From d13c2bccfbe7ad4475ce7a2a9da45b76c36d9697 Mon Sep 17 00:00:00 2001
From: Zoynels <5841616+Zoynels@users.noreply.github.com>
Date: Sat, 3 Aug 2019 22:07:05 +0600
Subject: [PATCH 1/3] Add: update dict of sheets before check

```python
import pandas as pd

df = pd.DataFrame(
    {
        "id": ["1", "2", "3", "4", "5"],
        "Feature1": ["A", "C", "E", "G", "I"],
        "Feature2": ["B", "D", "F", "H", "J"],
    },
    columns=["id", "Feature1", "Feature2"],
)
writer = pd.ExcelWriter(path="testOutput.xlsx", mode="a", engine="openpyxl")
df.to_excel(writer, sheet_name="Main")
writer.save()
writer.close()
```
#### Problem description

I have excel file with existing sheet "Main", and if I try to append some dataframe to this sheet, on first df.to_excel() pandas creates new sheet with name "Main1" and saves information to this new sheet. But if once to_excel(), than pandas add new sheet to dictionary and saves to existing sheet. So, on start appen pandas don't know nothing about existing sheets, and check in **_openpyxl.py** file don't realy work
```python
        if sheet_name in self.sheets:
            wks = self.sheets[sheet_name]
        else:
            wks = self.book.create_sheet()
            wks.title = sheet_name
            self.sheets[sheet_name] = wks
```
so I add new code to update sheetname-list before check.
---
 pandas/io/excel/_openpyxl.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index d8f5da5ab5bc6..a0ff1bf640278 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -397,6 +397,11 @@ def write_cells(
 
         _style_cache = {}
 
+        # Update sheet list
+        self.sheets = {}
+        for wks in self.book.worksheets:
+            self.sheets[wks.title] = wks
+
         if sheet_name in self.sheets:
             wks = self.sheets[sheet_name]
         else:

From 09f31bc780e41996e91713822368b8f1771e9e6f Mon Sep 17 00:00:00 2001
From: Zoynels <5841616+Zoynels@users.noreply.github.com>
Date: Wed, 7 Aug 2019 01:24:20 +0600
Subject: [PATCH 2/3] add tests

---
 pandas/tests/io/excel/test_openpyxl.py | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 79fc87a62ad08..87c6f3a0ce198 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -78,10 +78,11 @@ def test_write_cells_merge_styled(ext):
 
 
 @pytest.mark.parametrize(
-    "mode,expected", [("w", ["baz"]), ("a", ["foo", "bar", "baz"])]
+    "mode,expected", [("w", ["new_sheet"]), ("a", ["foo", "bar", "existing_sheet", "new_sheet"])]
 )
 def test_write_append_mode(ext, mode, expected):
-    df = DataFrame([1], columns=["baz"])
+    df_new_sheet = DataFrame([1], columns=["new_sheet"])
+    df_existing_sheet = DataFrame([1], columns=["existing_sheet"])
 
     with ensure_clean(ext) as f:
         wb = openpyxl.Workbook()
@@ -89,10 +90,13 @@ def test_write_append_mode(ext, mode, expected):
         wb.worksheets[0]["A1"].value = "foo"
         wb.create_sheet("bar")
         wb.worksheets[1]["A1"].value = "bar"
+        wb.create_sheet("existing_sheet")
         wb.save(f)
 
         writer = ExcelWriter(f, engine="openpyxl", mode=mode)
-        df.to_excel(writer, sheet_name="baz", index=False)
+        if mode == "a":
+            df_existing_sheet.to_excel(writer, sheet_name="existing_sheet", index=False)
+        df_new_sheet.to_excel(writer, sheet_name="new_sheet", index=False)
         writer.save()
 
         wb2 = openpyxl.load_workbook(f)

From 419e29417b688d03fee9b94ac707967fc9026596 Mon Sep 17 00:00:00 2001
From: Zoynels <5841616+Zoynels@users.noreply.github.com>
Date: Wed, 7 Aug 2019 19:24:44 +0600
Subject: [PATCH 3/3] PEP 8 change

---
 pandas/tests/io/excel/test_openpyxl.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/io/excel/test_openpyxl.py b/pandas/tests/io/excel/test_openpyxl.py
index 87c6f3a0ce198..c19376b1a3438 100644
--- a/pandas/tests/io/excel/test_openpyxl.py
+++ b/pandas/tests/io/excel/test_openpyxl.py
@@ -78,7 +78,8 @@ def test_write_cells_merge_styled(ext):
 
 
 @pytest.mark.parametrize(
-    "mode,expected", [("w", ["new_sheet"]), ("a", ["foo", "bar", "existing_sheet", "new_sheet"])]
+    "mode,expected", [("w", ["new_sheet"]),
+                      ("a", ["foo", "bar", "existing_sheet", "new_sheet"])]
 )
 def test_write_append_mode(ext, mode, expected):
     df_new_sheet = DataFrame([1], columns=["new_sheet"])