How to write to an open Excel file using Python?

If you're a Windows user there is a very easy way to do this. If we use the Win32 Library we can leverage the built-in Excel Object VBA model.

Now, I am not sure exactly how your data looks or where you want it in the workbook but I'll just assume you want it on the sheet that appears when you open the workbook.

For example, let's imagine I have a Panda's DataFrame that I want to write to an open Excel Workbook. It would like the following:

import win32com.client
import pandas as pd

# Create an instance of the Excel Application & make it visible.
ExcelApp = win32com.client.GetActiveObject("Excel.Application")
ExcelApp.Visible = True

# Open the desired workbook
workbook = ExcelApp.Workbooks.Open(r"<FILE_PATH>")

# Take the data frame object and convert it to a recordset array
rec_array = data_frame.to_records()

# Convert the Recordset Array to a list. This is because Excel doesn't recognize 
# Numpy datatypes.
rec_array = rec_array.tolist()

# It will look something like this now.
# [(1, 'Apple', Decimal('2'), 4.0), (2, 'Orange', Decimal('3'), 5.0), (3, 'Peach', 
# Decimal('5'), 5.0), (4, 'Pear', Decimal('6'), 5.0)]

# set the value property equal to the record array.
ExcelApp.Range("F2:I5").Value = rec_array

Again, there are a lot of things we have to keep in mind as to where we want it pasted, how the data is formatted and a whole host of other issues. However, at the end of the day, it is possible to write to an open Excel file using Python if you're a Windows' user.

Generally, two different processes shouldn't not be writing to the same file because it will cause synchronization issues.

A better way would be to close the existing file in parent process (aka VBA code) and pass the location of the workbook to python script.

The python script will open it and write the contents in the cell and exit.

No this is not possible because Excel files do not support concurrent access.