Pandas Excel Writer using Openpyxl with existing workbook

Question

I have code from a while ago that I am re-using for a new task. The task is to write a new DataFrame into a new sheet, into an existing excel file. But there is one part of the code that I do not understand, but it just makes the code "work".

working:

from openpyxl import load_workbook
import pandas as pd
file = r'YOUR_PATH_TO_EXCEL_HERE'

df1 = pd.DataFrame({'Data': [10, 20, 30, 20, 15, 30, 45]})
book = load_workbook(file)
writer = pd.ExcelWriter(file, engine='openpyxl')
writer.book = book # <---------------------------- piece i do not understand
df1.to_excel(writer, sheet_name='New', index=None)
writer.save()

The little line of writer.book=book has me stumped. Without that piece of code, the Excel file will delete all other sheets, except the sheet used in the sheetname= parameter in df1.to_excel.

i looked at xlsxwriter's documentation as well as openpyxl's, but cannot seem to figure out why that line gives me my expected output. Any ideas?

edit: i believe this post is where i got the original idea from.

Solution

In the source code of ExcelWriter, with openpyxl, it initializes empty workbook and delete all sheets. That's why you need to add it explicitly

class _OpenpyxlWriter(ExcelWriter):
    engine = 'openpyxl'
    supported_extensions = ('.xlsx', '.xlsm')

    def __init__(self, path, engine=None, **engine_kwargs):
        # Use the openpyxl module as the Excel writer.
        from openpyxl.workbook import Workbook

        super(_OpenpyxlWriter, self).__init__(path, **engine_kwargs)

        # Create workbook object with default optimized_write=True.
        self.book = Workbook()

        # Openpyxl 1.6.1 adds a dummy sheet. We remove it.
        if self.book.worksheets:
            try:
                self.book.remove(self.book.worksheets[0])
            except AttributeError:

                # compat
                self.book.remove_sheet(self.book.worksheets[0])