BUG: Fix to_excel storing decimals as strings instead of numbers (issue #49598) (#60230)

* Fix issue 50174

* Add release notes

* Use correct github issue number

Author: michiel-de-muynck

doc/source/whatsnew/v2.3.0.rst

@@ -133,7 +133,7 @@ MultiIndex
 
 I/O
 ^^^
--
+- :meth:`DataFrame.to_excel` was storing decimals as strings instead of numbers (:issue:`49598`)
 -
 
 Period

pandas/io/excel/_base.py

@@ -8,6 +8,7 @@
     Sequence,
 )
 import datetime
+from decimal import Decimal
 from functools import partial
 import os
 from textwrap import fill
@@ -43,6 +44,7 @@
 
 from pandas.core.dtypes.common import (
     is_bool,
+    is_decimal,
     is_file_like,
     is_float,
     is_integer,
@@ -1348,6 +1350,8 @@ def _value_with_fmt(
             val = float(val)
         elif is_bool(val):
             val = bool(val)
+        elif is_decimal(val):
+            val = Decimal(val)
         elif isinstance(val, datetime.datetime):
             fmt = self._datetime_format
         elif isinstance(val, datetime.date):

pandas/tests/io/excel/test_writers.py

@@ -3,6 +3,7 @@
     datetime,
     timedelta,
 )
+from decimal import Decimal
 from functools import partial
 from io import BytesIO
 import os
@@ -977,6 +978,36 @@ def test_to_excel_float_format(self, tmp_excel):
         )
         tm.assert_frame_equal(result, expected)
 
+    def test_to_excel_datatypes_preserved(self, tmp_excel):
+        # Test that when writing and reading Excel with dtype=object,
+        # datatypes are preserved, except Decimals which should be
+        # stored as floats
+
+        # see gh-49598
+        df = DataFrame(
+            [
+                [1.23, "1.23", Decimal("1.23")],
+                [4.56, "4.56", Decimal("4.56")],
+            ],
+            index=["A", "B"],
+            columns=["X", "Y", "Z"],
+        )
+        df.to_excel(tmp_excel)
+
+        with ExcelFile(tmp_excel) as reader:
+            result = pd.read_excel(reader, index_col=0, dtype=object)
+
+        expected = DataFrame(
+            [
+                [1.23, "1.23", 1.23],
+                [4.56, "4.56", 4.56],
+            ],
+            index=["A", "B"],
+            columns=["X", "Y", "Z"],
+            dtype=object,
+        )
+        tm.assert_frame_equal(result, expected)
+
     def test_to_excel_output_encoding(self, tmp_excel):
         # Avoid mixed inferred_type.
         df = DataFrame(