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/whatsnew/v2.1.0.rst

@@ -116,6 +116,7 @@ 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 +577,32 @@ def __init__(
         self.header = header
         self.merge_cells = merge_cells
         self.inf_rep = inf_rep
+        self._header_styledict[str, Any] = None
+        self._body_styledict[str, Any] = 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):
+        if not isinstance(val, dict):
+            return None
+        else:
+            self._header_style = val
+            return self._header_style
+
+    @property
+    def body_style(self):
+        return self._body_style
+
+    @body_style.setter
+    def body_style(self, val):
+        if not isinstance(val, dict):
+            return None
+        else:
+            self._body_style = val
+            return self._body_style
 
     def _format_value(self, val):
         if is_scalar(val) and missing.isna(val):

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.
@@ -254,6 +255,61 @@ def test_styler_to_excel_border_style(engine, border_style):
             assert s_cell == expected
 
 
+def test_styler_update_values():
+    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"},
+    }
+
+    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(1, 2).alignment.horizontal == "center"
+            assert wb["custom"].cell(1, 2).alignment.vertical == "top"
+
+            # Check border
+            wb["custom"].cell(1, 2).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"
+
+
+def test_styler_default_values():
+    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.color.value == 1
+            assert wb["custom"].cell(1, 1).alignment.horizontal is None
+            assert wb["custom"].cell(1, 1).alignment.vertical is None
+            assert wb["custom"].cell(1, 1).alignment.indent == 0.0
+
+            # Check border
+            wb["custom"].cell(1, 1).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")