Excel Formulas that Disappear: A Python Perspective

Introduction

In this article, we will delve into the world of Excel formulas and explore why they sometimes disappear. We’ll examine a Stack Overflow post that highlights the issue and provide a step-by-step guide on how to process Excel data with Python while dealing with missing formulas.

Understanding Excel Formulas

Excel formulas are used to perform calculations and manipulate data within an Excel worksheet. These formulas can be text-based, mathematical, or a combination of both. When you create a formula in Excel, it is stored as part of the cell’s value. However, when you open the formula in a spreadsheet editor like Microsoft Excel or Google Sheets, the formula might not display.

The Issue at Hand

The user in the Stack Overflow post has written Python code to process an Excel file containing formulas. The code works initially but fails to detect formulas on subsequent runs. The problem lies in how Excel stores and retrieves formulas from its worksheets.

Excel Formulas in Memory

When you open a spreadsheet, Excel loads the entire worksheet into memory. This includes all cell values, including formulas. However, when you save the spreadsheet, Excel only saves the final value of each cell, not the formula that generated it.

The Problem with Saving and Loading

The user’s code uses pd.read_excel() to load the Excel file into a Pandas DataFrame. When the data is loaded, the formulas are detected because they are part of the cell values. However, when the DataFrame is saved back to an Excel file using pd.ExcelWriter(), the formulas disappear because only the final value of each cell is saved.

The Solution: Using ExcelWriter with openpyxl

To solve this issue, we need to use a library that can handle Excel files in a way that preserves formulas. In this case, we’ll use the openpyxl library, which provides better support for Excel formulas compared to other libraries like Pandas.

First, let’s update our code to use openpyxl instead of Pandas:

import openpyxl

# Load the workbook
wb = openpyxl.load_workbook('site.xlsx')

# Select the first sheet
sheet = wb['Feuille1']

# Process the data as before
data = []
for row in range(2, 100):  # assuming there are at least 98 rows
    row_data = []
    for col in range(1, 5):
        cell_value = sheet.cell(row=row, column=col).value
        if cell_value == None:
            row_data.append(None)
        else:
            row_data.append(cell_value)
    data.append(row_data)

# Create a DataFrame from the data
import pandas as pd

df = pd.DataFrame(data[1:], columns=data[0])

# Process the data as before
total_ballon = df['Unnamed: 3'].sum()
b = df[['Unnamed: 0']].apply(lambda x: x.sum(), axis=1).tolist()

# Save the DataFrame back to an Excel file using openpyxl
with pd.ExcelWriter('site.xlsx', engine='openpyxl') as writer:
    writer.book = wb

    df.to_excel(writer, "Feuille2", index=False)
    writer.book.save('site.xlsx')
    writer.book.close()

How It Works

In this updated code, we load the workbook using openpyxl and select the first sheet. We then process the data as before, but instead of creating a Pandas DataFrame, we create an in-memory list of lists using Python lists.

We use the apply() method to calculate the sum of each column and convert it to a list. Finally, we save the DataFrame back to an Excel file using pd.ExcelWriter() with the openpyxl engine.

Conclusion

In this article, we’ve explored why Excel formulas sometimes disappear and how to process Excel data with Python while dealing with missing formulas. By using the openpyxl library, we can create a more robust solution that preserves formulas even after saving and loading an Excel file.

We hope this helps you tackle similar challenges in your own projects!

Last modified on 2024-09-30