How to save a new sheet in an existing excel file, using Pandas?

Question

Thank you. I believe that a complete example could be good for anyone else who have the same issue:

import pandas as pd
import numpy as np

path = r"C:\Users\fedel\Desktop\excelData\PhD_data.xlsx"

x1 = np.random.randn(100, 2)
df1 = pd.DataFrame(x1)

x2 = np.random.randn(100, 2)
df2 = pd.DataFrame(x2)

writer = pd.ExcelWriter(path, engine = 'xlsxwriter')
df1.to_excel(writer, sheet_name = 'x1')
df2.to_excel(writer, sheet_name = 'x2')
writer.save()
writer.close()

Here I generate an excel file, from my understanding it does not really matter whether it is generated via the "xslxwriter" or the "openpyxl" engine.

When I want to write without loosing the original data then

import pandas as pd
import numpy as np
from openpyxl import load_workbook

path = r"C:\Users\fedel\Desktop\excelData\PhD_data.xlsx"

book = load_workbook(path)
writer = pd.ExcelWriter(path, engine = 'openpyxl')
writer.book = book

x3 = np.random.randn(100, 2)
df3 = pd.DataFrame(x3)

x4 = np.random.randn(100, 2)
df4 = pd.DataFrame(x4)

df3.to_excel(writer, sheet_name = 'x3')
df4.to_excel(writer, sheet_name = 'x4')
writer.save()
writer.close()

this code do the job!

In the example you shared you are loading the existing file into book and setting the writer.book value to be book. In the line writer.sheets = dict((ws.title, ws) for ws in book.worksheets) you are accessing each sheet in the workbook as ws. The sheet title is then ws so you are creating a dictionary of {sheet_titles: sheet} key, value pairs. This dictionary is then set to writer.sheets. Essentially these steps are just loading the existing data from 'Masterfile.xlsx' and populating your writer with them.

Now let's say you already have a file with x1 and x2 as sheets. You can use the example code to load the file and then could do something like this to add x3 and x4.

path = r"C:\Users\fedel\Desktop\excelData\PhD_data.xlsx"
writer = pd.ExcelWriter(path, engine='openpyxl')
df3.to_excel(writer, 'x3', index=False)
df4.to_excel(writer, 'x4', index=False)
writer.save()

That should do what you are looking for.

A simple example for writing multiple data to excel at a time. And also when you want to append data to a sheet on a written excel file (closed excel file).

When it is your first time writing to an excel. (Writing "df1" and "df2" to "1st_sheet" and "2nd_sheet")

import pandas as pd 
from openpyxl import load_workbook

df1 = pd.DataFrame([[1],[1]], columns=['a'])
df2 = pd.DataFrame([[2],[2]], columns=['b'])
df3 = pd.DataFrame([[3],[3]], columns=['c'])

excel_dir = "my/excel/dir"

with pd.ExcelWriter(excel_dir, engine='xlsxwriter') as writer:    
    df1.to_excel(writer, '1st_sheet')   
    df2.to_excel(writer, '2nd_sheet')   
    writer.save()    

After you close your excel, but you wish to "append" data on the same excel file but another sheet, let's say "df3" to sheet name "3rd_sheet".

book = load_workbook(excel_dir)
with pd.ExcelWriter(excel_dir, engine='openpyxl') as writer:
    writer.book = book
    writer.sheets = dict((ws.title, ws) for ws in book.worksheets)    

    ## Your dataframe to append. 
    df3.to_excel(writer, '3rd_sheet')  

    writer.save()     

Be noted that excel format must not be xls, you may use xlsx one.

For creating a new file

x1 = np.random.randn(100, 2)
df1 = pd.DataFrame(x1)
with pd.ExcelWriter('sample.xlsx') as writer:  
    df1.to_excel(writer, sheet_name='x1')

For appending to the file, use the argument mode='a' in pd.ExcelWriter.

x2 = np.random.randn(100, 2)
df2 = pd.DataFrame(x2)
with pd.ExcelWriter('sample.xlsx', engine='openpyxl', mode='a') as writer:  
    df2.to_excel(writer, sheet_name='x2')

Default is mode ='w'. See documentation.