole Series or DataFrame (:issue:`53400`, :issue:`53601`).
 - Groupby aggregations (such as :meth:`DataFrameGroupby.sum`) now can preserve the dtype of the input instead of casting to ``float64`` (:issue:`44952`)
 - Improved error message when :meth:`DataFrameGroupBy.agg` failed (:issue:`52930`)
+- Made :attr:`ExcelFormatter.header_style` a class attribute instead of a property. Default styles for :meth:`DataFrame.to_excel` are set to None. (:issue:`52369`)
 - Many read/to_* functions, such as :meth:`DataFrame.to_pickle` and :func:`read_csv`, support forwarding compression arguments to lzma.LZMAFile (:issue:`52979`)
 - Performance improvement in :func:`concat` with homogeneous ``np.float64`` or ``np.float32`` dtypes (:issue:`52685`)
 - Performance improvement in :meth:`DataFrame.filter` when ``items`` is given (:issue:`52941`)

pandas/io/formats/excel.py

@@ -577,19 +577,26 @@ def __init__(
         self.header = header
         self.merge_cells = merge_cells
         self.inf_rep = inf_rep
+        self._header_style: dict[str, Any] | None = None
+        self._body_style: dict[str, Any] | None = None
 
     @property
-    def header_style(self) -> dict[str, dict[str, str | bool]]:
-        return {
-            "font": {"bold": True},
-            "borders": {
-                "top": "thin",
-                "right": "thin",
-                "bottom": "thin",
-                "left": "thin",
-            },
-            "alignment": {"horizontal": "center", "vertical": "top"},
-        }
+    def header_style(self):
+        return self._header_style
+
+    @header_style.setter
+    def header_style(self, val) -> None:
+        if isinstance(val, dict):
+            self._header_style = val
+
+    @property
+    def body_style(self):
+        return self._body_style
+
+    @body_style.setter
+    def body_style(self, val) -> None:
+        if isinstance(val, dict):
+            self._body_style = val
 
     def _format_value(self, val):
         if is_scalar(val) and missing.isna(val):
@@ -876,7 +883,7 @@ def _generate_body(self, coloffset: int) -> Iterable[ExcelCell]:
                     row=self.rowcounter + i,
                     col=colidx + coloffset,
                     val=val,
-                    style=None,
+                    style=self.body_style,
                     css_styles=getattr(self.styler, "ctx", None),
                     css_row=i,
                     css_col=colidx,

pandas/tests/io/excel/test_style.py

@@ -6,6 +6,7 @@
 
 import pandas.util._test_decorators as td
 
+import pandas as pd
 from pandas import (
     DataFrame,
     read_excel,
@@ -15,7 +16,7 @@
 from pandas.io.excel import ExcelWriter
 from pandas.io.formats.excel import ExcelFormatter
 
-pytest.importorskip("jinja2")
+# pytest.importorskip("jinja2")
 # jinja2 is currently required for Styler.__init__(). Technically Styler.to_excel
 # could compute styles and render to excel without jinja2, since there is no
 # 'template' file, but this needs the import error to delayed until render time.
@@ -35,7 +36,7 @@ def assert_equal_cell_styles(cell1, cell2):
     "engine",
     ["xlsxwriter", "openpyxl"],
 )
-def test_styler_to_excel_unstyled(engine):
+def test_styler_to_excel_nostyle(engine):
     # compare DataFrame.to_excel and Styler.to_excel when no styles applied
     pytest.importorskip(engine)
     df = DataFrame(np.random.randn(2, 2))
@@ -254,6 +255,125 @@ def test_styler_to_excel_border_style(engine, border_style):
             assert s_cell == expected
 
 
+def test_styler_update_values():
+    # GH 53973
+    openpyxl = pytest.importorskip("openpyxl")
+    df = DataFrame([{"A": 1, "B": 2, "C": 3}, {"A": 1, "B": 2, "C": 3}])
+    style = {
+        "font": {"bold": True},
+        "borders": {
+            "top": "thin",
+            "right": "thin",
+            "bottom": "thin",
+            "left": "thin",
+        },
+        "alignment": {"horizontal": "center", "vertical": "top"},
+    }
+
+    with tm.ensure_clean(".xlsx") as path:
+        with ExcelWriter(path, engine="openpyxl") as writer:
+            # write to sheet 'custom'
+            pd.io.formats.excel.ExcelFormatter.header_style = style
+            pd.io.formats.excel.ExcelFormatter.body_style = style
+            df.to_excel(writer, sheet_name="custom")
+
+            # write to sheet 'default'
+            pd.io.formats.excel.ExcelFormatter.header_style = None
+            pd.io.formats.excel.ExcelFormatter.body_style = None
+            df.to_excel(writer, sheet_name="default")
+
+        with contextlib.closing(openpyxl.load_workbook(path)) as wb:
+            # Check font, spacing, indentation
+            assert wb["custom"].cell(1, 2).font.bold is True
+            assert wb["custom"].cell(1, 2).alignment.horizontal == "center"
+            assert wb["custom"].cell(1, 2).alignment.vertical == "top"
+            assert wb["default"].cell(1, 2).font.bold is False
+            assert wb["default"].cell(1, 2).alignment.horizontal is None
+            assert wb["default"].cell(1, 2).alignment.vertical is None
+
+            # Check border
+            assert wb["custom"].cell(1, 2).border.bottom.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.top.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.left.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.right.border_style == "thin"
+            assert wb["default"].cell(1, 2).border.bottom.border_style is None
+            assert wb["default"].cell(1, 2).border.top.border_style is None
+            assert wb["default"].cell(1, 2).border.left.border_style is None
+            assert wb["default"].cell(1, 2).border.right.border_style is None
+
+
+def test_styler_custom_values():
+    # GH 53973
+    openpyxl = pytest.importorskip("openpyxl")
+    pd.io.formats.excel.ExcelFormatter.header_style = {
+        "font": {"bold": True},
+        "borders": {
+            "top": "thin",
+            "right": "thin",
+            "bottom": "thin",
+            "left": "thin",
+        },
+        "alignment": {"horizontal": "center", "vertical": "top"},
+    }
+
+    pd.io.formats.excel.ExcelFormatter.body_style = {
+        "font": {"bold": False},
+        "borders": {
+            "top": "thin",
+            "right": "thin",
+            "bottom": "thin",
+            "left": "thin",
+        },
+        "alignment": {"horizontal": "right", "vertical": "top"},
+    }
+
+    df = DataFrame([{"A": 1, "B": 2, "C": 3}, {"A": 1, "B": 2, "C": 3}])
+    with tm.ensure_clean(".xlsx") as path:
+        with ExcelWriter(path, engine="openpyxl") as writer:
+            df.to_excel(writer, sheet_name="custom")
+
+        with contextlib.closing(openpyxl.load_workbook(path)) as wb:
+            # Check font, spacing, indentation
+            assert wb["custom"].cell(1, 2).font.bold is True
+            assert wb["custom"].cell(2, 2).font.bold is False
+            assert wb["custom"].cell(1, 2).alignment.horizontal == "center"
+            assert wb["custom"].cell(1, 2).alignment.vertical == "top"
+            assert wb["custom"].cell(2, 2).alignment.horizontal == "right"
+            assert wb["custom"].cell(2, 2).alignment.vertical == "top"
+
+            # Check border
+            assert wb["custom"].cell(1, 2).border.bottom.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.top.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.left.border_style == "thin"
+            assert wb["custom"].cell(1, 2).border.right.border_style == "thin"
+            assert wb["custom"].cell(2, 2).border.bottom.border_style == "thin"
+            assert wb["custom"].cell(2, 2).border.top.border_style == "thin"
+            assert wb["custom"].cell(2, 2).border.left.border_style == "thin"
+            assert wb["custom"].cell(2, 2).border.right.border_style == "thin"
+
+
+def test_styler_default_values():
+    # GH 53973
+    openpyxl = pytest.importorskip("openpyxl")
+
+    df = DataFrame([{"A": 1, "B": 2, "C": 3}, {"A": 1, "B": 2, "C": 3}])
+    with tm.ensure_clean(".xlsx") as path:
+        with ExcelWriter(path, engine="openpyxl") as writer:
+            df.to_excel(writer, sheet_name="custom")
+
+        with contextlib.closing(openpyxl.load_workbook(path)) as wb:
+            # Check font, spacing, indentation
+            assert wb["custom"].cell(1, 1).font.bold is False
+            assert wb["custom"].cell(1, 1).alignment.horizontal is None
+            assert wb["custom"].cell(1, 1).alignment.vertical is None
+
+            # Check border
+            assert wb["custom"].cell(1, 1).border.bottom.color is None
+            assert wb["custom"].cell(1, 1).border.top.color is None
+            assert wb["custom"].cell(1, 1).border.left.color is None
+            assert wb["custom"].cell(1, 1).border.right.color is None
+
+
 def test_styler_custom_converter():
     openpyxl = pytest.importorskip("openpyxl")