From 4b73d6a3aa83a7e5b466a81bfcc65f4c40e838bd Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 1 Mar 2021 21:31:27 +0000
Subject: [PATCH 01/15] ENH: Add if_exists parameter to ExcelWriter

---
 doc/source/whatsnew/v1.3.0.rst         |  1 +
 pandas/io/excel/_base.py               | 23 +++++++++++++
 pandas/io/excel/_openpyxl.py           | 25 ++++++++++++--
 pandas/tests/io/excel/test_openpyxl.py | 47 ++++++++++++++++++++++++++
 4 files changed, 94 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index f7204ceb9d412..2f99a28275741 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -125,6 +125,7 @@ Other enhancements
 - :func:`to_numeric` now supports downcasting of nullable ``ExtensionDtype`` objects (:issue:`33013`)
 - Add support for dict-like names in :class:`MultiIndex.set_names` and :class:`MultiIndex.rename` (:issue:`20421`)
 - :func:`pandas.read_excel` can now auto detect .xlsb files (:issue:`35416`)
+- :class:`pandas.ExcelWriter` now accepts an ``if_exists`` parameter to control the behaviour of append mode when writing to existing sheets (:issue:`40230`)
 - :meth:`.Rolling.sum`, :meth:`.Expanding.sum`, :meth:`.Rolling.mean`, :meth:`.Expanding.mean`, :meth:`.Rolling.median`, :meth:`.Expanding.median`, :meth:`.Rolling.max`, :meth:`.Expanding.max`, :meth:`.Rolling.min`, and :meth:`.Expanding.min` now support ``Numba`` execution with the ``engine`` keyword (:issue:`38895`)
 - :meth:`DataFrame.apply` can now accept NumPy unary operators as strings, e.g. ``df.apply("sqrt")``, which was already the case for :meth:`Series.apply` (:issue:`39116`)
 - :meth:`DataFrame.apply` can now accept non-callable DataFrame properties as strings, e.g. ``df.apply("size")``, which was already the case for :meth:`Series.apply` (:issue:`39116`)
diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index 9ad589d4583c6..c777d9326382e 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -666,6 +666,16 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         be parsed by ``fsspec``, e.g., starting "s3://", "gcs://".
 
         .. versionadded:: 1.2.0
+    if_exists : {'new_sheet', 'overwrite_sheet', 'overwrite_cells'}, default 'new_sheet'
+            How to behave when trying to write to a sheet that already
+            exists (append mode only).
+
+            * new_sheet: Create a new sheet with a different name.
+            * overwrite_sheet: Delete the contents of the sheet, then write to it.
+            * overwrite_cells: Write directly to the named sheet
+              without deleting the previous contents.
+
+        .. versionadded:: 1.3.0
 
     Attributes
     ----------
@@ -834,6 +844,7 @@ def __init__(
         datetime_format=None,
         mode: str = "w",
         storage_options: StorageOptions = None,
+        if_exists: Optional[str] = None,
         **engine_kwargs,
     ):
         # validate that this engine can handle the extension
@@ -868,6 +879,18 @@ def __init__(
 
         self.mode = mode
 
+        if if_exists and "r+" not in mode:
+            raise ValueError("if_exists is only valid in append mode (mode='a')")
+        if if_exists is not None and if_exists not in {
+            "new_sheet",
+            "overwrite_sheet",
+            "overwrite_cells",
+        }:
+            raise ValueError(f"'{if_exists}' is not valid for if_exists")
+        if if_exists is None and "r+" in mode:
+            if_exists = "new_sheet"
+        self.if_exists = if_exists
+
     def __fspath__(self):
         return getattr(self.handles.handle, "name", "")
 
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index be2c9b919a5c3..7ae4e866bb235 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -37,13 +37,18 @@ def __init__(
         engine=None,
         mode: str = "w",
         storage_options: StorageOptions = None,
+        if_exists: Optional[str] = None,
         **engine_kwargs,
     ):
         # Use the openpyxl module as the Excel writer.
         from openpyxl.workbook import Workbook
 
         super().__init__(
-            path, mode=mode, storage_options=storage_options, **engine_kwargs
+            path,
+            mode=mode,
+            storage_options=storage_options,
+            if_exists=if_exists,
+            **engine_kwargs,
         )
 
         # ExcelWriter replaced "a" by "r+" to allow us to first read the excel file from
@@ -53,6 +58,8 @@ def __init__(
 
             self.book = load_workbook(self.handles.handle)
             self.handles.handle.seek(0)
+            self.sheets = {name: self.book[name] for name in self.book.sheetnames}
+
         else:
             # Create workbook object with default optimized_write=True.
             self.book = Workbook()
@@ -412,7 +419,21 @@ def write_cells(
         _style_cache: Dict[str, Dict[str, Serialisable]] = {}
 
         if sheet_name in self.sheets:
-            wks = self.sheets[sheet_name]
+            if "r+" in self.mode:
+                if self.if_exists == "new_sheet":
+                    wks = self.book.create_sheet()
+                    # openpyxl will create a name for the new sheet by appending digits
+                    wks.title = sheet_name
+                    self.sheets[wks.title] = wks
+                elif self.if_exists == "overwrite_sheet":
+                    wks = self.sheets[sheet_name]
+                    wks.delete_cols(1, wks.max_column)
+                elif self.if_exists == "overwrite_cells" or self.if_exists is None:
+                    wks = self.sheets[sheet_name]
+                else:
+                    raise ValueError(f"'{self.if_exists}' is not valid for if_exists")
+            else:
+                wks = self.sheets[sheet_name]
         else:
             wks = self.book.create_sheet()
             wks.title = sheet_name
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 9010f978d268d..3fd7d6727d14b 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -1,4 +1,5 @@
 from pathlib import Path
+import re
 
 import numpy as np
 import pytest
@@ -109,6 +110,52 @@ def test_write_append_mode(ext, mode, expected):
             assert wb2.worksheets[index]["A1"].value == cell_value
 
 
+@pytest.mark.parametrize(
+    "if_exists,num_sheets,expected",
+    [
+        ("new_sheet", 2, ["apple", "banana"]),
+        ("overwrite_sheet", 1, ["pear"]),
+        ("overwrite_cells", 1, ["pear", "banana"]),
+    ],
+)
+def test_if_exists_append_modes(ext, if_exists, num_sheets, expected):
+    # GH 40230
+    df1 = DataFrame({"fruit": ["apple", "banana"]})
+    df2 = DataFrame({"fruit": ["pear"]})
+
+    with tm.ensure_clean(ext) as f:
+        with pd.ExcelWriter(f, engine="openpyxl", mode="w") as writer:
+            df1.to_excel(writer, sheet_name="foo", index=False)
+        with pd.ExcelWriter(
+            f, engine="openpyxl", mode="a", if_exists=if_exists
+        ) as writer:
+            df2.to_excel(writer, sheet_name="foo", index=False)
+
+        wb = openpyxl.load_workbook(f)
+        assert len(wb.sheetnames) == num_sheets
+        assert wb.sheetnames[0] == "foo"
+        result = pd.read_excel(wb, "foo", engine="openpyxl")
+        assert list(result["fruit"]) == expected
+        if len(wb.sheetnames) == 2:
+            # atm the name given for the second sheet will be "foo1"
+            # but we don't want the test to fail if openpyxl changes this
+            result = pd.read_excel(wb, wb.sheetnames[1], engine="openpyxl")
+            assert result.equals(df2)
+        wb.close()
+
+
+def test_if_exists_raises(ext):
+    if_exists_msg = "if_exists is only valid in append mode (mode='a')"
+    invalid_msg = "'invalid' is not valid for if_exists"
+
+    with tm.ensure_clean(ext) as f:
+        with pytest.raises(ValueError, match=re.escape(if_exists_msg)):
+            ExcelWriter(f, engine="openpyxl", mode="w", if_exists="new_sheet")
+    with tm.ensure_clean(ext) as f:
+        with pytest.raises(ValueError, match=invalid_msg):
+            ExcelWriter(f, engine="openpyxl", mode="a", if_exists="invalid")
+
+
 def test_to_excel_with_openpyxl_engine(ext):
     # GH 29854
     with tm.ensure_clean(ext) as filename:

From e8716c7eda81dfba7a2485a4a35ca92fa7052861 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Fri, 5 Mar 2021 19:26:33 +0000
Subject: [PATCH 02/15] Rename parameter to if_sheet_exists

---
 doc/source/whatsnew/v1.3.0.rst         |  2 +-
 pandas/io/excel/_base.py               | 38 +++++++++++++------------
 pandas/io/excel/_openpyxl.py           | 16 +++++++----
 pandas/tests/io/excel/test_openpyxl.py | 39 +++++++++++++++++---------
 4 files changed, 56 insertions(+), 39 deletions(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 4e1bda437d5e4..6e4a22205b745 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -125,7 +125,7 @@ Other enhancements
 - :func:`to_numeric` now supports downcasting of nullable ``ExtensionDtype`` objects (:issue:`33013`)
 - Add support for dict-like names in :class:`MultiIndex.set_names` and :class:`MultiIndex.rename` (:issue:`20421`)
 - :func:`pandas.read_excel` can now auto detect .xlsb files (:issue:`35416`)
-- :class:`pandas.ExcelWriter` now accepts an ``if_exists`` parameter to control the behaviour of append mode when writing to existing sheets (:issue:`40230`)
+- :class:`pandas.ExcelWriter` now accepts an ``if_sheet_exists`` parameter to control the behaviour of append mode when writing to existing sheets (:issue:`40230`)
 - :meth:`.Rolling.sum`, :meth:`.Expanding.sum`, :meth:`.Rolling.mean`, :meth:`.Expanding.mean`, :meth:`.Rolling.median`, :meth:`.Expanding.median`, :meth:`.Rolling.max`, :meth:`.Expanding.max`, :meth:`.Rolling.min`, and :meth:`.Expanding.min` now support ``Numba`` execution with the ``engine`` keyword (:issue:`38895`)
 - :meth:`DataFrame.apply` can now accept NumPy unary operators as strings, e.g. ``df.apply("sqrt")``, which was already the case for :meth:`Series.apply` (:issue:`39116`)
 - :meth:`DataFrame.apply` can now accept non-callable DataFrame properties as strings, e.g. ``df.apply("size")``, which was already the case for :meth:`Series.apply` (:issue:`39116`)
diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index c777d9326382e..d96a65d574c49 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -666,14 +666,15 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         be parsed by ``fsspec``, e.g., starting "s3://", "gcs://".
 
         .. versionadded:: 1.2.0
-    if_exists : {'new_sheet', 'overwrite_sheet', 'overwrite_cells'}, default 'new_sheet'
-            How to behave when trying to write to a sheet that already
-            exists (append mode only).
+    if_sheet_exists : {'avoid', 'replace', 'overwrite', 'fail'}, default 'avoid'
+        How to behave when trying to write to a sheet that already
+        exists (append mode only).
 
-            * new_sheet: Create a new sheet with a different name.
-            * overwrite_sheet: Delete the contents of the sheet, then write to it.
-            * overwrite_cells: Write directly to the named sheet
-              without deleting the previous contents.
+        * avoid: Create a new sheet with a different name.
+        * replace: Delete the contents of the sheet before writing to it.
+        * overwrite: Write directly to the named sheet
+          without deleting the previous contents.
+        * fail: raise a ValueError.
 
         .. versionadded:: 1.3.0
 
@@ -844,7 +845,7 @@ def __init__(
         datetime_format=None,
         mode: str = "w",
         storage_options: StorageOptions = None,
-        if_exists: Optional[str] = None,
+        if_sheet_exists: Optional[str] = None,
         **engine_kwargs,
     ):
         # validate that this engine can handle the extension
@@ -879,17 +880,18 @@ def __init__(
 
         self.mode = mode
 
-        if if_exists and "r+" not in mode:
-            raise ValueError("if_exists is only valid in append mode (mode='a')")
-        if if_exists is not None and if_exists not in {
-            "new_sheet",
-            "overwrite_sheet",
-            "overwrite_cells",
+        if if_sheet_exists and "r+" not in mode:
+            raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
+        if if_sheet_exists is not None and if_sheet_exists not in {
+            "avoid",
+            "replace",
+            "overwrite",
+            "fail",
         }:
-            raise ValueError(f"'{if_exists}' is not valid for if_exists")
-        if if_exists is None and "r+" in mode:
-            if_exists = "new_sheet"
-        self.if_exists = if_exists
+            raise ValueError(f"'{if_sheet_exists}' is not valid for if_sheet_exists")
+        if if_sheet_exists is None and "r+" in mode:
+            if_sheet_exists = "avoid"
+        self.if_sheet_exists = if_sheet_exists
 
     def __fspath__(self):
         return getattr(self.handles.handle, "name", "")
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 7ae4e866bb235..8170797e98e7c 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -37,7 +37,7 @@ def __init__(
         engine=None,
         mode: str = "w",
         storage_options: StorageOptions = None,
-        if_exists: Optional[str] = None,
+        if_sheet_exists: Optional[str] = None,
         **engine_kwargs,
     ):
         # Use the openpyxl module as the Excel writer.
@@ -47,7 +47,7 @@ def __init__(
             path,
             mode=mode,
             storage_options=storage_options,
-            if_exists=if_exists,
+            if_sheet_exists=if_sheet_exists,
             **engine_kwargs,
         )
 
@@ -420,18 +420,22 @@ def write_cells(
 
         if sheet_name in self.sheets:
             if "r+" in self.mode:
-                if self.if_exists == "new_sheet":
+                if self.if_sheet_exists == "avoid":
                     wks = self.book.create_sheet()
                     # openpyxl will create a name for the new sheet by appending digits
                     wks.title = sheet_name
                     self.sheets[wks.title] = wks
-                elif self.if_exists == "overwrite_sheet":
+                elif self.if_sheet_exists == "replace":
                     wks = self.sheets[sheet_name]
                     wks.delete_cols(1, wks.max_column)
-                elif self.if_exists == "overwrite_cells" or self.if_exists is None:
+                elif self.if_sheet_exists == "overwrite":
                     wks = self.sheets[sheet_name]
+                elif self.if_sheet_exists == "fail":
+                    raise ValueError(f"Sheet '{sheet_name}' already exists.")
                 else:
-                    raise ValueError(f"'{self.if_exists}' is not valid for if_exists")
+                    raise ValueError(
+                        f"'{self.if_sheet_exists}' is not valid for if_sheet_exists"
+                    )
             else:
                 wks = self.sheets[sheet_name]
         else:
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 3fd7d6727d14b..06a6276fa3a60 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -111,23 +111,23 @@ def test_write_append_mode(ext, mode, expected):
 
 
 @pytest.mark.parametrize(
-    "if_exists,num_sheets,expected",
+    "if_sheet_exists,num_sheets,expected",
     [
-        ("new_sheet", 2, ["apple", "banana"]),
-        ("overwrite_sheet", 1, ["pear"]),
-        ("overwrite_cells", 1, ["pear", "banana"]),
+        ("avoid", 2, ["apple", "banana"]),
+        (None, 2, ["apple", "banana"]),
+        ("replace", 1, ["pear"]),
+        ("overwrite", 1, ["pear", "banana"]),
     ],
 )
-def test_if_exists_append_modes(ext, if_exists, num_sheets, expected):
+def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected):
     # GH 40230
     df1 = DataFrame({"fruit": ["apple", "banana"]})
     df2 = DataFrame({"fruit": ["pear"]})
 
     with tm.ensure_clean(ext) as f:
-        with pd.ExcelWriter(f, engine="openpyxl", mode="w") as writer:
-            df1.to_excel(writer, sheet_name="foo", index=False)
+        df1.to_excel(f, engine="openpyxl", sheet_name="foo", index=False)
         with pd.ExcelWriter(
-            f, engine="openpyxl", mode="a", if_exists=if_exists
+            f, engine="openpyxl", mode="a", if_sheet_exists=if_sheet_exists
         ) as writer:
             df2.to_excel(writer, sheet_name="foo", index=False)
 
@@ -144,16 +144,27 @@ def test_if_exists_append_modes(ext, if_exists, num_sheets, expected):
         wb.close()
 
 
-def test_if_exists_raises(ext):
-    if_exists_msg = "if_exists is only valid in append mode (mode='a')"
-    invalid_msg = "'invalid' is not valid for if_exists"
+def test_if_sheet_exists_raises(ext):
+    mode_msg = "if_sheet_exists is only valid in append mode (mode='a')"
+    invalid_msg = "'invalid' is not valid for if_sheet_exists"
+    fail_msg = "Sheet 'foo' already exists."
+    df = DataFrame({"fruit": ["pear"]})
 
     with tm.ensure_clean(ext) as f:
-        with pytest.raises(ValueError, match=re.escape(if_exists_msg)):
-            ExcelWriter(f, engine="openpyxl", mode="w", if_exists="new_sheet")
+        with pytest.raises(ValueError, match=re.escape(mode_msg)):
+            ExcelWriter(f, engine="openpyxl", mode="w", if_sheet_exists="new_sheet")
+
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=invalid_msg):
-            ExcelWriter(f, engine="openpyxl", mode="a", if_exists="invalid")
+            ExcelWriter(f, engine="openpyxl", mode="a", if_sheet_exists="invalid")
+
+    with tm.ensure_clean(ext) as f:
+        with pytest.raises(ValueError, match=fail_msg):
+            df.to_excel(f, "foo", engine="openpyxl")
+            with pd.ExcelWriter(
+                f, engine="openpyxl", mode="a", if_sheet_exists="fail"
+            ) as writer:
+                df.to_excel(writer, sheet_name="foo")
 
 
 def test_to_excel_with_openpyxl_engine(ext):

From 71104f6723e806253d85ee7245bb9fb2e496c5f3 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Sat, 6 Mar 2021 13:23:05 +0000
Subject: [PATCH 03/15] Update avoid -> new in ExcelWriter if_sheet_exists

---
 pandas/io/excel/_base.py                 | 16 ++++++----------
 pandas/io/excel/_openpyxl.py             |  2 +-
 pandas/tests/io/excel/test_openpyxl.py   |  4 ++--
 pandas/tests/io/excel/test_xlsxwriter.py |  4 ++++
 4 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index d96a65d574c49..f89aa3d7f55bb 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -666,11 +666,11 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         be parsed by ``fsspec``, e.g., starting "s3://", "gcs://".
 
         .. versionadded:: 1.2.0
-    if_sheet_exists : {'avoid', 'replace', 'overwrite', 'fail'}, default 'avoid'
+    if_sheet_exists : {'new', 'replace', 'overwrite', 'fail'}, default 'new'
         How to behave when trying to write to a sheet that already
         exists (append mode only).
 
-        * avoid: Create a new sheet with a different name.
+        * new: Create a new sheet with a different name.
         * replace: Delete the contents of the sheet before writing to it.
         * overwrite: Write directly to the named sheet
           without deleting the previous contents.
@@ -880,17 +880,13 @@ def __init__(
 
         self.mode = mode
 
+        ise_valid = [None, "new", "replace", "overwrite", "fail"]
+        if if_sheet_exists not in ise_valid:
+            raise ValueError(f"'{if_sheet_exists}' is not valid for if_sheet_exists")
         if if_sheet_exists and "r+" not in mode:
             raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
-        if if_sheet_exists is not None and if_sheet_exists not in {
-            "avoid",
-            "replace",
-            "overwrite",
-            "fail",
-        }:
-            raise ValueError(f"'{if_sheet_exists}' is not valid for if_sheet_exists")
         if if_sheet_exists is None and "r+" in mode:
-            if_sheet_exists = "avoid"
+            if_sheet_exists = "new"
         self.if_sheet_exists = if_sheet_exists
 
     def __fspath__(self):
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 8170797e98e7c..086ce2d72c0dd 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -420,7 +420,7 @@ def write_cells(
 
         if sheet_name in self.sheets:
             if "r+" in self.mode:
-                if self.if_sheet_exists == "avoid":
+                if self.if_sheet_exists in "new":
                     wks = self.book.create_sheet()
                     # openpyxl will create a name for the new sheet by appending digits
                     wks.title = sheet_name
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 06a6276fa3a60..1171beb048fa5 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -113,7 +113,7 @@ def test_write_append_mode(ext, mode, expected):
 @pytest.mark.parametrize(
     "if_sheet_exists,num_sheets,expected",
     [
-        ("avoid", 2, ["apple", "banana"]),
+        ("new", 2, ["apple", "banana"]),
         (None, 2, ["apple", "banana"]),
         ("replace", 1, ["pear"]),
         ("overwrite", 1, ["pear", "banana"]),
@@ -152,7 +152,7 @@ def test_if_sheet_exists_raises(ext):
 
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=re.escape(mode_msg)):
-            ExcelWriter(f, engine="openpyxl", mode="w", if_sheet_exists="new_sheet")
+            ExcelWriter(f, engine="openpyxl", mode="w", if_sheet_exists="new")
 
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=invalid_msg):
diff --git a/pandas/tests/io/excel/test_xlsxwriter.py b/pandas/tests/io/excel/test_xlsxwriter.py
index 6de378f6a3d3e..08514c6b39a4d 100644
--- a/pandas/tests/io/excel/test_xlsxwriter.py
+++ b/pandas/tests/io/excel/test_xlsxwriter.py
@@ -1,3 +1,4 @@
+import re
 import warnings
 
 import pytest
@@ -57,7 +58,10 @@ def test_column_format(ext):
 
 def test_write_append_mode_raises(ext):
     msg = "Append mode is not supported with xlsxwriter!"
+    ise_msg = "if_sheet_exists is only valid in append mode (mode='a')"
 
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=msg):
             ExcelWriter(f, engine="xlsxwriter", mode="a")
+        with pytest.raises(ValueError, match=re.escape(ise_msg)):
+            ExcelWriter(f, engine="xlsxwriter", if_sheet_exists="replace")

From ae5b385d2b0f207c074f997467ea215d6fd9c96a Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Sat, 6 Mar 2021 13:46:10 +0000
Subject: [PATCH 04/15] use assert_frame_equal instead of result.equals

---
 pandas/tests/io/excel/test_openpyxl.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 1171beb048fa5..b4c5bc907c0d4 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -140,7 +140,7 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
             # atm the name given for the second sheet will be "foo1"
             # but we don't want the test to fail if openpyxl changes this
             result = pd.read_excel(wb, wb.sheetnames[1], engine="openpyxl")
-            assert result.equals(df2)
+            tm.assert_frame_equal(result, df2)
         wb.close()
 
 

From 6baec67643923ae93b685bdec01105b51f1c0a94 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Sat, 6 Mar 2021 14:38:49 +0000
Subject: [PATCH 05/15] fix mypy error

---
 pandas/io/excel/_openpyxl.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 086ce2d72c0dd..3d663f49c35c6 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -420,7 +420,7 @@ def write_cells(
 
         if sheet_name in self.sheets:
             if "r+" in self.mode:
-                if self.if_sheet_exists in "new":
+                if self.if_sheet_exists == "new":
                     wks = self.book.create_sheet()
                     # openpyxl will create a name for the new sheet by appending digits
                     wks.title = sheet_name

From 3a2a54f475bfd55f47eb6dcbbdc046c02ab9c6d6 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 8 Mar 2021 19:27:16 +0000
Subject: [PATCH 06/15] Update docstring to better describe 'new' behaviour

---
 pandas/io/excel/_base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index f89aa3d7f55bb..bcf04b73cac5a 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -670,7 +670,7 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         How to behave when trying to write to a sheet that already
         exists (append mode only).
 
-        * new: Create a new sheet with a different name.
+        * new: Create a new sheet, with a name determined by the engine.
         * replace: Delete the contents of the sheet before writing to it.
         * overwrite: Write directly to the named sheet
           without deleting the previous contents.

From fcbab057eb44b783d5617e5edc272ca04e30876c Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 8 Mar 2021 19:32:53 +0000
Subject: [PATCH 07/15] Parameterize test

---
 pandas/tests/io/excel/test_openpyxl.py | 31 +++++++++++++-------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index b4c5bc907c0d4..ab0c2b78cb3cd 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -144,25 +144,26 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
         wb.close()
 
 
-def test_if_sheet_exists_raises(ext):
-    mode_msg = "if_sheet_exists is only valid in append mode (mode='a')"
-    invalid_msg = "'invalid' is not valid for if_sheet_exists"
-    fail_msg = "Sheet 'foo' already exists."
+@pytest.mark.parametrize(
+    "mode,if_sheet_exists,msg",
+    [
+        ("w", "new", "if_sheet_exists is only valid in append mode (mode='a')"),
+        ("a", "invalid", "'invalid' is not valid for if_sheet_exists"),
+        (
+            "a",
+            "fail",
+            "Sheet 'foo' already exists and if_sheet_exists is set to 'fail'.",
+        ),
+    ],
+)
+def test_if_sheet_exists_raises(ext, mode, if_sheet_exists, msg):
+    # GH 40230
     df = DataFrame({"fruit": ["pear"]})
-
-    with tm.ensure_clean(ext) as f:
-        with pytest.raises(ValueError, match=re.escape(mode_msg)):
-            ExcelWriter(f, engine="openpyxl", mode="w", if_sheet_exists="new")
-
-    with tm.ensure_clean(ext) as f:
-        with pytest.raises(ValueError, match=invalid_msg):
-            ExcelWriter(f, engine="openpyxl", mode="a", if_sheet_exists="invalid")
-
     with tm.ensure_clean(ext) as f:
-        with pytest.raises(ValueError, match=fail_msg):
+        with pytest.raises(ValueError, match=re.escape(msg)):
             df.to_excel(f, "foo", engine="openpyxl")
             with pd.ExcelWriter(
-                f, engine="openpyxl", mode="a", if_sheet_exists="fail"
+                f, engine="openpyxl", mode=mode, if_sheet_exists=if_sheet_exists
             ) as writer:
                 df.to_excel(writer, sheet_name="foo")
 

From 5bcd6e3731e277fca26a714e6f9bd8be9554263e Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 8 Mar 2021 19:35:45 +0000
Subject: [PATCH 08/15] Add raises test for other engines

---
 pandas/tests/io/excel/test_writers.py    | 25 ++++++++++++++++++++++++
 pandas/tests/io/excel/test_xlsxwriter.py |  4 ----
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/io/excel/test_writers.py b/pandas/tests/io/excel/test_writers.py
index d8448736c7cc8..ea20857a5bc62 100644
--- a/pandas/tests/io/excel/test_writers.py
+++ b/pandas/tests/io/excel/test_writers.py
@@ -6,6 +6,7 @@
 from functools import partial
 from io import BytesIO
 import os
+import re
 
 import numpy as np
 import pytest
@@ -1398,3 +1399,27 @@ def test_excelwriter_fspath(self):
         with tm.ensure_clean("foo.xlsx") as path:
             with ExcelWriter(path) as writer:
                 assert os.fspath(writer) == str(path)
+
+
+@pytest.mark.parametrize(
+    "engine,ext",
+    [
+        pytest.param(
+            "xlwt", ".xls", marks=[td.skip_if_no("xlwt"), td.skip_if_no("xlrd")]
+        ),
+        pytest.param(
+            "xlsxwriter",
+            ".xlsx",
+            marks=[td.skip_if_no("xlsxwriter"), td.skip_if_no("xlrd")],
+        ),
+        pytest.param("odf", ".ods", marks=td.skip_if_no("odf")),
+    ],
+)
+@pytest.mark.usefixtures("set_engine")
+def test_if_sheet_exists_raises(ext):
+    # GH 40230
+    msg = "if_sheet_exists is only valid in append mode (mode='a')"
+
+    with tm.ensure_clean(ext) as f:
+        with pytest.raises(ValueError, match=re.escape(msg)):
+            ExcelWriter(f, if_sheet_exists="replace")
diff --git a/pandas/tests/io/excel/test_xlsxwriter.py b/pandas/tests/io/excel/test_xlsxwriter.py
index 08514c6b39a4d..6de378f6a3d3e 100644
--- a/pandas/tests/io/excel/test_xlsxwriter.py
+++ b/pandas/tests/io/excel/test_xlsxwriter.py
@@ -1,4 +1,3 @@
-import re
 import warnings
 
 import pytest
@@ -58,10 +57,7 @@ def test_column_format(ext):
 
 def test_write_append_mode_raises(ext):
     msg = "Append mode is not supported with xlsxwriter!"
-    ise_msg = "if_sheet_exists is only valid in append mode (mode='a')"
 
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=msg):
             ExcelWriter(f, engine="xlsxwriter", mode="a")
-        with pytest.raises(ValueError, match=re.escape(ise_msg)):
-            ExcelWriter(f, engine="xlsxwriter", if_sheet_exists="replace")

From 47527c6abfe17f8159cbb5afb6ca0c130b683fcd Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 8 Mar 2021 20:02:23 +0000
Subject: [PATCH 09/15] More detailed error message

---
 pandas/io/excel/_base.py               | 5 ++++-
 pandas/io/excel/_openpyxl.py           | 3 ++-
 pandas/tests/io/excel/test_openpyxl.py | 7 ++++++-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index bcf04b73cac5a..faa711cc7f020 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -882,7 +882,10 @@ def __init__(
 
         ise_valid = [None, "new", "replace", "overwrite", "fail"]
         if if_sheet_exists not in ise_valid:
-            raise ValueError(f"'{if_sheet_exists}' is not valid for if_sheet_exists")
+            raise ValueError(
+                f"'{if_sheet_exists}' is not valid for if_sheet_exists. "
+                "Valid options are 'new', 'replace', 'overwrite' and 'fail'."
+            )
         if if_sheet_exists and "r+" not in mode:
             raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
         if if_sheet_exists is None and "r+" in mode:
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 3d663f49c35c6..c56e973b8663f 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -434,7 +434,8 @@ def write_cells(
                     raise ValueError(f"Sheet '{sheet_name}' already exists.")
                 else:
                     raise ValueError(
-                        f"'{self.if_sheet_exists}' is not valid for if_sheet_exists"
+                        f"'{self.if_sheet_exists}' is not valid for if_sheet_exists. "
+                        "Valid options are 'new', 'replace', 'overwrite' and 'fail'."
                     )
             else:
                 wks = self.sheets[sheet_name]
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index ab0c2b78cb3cd..c0af852a074c8 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -148,7 +148,12 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
     "mode,if_sheet_exists,msg",
     [
         ("w", "new", "if_sheet_exists is only valid in append mode (mode='a')"),
-        ("a", "invalid", "'invalid' is not valid for if_sheet_exists"),
+        (
+            "a",
+            "invalid",
+            "'invalid' is not valid for if_sheet_exists. Valid options "
+            "are 'new', 'replace', 'overwrite' and 'fail'.",
+        ),
         (
             "a",
             "fail",

From 060d754fded57eb32c95aebca7b9e91cdbef931a Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Mon, 8 Mar 2021 20:03:30 +0000
Subject: [PATCH 10/15] Remove redundant code, delete sheet when replacing

---
 pandas/io/excel/_openpyxl.py | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index c56e973b8663f..4a5d6c787c687 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -418,20 +418,21 @@ def write_cells(
 
         _style_cache: Dict[str, Dict[str, Serialisable]] = {}
 
-        if sheet_name in self.sheets:
+        if sheet_name in self.sheets and self.if_sheet_exists != "new":
             if "r+" in self.mode:
-                if self.if_sheet_exists == "new":
-                    wks = self.book.create_sheet()
-                    # openpyxl will create a name for the new sheet by appending digits
-                    wks.title = sheet_name
-                    self.sheets[wks.title] = wks
-                elif self.if_sheet_exists == "replace":
-                    wks = self.sheets[sheet_name]
-                    wks.delete_cols(1, wks.max_column)
+                if self.if_sheet_exists == "replace":
+                    old_wks = self.sheets[sheet_name]
+                    target_index = self.book.index(old_wks)
+                    del self.book[sheet_name]
+                    wks = self.book.create_sheet(sheet_name, target_index)
+                    self.sheets[sheet_name] = wks
                 elif self.if_sheet_exists == "overwrite":
                     wks = self.sheets[sheet_name]
                 elif self.if_sheet_exists == "fail":
-                    raise ValueError(f"Sheet '{sheet_name}' already exists.")
+                    raise ValueError(
+                        f"Sheet '{sheet_name}' already exists and "
+                        f"if_sheet_exists is set to 'fail'."
+                    )
                 else:
                     raise ValueError(
                         f"'{self.if_sheet_exists}' is not valid for if_sheet_exists. "

From d8f0be1dbc2ab43cf7454842be560b03a85d18d7 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Tue, 9 Mar 2021 17:38:22 +0000
Subject: [PATCH 11/15] Tidy up raise tests

---
 pandas/tests/io/excel/test_openpyxl.py |  9 +++-----
 pandas/tests/io/excel/test_writers.py  | 32 +++++++-------------------
 2 files changed, 11 insertions(+), 30 deletions(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index c0af852a074c8..8f00916d1bb4c 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -145,30 +145,27 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
 
 
 @pytest.mark.parametrize(
-    "mode,if_sheet_exists,msg",
+    "if_sheet_exists,msg",
     [
-        ("w", "new", "if_sheet_exists is only valid in append mode (mode='a')"),
         (
-            "a",
             "invalid",
             "'invalid' is not valid for if_sheet_exists. Valid options "
             "are 'new', 'replace', 'overwrite' and 'fail'.",
         ),
         (
-            "a",
             "fail",
             "Sheet 'foo' already exists and if_sheet_exists is set to 'fail'.",
         ),
     ],
 )
-def test_if_sheet_exists_raises(ext, mode, if_sheet_exists, msg):
+def test_if_sheet_exists_raises(ext, if_sheet_exists, msg):
     # GH 40230
     df = DataFrame({"fruit": ["pear"]})
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=re.escape(msg)):
             df.to_excel(f, "foo", engine="openpyxl")
             with pd.ExcelWriter(
-                f, engine="openpyxl", mode=mode, if_sheet_exists=if_sheet_exists
+                f, engine="openpyxl", mode="a", if_sheet_exists=if_sheet_exists
             ) as writer:
                 df.to_excel(writer, sheet_name="foo")
 
diff --git a/pandas/tests/io/excel/test_writers.py b/pandas/tests/io/excel/test_writers.py
index ea20857a5bc62..2c3a7a9648cf1 100644
--- a/pandas/tests/io/excel/test_writers.py
+++ b/pandas/tests/io/excel/test_writers.py
@@ -1325,6 +1325,14 @@ def test_excel_duplicate_columns_with_names(self, path):
         expected = DataFrame([[0, 10, 0], [1, 11, 1]], columns=["A", "B", "A.1"])
         tm.assert_frame_equal(result, expected)
 
+    def test_if_sheet_exists_raises(self, ext):
+        # GH 40230
+        msg = "if_sheet_exists is only valid in append mode (mode='a')"
+
+        with tm.ensure_clean(ext) as f:
+            with pytest.raises(ValueError, match=re.escape(msg)):
+                ExcelWriter(f, if_sheet_exists="replace")
+
 
 class TestExcelWriterEngineTests:
     @pytest.mark.parametrize(
@@ -1399,27 +1407,3 @@ def test_excelwriter_fspath(self):
         with tm.ensure_clean("foo.xlsx") as path:
             with ExcelWriter(path) as writer:
                 assert os.fspath(writer) == str(path)
-
-
-@pytest.mark.parametrize(
-    "engine,ext",
-    [
-        pytest.param(
-            "xlwt", ".xls", marks=[td.skip_if_no("xlwt"), td.skip_if_no("xlrd")]
-        ),
-        pytest.param(
-            "xlsxwriter",
-            ".xlsx",
-            marks=[td.skip_if_no("xlsxwriter"), td.skip_if_no("xlrd")],
-        ),
-        pytest.param("odf", ".ods", marks=td.skip_if_no("odf")),
-    ],
-)
-@pytest.mark.usefixtures("set_engine")
-def test_if_sheet_exists_raises(ext):
-    # GH 40230
-    msg = "if_sheet_exists is only valid in append mode (mode='a')"
-
-    with tm.ensure_clean(ext) as f:
-        with pytest.raises(ValueError, match=re.escape(msg)):
-            ExcelWriter(f, if_sheet_exists="replace")

From e31a59edd30e25f6242a152192975397bbc9dd4f Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Sat, 13 Mar 2021 17:07:39 +0000
Subject: [PATCH 12/15] Fix inconsistent namespacing

---
 pandas/tests/io/excel/test_openpyxl.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 8f00916d1bb4c..8366b890e7fe3 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -126,7 +126,7 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
 
     with tm.ensure_clean(ext) as f:
         df1.to_excel(f, engine="openpyxl", sheet_name="foo", index=False)
-        with pd.ExcelWriter(
+        with ExcelWriter(
             f, engine="openpyxl", mode="a", if_sheet_exists=if_sheet_exists
         ) as writer:
             df2.to_excel(writer, sheet_name="foo", index=False)
@@ -164,7 +164,7 @@ def test_if_sheet_exists_raises(ext, if_sheet_exists, msg):
     with tm.ensure_clean(ext) as f:
         with pytest.raises(ValueError, match=re.escape(msg)):
             df.to_excel(f, "foo", engine="openpyxl")
-            with pd.ExcelWriter(
+            with ExcelWriter(
                 f, engine="openpyxl", mode="a", if_sheet_exists=if_sheet_exists
             ) as writer:
                 df.to_excel(writer, sheet_name="foo")

From 338b6d5e8186af4f07221b084eeb868a730a2cee Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Sun, 21 Mar 2021 15:38:43 +0000
Subject: [PATCH 13/15] Rename fail to error

---
 pandas/io/excel/_base.py               | 10 +++++-----
 pandas/io/excel/_openpyxl.py           |  6 +++---
 pandas/tests/io/excel/test_openpyxl.py | 15 ++++++++++-----
 3 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index a3932773a14e3..5526ad1109a81 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -667,7 +667,7 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         be parsed by ``fsspec``, e.g., starting "s3://", "gcs://".
 
         .. versionadded:: 1.2.0
-    if_sheet_exists : {'new', 'replace', 'overwrite', 'fail'}, default 'new'
+    if_sheet_exists : {'new', 'replace', 'overwrite', 'error'}, default 'error'
         How to behave when trying to write to a sheet that already
         exists (append mode only).
 
@@ -675,7 +675,7 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         * replace: Delete the contents of the sheet before writing to it.
         * overwrite: Write directly to the named sheet
           without deleting the previous contents.
-        * fail: raise a ValueError.
+        * error: raise a ValueError.
 
         .. versionadded:: 1.3.0
     engine_kwargs : dict, optional
@@ -911,16 +911,16 @@ def __init__(
 
         self.mode = mode
 
-        ise_valid = [None, "new", "replace", "overwrite", "fail"]
+        ise_valid = [None, "new", "replace", "overwrite", "error"]
         if if_sheet_exists not in ise_valid:
             raise ValueError(
                 f"'{if_sheet_exists}' is not valid for if_sheet_exists. "
-                "Valid options are 'new', 'replace', 'overwrite' and 'fail'."
+                "Valid options are 'new', 'replace', 'overwrite' and 'error'."
             )
         if if_sheet_exists and "r+" not in mode:
             raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
         if if_sheet_exists is None and "r+" in mode:
-            if_sheet_exists = "new"
+            if_sheet_exists = "error"
         self.if_sheet_exists = if_sheet_exists
 
     def __fspath__(self):
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 038b7f97374a9..2bfc2a88599f7 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -431,15 +431,15 @@ def write_cells(
                     self.sheets[sheet_name] = wks
                 elif self.if_sheet_exists == "overwrite":
                     wks = self.sheets[sheet_name]
-                elif self.if_sheet_exists == "fail":
+                elif self.if_sheet_exists == "error":
                     raise ValueError(
                         f"Sheet '{sheet_name}' already exists and "
-                        f"if_sheet_exists is set to 'fail'."
+                        f"if_sheet_exists is set to 'error'."
                     )
                 else:
                     raise ValueError(
                         f"'{self.if_sheet_exists}' is not valid for if_sheet_exists. "
-                        "Valid options are 'new', 'replace', 'overwrite' and 'fail'."
+                        "Valid options are 'new', 'replace', 'overwrite' and 'error'."
                     )
             else:
                 wks = self.sheets[sheet_name]
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 8366b890e7fe3..a03400ba03550 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -114,7 +114,6 @@ def test_write_append_mode(ext, mode, expected):
     "if_sheet_exists,num_sheets,expected",
     [
         ("new", 2, ["apple", "banana"]),
-        (None, 2, ["apple", "banana"]),
         ("replace", 1, ["pear"]),
         ("overwrite", 1, ["pear", "banana"]),
     ],
@@ -150,11 +149,15 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
         (
             "invalid",
             "'invalid' is not valid for if_sheet_exists. Valid options "
-            "are 'new', 'replace', 'overwrite' and 'fail'.",
+            "are 'new', 'replace', 'overwrite' and 'error'.",
         ),
         (
-            "fail",
-            "Sheet 'foo' already exists and if_sheet_exists is set to 'fail'.",
+            "error",
+            "Sheet 'foo' already exists and if_sheet_exists is set to 'error'.",
+        ),
+        (
+            None,
+            "Sheet 'foo' already exists and if_sheet_exists is set to 'error'.",
         ),
     ],
 )
@@ -236,7 +239,9 @@ def test_append_mode_file(ext):
     with tm.ensure_clean(ext) as f:
         df.to_excel(f, engine="openpyxl")
 
-        with ExcelWriter(f, mode="a", engine="openpyxl") as writer:
+        with ExcelWriter(
+            f, mode="a", engine="openpyxl", if_sheet_exists="new"
+        ) as writer:
             df.to_excel(writer)
 
         # make sure that zip files are not concatenated by making sure that

From b2ad90bc0de7d4781b561cbe14459b4de9c027c8 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Tue, 13 Apr 2021 20:22:43 +0100
Subject: [PATCH 14/15] Remove overwrite option

---
 pandas/io/excel/_base.py               | 12 +++++-------
 pandas/io/excel/_openpyxl.py           |  4 +---
 pandas/tests/io/excel/test_openpyxl.py |  5 +----
 3 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index 3fff4124caa91..1536c760cd4b8 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -664,15 +664,13 @@ class ExcelWriter(metaclass=abc.ABCMeta):
         be parsed by ``fsspec``, e.g., starting "s3://", "gcs://".
 
         .. versionadded:: 1.2.0
-    if_sheet_exists : {'new', 'replace', 'overwrite', 'error'}, default 'error'
+    if_sheet_exists : {'error', 'new', 'replace'}, default 'error'
         How to behave when trying to write to a sheet that already
         exists (append mode only).
 
+        * error: raise a ValueError.
         * new: Create a new sheet, with a name determined by the engine.
         * replace: Delete the contents of the sheet before writing to it.
-        * overwrite: Write directly to the named sheet
-          without deleting the previous contents.
-        * error: raise a ValueError.
 
         .. versionadded:: 1.3.0
     engine_kwargs : dict, optional
@@ -771,6 +769,7 @@ def __new__(
         datetime_format=None,
         mode: str = "w",
         storage_options: StorageOptions = None,
+        if_sheet_exists: str | None = None,
         engine_kwargs: dict | None = None,
         **kwargs,
     ):
@@ -908,11 +907,10 @@ def __init__(
 
         self.mode = mode
 
-        ise_valid = [None, "new", "replace", "overwrite", "error"]
-        if if_sheet_exists not in ise_valid:
+        if if_sheet_exists not in [None, "error", "new", "replace"]:
             raise ValueError(
                 f"'{if_sheet_exists}' is not valid for if_sheet_exists. "
-                "Valid options are 'new', 'replace', 'overwrite' and 'error'."
+                "Valid options are 'error', 'new' and 'replace'."
             )
         if if_sheet_exists and "r+" not in mode:
             raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 555dd6cca9671..a99f8e2625602 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -426,8 +426,6 @@ def write_cells(
                     del self.book[sheet_name]
                     wks = self.book.create_sheet(sheet_name, target_index)
                     self.sheets[sheet_name] = wks
-                elif self.if_sheet_exists == "overwrite":
-                    wks = self.sheets[sheet_name]
                 elif self.if_sheet_exists == "error":
                     raise ValueError(
                         f"Sheet '{sheet_name}' already exists and "
@@ -436,7 +434,7 @@ def write_cells(
                 else:
                     raise ValueError(
                         f"'{self.if_sheet_exists}' is not valid for if_sheet_exists. "
-                        "Valid options are 'new', 'replace', 'overwrite' and 'error'."
+                        "Valid options are 'error', 'new' and 'replace'."
                     )
             else:
                 wks = self.sheets[sheet_name]
diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index a03400ba03550..62f567457c3ab 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -115,7 +115,6 @@ def test_write_append_mode(ext, mode, expected):
     [
         ("new", 2, ["apple", "banana"]),
         ("replace", 1, ["pear"]),
-        ("overwrite", 1, ["pear", "banana"]),
     ],
 )
 def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected):
@@ -136,8 +135,6 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
         result = pd.read_excel(wb, "foo", engine="openpyxl")
         assert list(result["fruit"]) == expected
         if len(wb.sheetnames) == 2:
-            # atm the name given for the second sheet will be "foo1"
-            # but we don't want the test to fail if openpyxl changes this
             result = pd.read_excel(wb, wb.sheetnames[1], engine="openpyxl")
             tm.assert_frame_equal(result, df2)
         wb.close()
@@ -149,7 +146,7 @@ def test_if_sheet_exists_append_modes(ext, if_sheet_exists, num_sheets, expected
         (
             "invalid",
             "'invalid' is not valid for if_sheet_exists. Valid options "
-            "are 'new', 'replace', 'overwrite' and 'error'.",
+            "are 'error', 'new' and 'replace'.",
         ),
         (
             "error",

From 49c4d3be5f5e614d45c63443e152c7b2d06e4b24 Mon Sep 17 00:00:00 2001
From: Mike Roberts <42875462+mrob95@users.noreply.github.com>
Date: Wed, 21 Apr 2021 20:53:20 +0100
Subject: [PATCH 15/15] Remove redundant mode check

---
 pandas/io/excel/_base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index 1536c760cd4b8..5796d77a2d027 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -914,7 +914,7 @@ def __init__(
             )
         if if_sheet_exists and "r+" not in mode:
             raise ValueError("if_sheet_exists is only valid in append mode (mode='a')")
-        if if_sheet_exists is None and "r+" in mode:
+        if if_sheet_exists is None:
             if_sheet_exists = "error"
         self.if_sheet_exists = if_sheet_exists