Fix: Excel Chart Formatting With Win32com & Openpyxl

Hey guys,

Have you ever encountered a frustrating issue where your Excel chart formatting, which works perfectly fine with win32com, suddenly breaks down when you upgrade your openpyxl library to version 3.1.4 or later? If so, you're not alone! This article dives deep into this specific bug, providing a comprehensive explanation, steps to reproduce it, and potential workarounds. We'll break down the technical details in a friendly and easy-to-understand manner, so stick around!

Understanding the Issue

The core problem lies in the interaction between openpyxl, a Python library for reading and writing Excel files, and win32com, a Python library that allows you to interact with Windows COM (Component Object Model) objects – in this case, Excel itself. Specifically, the issue manifests when you try to access the TickLabels of the Y-axis of a chart created using openpyxl and then manipulated using win32com.

When using openpyxl versions 3.1.3 and earlier, this process works flawlessly. However, starting with version 3.1.4, an error occurs during the TickLabels access, indicating that these labels are either missing or inaccessible. This can be a significant headache for anyone relying on win32com for advanced chart formatting or automation tasks in Excel. This bug can be particularly troublesome because it doesn't immediately break the chart creation or saving process. Instead, it surfaces only when the code attempts to interact with the chart's axis labels using win32com. This delayed error can make debugging more challenging, as the initial steps of chart creation might seem successful, leading to confusion when the TickLabels access fails later on.

The error message you'll likely encounter looks something like this:

TickLabels アクセス時に例外: (-2147352567, '例外が発生しました。', (0, None, None, None, 0, -2147467259), None)

This cryptic message essentially translates to "An exception occurred while accessing TickLabels." While not very informative on its own, it points directly to the source of the problem: the inability to retrieve the tick labels of the chart's Y-axis.

This issue is particularly relevant in scenarios where you need to dynamically adjust or customize the axis labels of your Excel charts. For example, you might want to change the number format, font, or alignment of the labels based on specific data or user preferences. If you're using win32com to accomplish these tasks, this bug in openpyxl can effectively block your progress.

Steps to Reproduce the Bug

To illustrate the problem and help you confirm if you're experiencing the same issue, let's walk through a step-by-step reproduction scenario. This example is based on the unit test mentioned in the original bug report, which provides a concise and reliable way to trigger the error.

1. Set up the Environment: First, ensure you have Python installed (version 3.9 or later is recommended) along with the necessary libraries: openpyxl and pywin32 (for win32com support). You can install these using pip:

   pip install openpyxl pywin32
   

   It's crucial to have openpyxl version 3.1.4 or later to observe the bug. If you have an older version, you can upgrade using:

   pip install --upgrade openpyxl
   

2. Create a Python Script: Now, create a Python script (e.g., test_chart_bug.py) with the following code:

import openpyxl
from openpyxl.chart import LineChart, Series, Reference
from openpyxl.utils import get_column_letter
import win32com.client

def create_excel_with_chart(filename):
    wb = openpyxl.Workbook()
    ws = wb.active

    # Sample data
    rows = [
        ['Date', 'Sales'],
        ['2023-01-01', 100],
        ['2023-01-02', 150],
        ['2023-01-03', 120],
        ['2023-01-04', 180],
    ]

    for row_idx, row_data in enumerate(rows, 1):
        for col_idx, cell_value in enumerate(row_data, 1):
            ws.cell(row=row_idx, column=col_idx, value=cell_value)

    # Create a chart
    chart = LineChart()
    chart.title = "Sales Trend"
    chart.x_axis.title = "Date"
    chart.y_axis.title = "Sales"

    data = Reference(ws, min_col=2, min_row=1, max_col=2, max_row=len(rows))
    dates = Reference(ws, min_col=1, min_row=2, max_col=1, max_row=len(rows))
    series = Series(data, xvalues=dates, title="Sales")
    chart.append(series)

    ws.add_chart(chart, "E2")

    wb.save(filename)

def access_chart_tick_labels(filename):
    excel = win32com.client.Dispatch("Excel.Application")
    excel.Visible = False  # Keep Excel hidden
    wb = excel.Workbooks.Open(filename)
    ws = wb.Sheets(1)
    chart = ws.ChartObjects(1).Chart

    # This line will raise the error with openpyxl >= 3.1.4
    try:
        tick_labels = chart.Axes(2).TickLabels  # 2 represents the Y-axis
        print("TickLabels accessed successfully!")
    except Exception as e:
        print(f"Error accessing TickLabels: {e}")

    wb.Close(False)  # Close without saving
    excel.Quit()


if __name__ == "__main__":
    filename = "test_chart.xlsx"
    create_excel_with_chart(filename)
    access_chart_tick_labels(filename)
    print(f"Excel file '{filename}' created and tested.")

3. Run the Script: Execute the script from your terminal:

   python test_chart_bug.py
   

4. Observe the Error: If you're running openpyxl 3.1.4 or later, you should see the "Error accessing TickLabels" message in the console output, along with the exception details. This confirms that you've successfully reproduced the bug.

Breaking Down the Code

Let's quickly explain what the Python script does:

• create_excel_with_chart(filename): This function uses openpyxl to create a new Excel workbook, adds some sample data (dates and sales figures), and generates a line chart based on this data. The chart is then embedded into the worksheet.
• access_chart_tick_labels(filename): This function is where the magic (or rather, the bug) happens. It uses win32com to open the Excel file created in the previous step. It then navigates to the chart and attempts to access the TickLabels property of the Y-axis (chart.Axes(2).TickLabels). This is the line that triggers the error when using openpyxl 3.1.4 or later.
• if __name__ == "__main__":: This block executes the main logic of the script: it calls the create_excel_with_chart function to create the Excel file, then calls access_chart_tick_labels to attempt accessing the tick labels and demonstrate the bug.

By running this script, you can clearly see the issue in action and confirm that it aligns with the described problem.

Expected Behavior

In an ideal scenario, the unit test should execute without any errors. Specifically, the access_chart_tick_labels function should successfully access the TickLabels property of the Y-axis without raising an exception. This is the behavior observed when using openpyxl versions prior to 3.1.4.

When the code works as expected, the console output should look something like this:

TickLabels accessed successfully!
Excel file 'test_chart.xlsx' created and tested.

This indicates that the win32com interaction with the openpyxl-generated chart is functioning correctly, and you can proceed with further chart manipulations or customizations using win32com.

Actual Behavior

As we've already discussed, the actual behavior when running the test with openpyxl 3.1.4 or later is quite different. Instead of successfully accessing the TickLabels, the script throws an exception, and the console output displays an error message similar to the one shown earlier:

Error accessing TickLabels: (-2147352567, '例外が発生しました。', (0, None, None, None, 0, -2147467259), None)
Excel file 'test_chart.xlsx' created and tested.

This error confirms that the bug is present and that accessing the TickLabels property using win32com is failing. This can have significant implications for any code that relies on this functionality, as it will prevent you from programmatically modifying or customizing the axis labels of your charts.

Environment Details

Understanding the environment in which the bug occurs is crucial for troubleshooting and identifying potential workarounds. Here's a breakdown of the key environment factors:

• Operating System: The bug has been confirmed on Windows 11. While it's possible that it might also occur on other versions of Windows, this is the primary environment where it has been observed and reported.
• openpyxl Version: This is the most critical factor. The bug specifically affects openpyxl versions 3.1.4 and later. Versions prior to 3.1.4 are not affected.
• Python Version: The Python version is not directly relevant to the bug. The issue has been observed with Python 3.9, but it's likely to occur with other Python versions as well.
• molass-legacy: This library is not directly related to the bug. It was mentioned in the original bug report as the context in which the issue was discovered, but it's not a dependency for reproducing the bug itself.

Potential Causes and Workarounds

While the exact root cause of the bug is still under investigation, some potential explanations have been proposed:

• Changes in openpyxl's Chart Structure: It's possible that the internal structure of charts generated by openpyxl was modified in version 3.1.4, leading to compatibility issues with win32com's way of accessing chart elements.
• COM Object Handling: There might be a change in how openpyxl handles COM objects related to charts, causing win32com to fail when accessing specific properties like TickLabels.

Unfortunately, a definitive workaround or fix is not yet available. However, here are some potential approaches you can consider:

1. Downgrade openpyxl: If you need to maintain compatibility with win32com and the TickLabels access is crucial for your workflow, the most straightforward solution is to downgrade openpyxl to version 3.1.3:

   pip install openpyxl==3.1.3
   

   This will revert to the working version and allow your code to function as expected. However, keep in mind that you'll be missing out on any bug fixes or new features introduced in later openpyxl versions.

2. Explore Alternative Chart Manipulation Methods: Instead of relying on win32com for chart formatting, consider using openpyxl's built-in chart manipulation capabilities. While this might require rewriting some of your code, it could provide a more robust and compatible solution in the long run. openpyxl offers a wide range of options for customizing chart elements, including axis labels, titles, and data series.

3. Investigate Other Libraries: There might be other Python libraries that offer better compatibility with openpyxl for advanced chart formatting. Explore alternatives like xlsxwriter or other libraries that provide chart manipulation functionalities.

4. Monitor openpyxl Issue Trackers: Keep an eye on the openpyxl project's issue trackers (e.g., GitHub issues) for updates on this bug. The developers might be working on a fix, and a future release might address the compatibility issue. Actively following the issue can give you insights into the progress and potential timelines for a resolution.

Conclusion

The bug affecting Excel chart formatting with win32com and openpyxl 3.1.4+ is a significant issue for those relying on these libraries for advanced Excel automation. While a definitive solution is still in the works, understanding the problem, its causes, and potential workarounds can help you mitigate its impact on your projects. Consider downgrading openpyxl, exploring alternative chart manipulation methods, or monitoring the openpyxl issue trackers for updates. By staying informed and proactive, you can navigate this bug and continue to leverage the power of Python for Excel automation. Guys, keep coding and stay tuned for more updates on this issue!