BUG: Fix date_format and datetime_format arguments to ExcelWriter being ignored

doc/source/whatsnew/v1.5.0.rst

@@ -850,6 +850,8 @@ I/O
 - Bug in :func:`read_sas` returned ``None`` rather than an empty DataFrame for SAS7BDAT files with zero rows (:issue:`18198`)
 - Bug in :class:`StataWriter` where value labels were always written with default encoding (:issue:`46750`)
 - Bug in :class:`StataWriterUTF8` where some valid characters were removed from variable names (:issue:`47276`)
+- Bug in :class:`ExcelWriter` not respecting ``date_format`` and ``datetime_format`` (:issue:`44284`)
+-
 
 Period
 ^^^^^^

pandas/io/excel/_odswriter.py

@@ -33,7 +33,7 @@ def __init__(
         path: FilePath | WriteExcelBuffer | ExcelWriter,
         engine: str | None = None,
         date_format: str | None = None,
-        datetime_format=None,
+        datetime_format: str | None = None,
         mode: str = "w",
         storage_options: StorageOptions = None,
         if_sheet_exists: str | None = None,
@@ -47,6 +47,8 @@ def __init__(
 
         super().__init__(
             path,
+            date_format=date_format,
+            datetime_format=datetime_format,
             mode=mode,
             storage_options=storage_options,
             if_sheet_exists=if_sheet_exists,

pandas/io/excel/_openpyxl.py

@@ -58,6 +58,8 @@ def __init__(
 
         super().__init__(
             path,
+            date_format=date_format,
+            datetime_format=datetime_format,
             mode=mode,
             storage_options=storage_options,
             if_sheet_exists=if_sheet_exists,

pandas/io/excel/_xlwt.py

@@ -51,6 +51,8 @@ def __init__(
 
         super().__init__(
             path,
+            date_format=date_format,
+            datetime_format=datetime_format,
             mode=mode,
             storage_options=storage_options,
             if_sheet_exists=if_sheet_exists,

pandas/tests/io/excel/test_openpyxl.py

@@ -1,4 +1,8 @@
 import contextlib
+from datetime import (
+    date,
+    datetime,
+)
 from pathlib import Path
 import re
 
@@ -10,6 +14,7 @@
 import pandas._testing as tm
 
 from pandas.io.excel import (
+    ExcelFile,
     ExcelWriter,
     _OpenpyxlWriter,
 )
@@ -396,3 +401,38 @@ def test_ints_spelled_with_decimals(datapath, ext):
     result = pd.read_excel(path)
     expected = DataFrame(range(2, 12), columns=[1])
     tm.assert_frame_equal(result, expected)
+
+
+def test_write_date_datetime_format(ext):
+    # see gh-44284
+    #
+    # Test that custom date/datetime formats are respected
+    # by inspecting formatting info in written file.
+    df = DataFrame(
+        [
+            [date(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+            [date(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
+        ],
+        index=["X", "Y"],
+        columns=["DATE", "DATETIME"],
+    )
+
+    with tm.ensure_clean(ext) as f:
+        with ExcelWriter(
+            f,
+            engine="xlsxwriter",
+            date_format="DD.MM.YYYY",
+            datetime_format="DD.MM.YYYY HH-MM-SS",
+        ) as writer:
+            df.to_excel(writer, "test1")
+
+        with ExcelFile(f) as reader:
+            ws = reader.book["test1"]
+
+            date_cells = (ws["B2"], ws["B3"])
+            assert all(cell.number_format == "DD.MM.YYYY" for cell in date_cells)
+
+            datetime_cells = (ws["C2"], ws["C3"])
+            assert all(
+                cell.number_format == "DD.MM.YYYY HH-MM-SS" for cell in datetime_cells
+            )

pandas/tests/io/excel/test_writers.py

@@ -652,25 +652,27 @@ def test_excel_roundtrip_datetime(self, merge_cells, tsframe, path):
 
         tm.assert_frame_equal(tsframe, recons)
 
-    def test_excel_date_datetime_format(self, ext, path):
+    def test_excel_roundtrip_date_datetime_format(self, ext, path):
         # see gh-4133
         #
-        # Excel output format strings
+        # Test that df written with custom date/datetime format strings
+        # is read back the same as if written using the default formats,
+        # and that the values are still recognised as dates/datetimes.
         df = DataFrame(
             [
-                [date(2014, 1, 31), date(1999, 9, 24)],
-                [datetime(1998, 5, 26, 23, 33, 4), datetime(2014, 2, 28, 13, 5, 13)],
+                [date(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+                [date(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
             ],
-            index=["DATE", "DATETIME"],
-            columns=["X", "Y"],
+            index=["X", "Y"],
+            columns=["DATE", "DATETIME"],
         )
         df_expected = DataFrame(
             [
-                [datetime(2014, 1, 31), datetime(1999, 9, 24)],
-                [datetime(1998, 5, 26, 23, 33, 4), datetime(2014, 2, 28, 13, 5, 13)],
+                [datetime(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+                [datetime(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
             ],
-            index=["DATE", "DATETIME"],
-            columns=["X", "Y"],
+            index=["X", "Y"],
+            columns=["DATE", "DATETIME"],
         )
 
         with tm.ensure_clean(ext) as filename2:

pandas/tests/io/excel/test_xlsxwriter.py

@@ -1,4 +1,8 @@
 import contextlib
+from datetime import (
+    date,
+    datetime,
+)
 import re
 import warnings
 
@@ -7,7 +11,10 @@
 from pandas import DataFrame
 import pandas._testing as tm
 
-from pandas.io.excel import ExcelWriter
+from pandas.io.excel import (
+    ExcelFile,
+    ExcelWriter,
+)
 
 xlsxwriter = pytest.importorskip("xlsxwriter")
 
@@ -92,3 +99,38 @@ def test_book_and_sheets_consistent(ext):
             assert writer.sheets == {}
             sheet = writer.book.add_worksheet("test_name")
             assert writer.sheets == {"test_name": sheet}
+
+
+def test_write_date_datetime_format(ext):
+    # see gh-44284
+    #
+    # Test that custom date/datetime formats are respected
+    # by inspecting formatting info in written file.
+    df = DataFrame(
+        [
+            [date(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+            [date(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
+        ],
+        index=["X", "Y"],
+        columns=["DATE", "DATETIME"],
+    )
+
+    with tm.ensure_clean(ext) as f:
+        with ExcelWriter(
+            f,
+            engine="xlsxwriter",
+            date_format="DD.MM.YYYY",
+            datetime_format="DD.MM.YYYY HH-MM-SS",
+        ) as writer:
+            df.to_excel(writer, "test1")
+
+        with ExcelFile(f) as reader:
+            ws = reader.book["test1"]
+
+            date_cells = (ws["B2"], ws["B3"])
+            assert all(cell.number_format == "DD.MM.YYYY" for cell in date_cells)
+
+            datetime_cells = (ws["C2"], ws["C3"])
+            assert all(
+                cell.number_format == "DD.MM.YYYY HH-MM-SS" for cell in datetime_cells
+            )

pandas/tests/io/excel/test_xlwt.py

@@ -1,3 +1,7 @@
+from datetime import (
+    date,
+    datetime,
+)
 import re
 
 import numpy as np
@@ -144,3 +148,49 @@ def test_deprecated_attr(ext, attr):
             msg = f"{attr} is not part of the public API"
             with tm.assert_produces_warning(FutureWarning, match=msg):
                 getattr(writer, attr)
+
+
+def test_write_date_datetime_format(ext):
+    # see gh-44284
+    #
+    # Test that custom date/datetime formats are respected
+    # by inspecting formatting info in written file.
+    xlrd = pytest.importorskip("xlrd")
+
+    df = DataFrame(
+        [
+            [date(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+            [date(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
+        ],
+        index=["X", "Y"],
+        columns=["DATE", "DATETIME"],
+    )
+
+    with tm.ensure_clean(ext) as f:
+        with ExcelWriter(
+            f,
+            engine="xlwt",
+            date_format="DD.MM.YYYY",
+            datetime_format="DD.MM.YYYY HH-MM-SS",
+        ) as writer:
+            df.to_excel(writer, "test1")
+
+        # formatting_info defaults to False
+        # so have to use xlrd.open_workbook() directly
+        with xlrd.open_workbook(f, formatting_info=True) as book:
+            sh = book["test1"]
+            xf_list = book.xf_list
+            format_map = book.format_map
+
+            date_cells = (sh[1, 1], sh[2, 1])
+            assert all(
+                format_map[xf_list[cell.xf_index].format_key].format_str == "DD.MM.YYYY"
+                for cell in date_cells
+            )
+
+            datetime_cells = (sh[1, 2], sh[2, 2])
+            assert all(
+                format_map[xf_list[cell.xf_index].format_key].format_str
+                == "DD.MM.YYYY HH-MM-SS"
+                for cell in datetime_cells
+            )