If you’ve been working with Excel files in Python using openpyxl, you might have encountered a common error: "TypeError: Expected <class 'openpyxl.styles.fills.Fill'>". This error can feel frustrating, especially when it blocks you from moving forward with your project. But don’t worry—I’ve been there, and I’m here to help you through it.

In this post, we’ll break down why this error happens, how to fix it, and share some real-world solutions from the Python community. By the end of this article, you’ll have all the tools you need to handle this error efficiently and get back to your work with Excel files.

Why Does This Error Occur?

First, let's understand what this error means. The message "TypeError: Expected <class 'openpyxl.styles.fills.Fill'>" suggests that openpyxl expects a specific type of object when it’s working with the Fill attribute, which is responsible for cell background styles (like solid colors, patterns, etc.).

Typically, this error occurs in the following scenarios:

  • When you're trying to apply a fill style to a cell incorrectly.
  • When there's an incompatibility between pandas and openpyxl.
  • When you’ve got incorrect or missing arguments in your code.

In most cases, the root cause is related to how the Fill class from openpyxl is being used or passed to the cell styling function.

Common Scenarios and Solutions

Here are the most common situations where this error occurs, along with step-by-step solutions.

1. Incorrect Use of PatternFill

One of the primary reasons for encountering this error is the improper use of the PatternFill class from openpyxl. If you try to assign a fill style to a cell without providing the correct fill parameters, the error will pop up.


from openpyxl import Workbook
from openpyxl.styles import PatternFill

wb = Workbook()
ws = wb.active

# Trying to apply a fill with missing parameters
fill = PatternFill()  # This raises the error
ws['A1'].fill = fill

Solution:

To fix this, you need to pass the required arguments to the PatternFill function. For example, specifying a solid fill and a color:


from openpyxl import Workbook
from openpyxl.styles import PatternFill

wb = Workbook()
ws = wb.active

# Correct way to apply a solid fill
fill = PatternFill(fill_type="solid", fgColor="FFFF00")  # Yellow color
ws['A1'].fill = fill

wb.save("styled_file.xlsx")

In this example, by providing a valid fill type ("solid") and a foreground color ("FFFF00"), we resolve the error.

2. Version Mismatch Between pandas and openpyxl

Another common source of the error is when using pandas to export dataframes to Excel files via openpyxl. In some cases, a mismatch between the versions of these libraries can trigger the error.

Let’s say you are trying to save a pandas DataFrame to an Excel file and this error arises:


import pandas as pd

# Creating a DataFrame
df = pd.DataFrame({
    'Name': ['John', 'Doe', 'Jane'],
    'Age': [28, 34, 29]
})

# Saving to Excel with openpyxl
df.to_excel('output.xlsx', engine='openpyxl')

Solution:

To solve this, ensure both pandas and openpyxl are updated to their latest versions. You can do this with:


pip install --upgrade pandas openpyxl

This often resolves compatibility issues between these libraries.

3. Incorrect Object Assignment in Cell Styling

Sometimes, the error arises because of incorrect object assignment when styling cells. For example, you might be passing an unsupported object to a cell’s fill property.


# Incorrect code
ws['B1'].fill = 'solid'  # This is wrong; it expects a Fill object

Solution:

Always make sure that you're passing a valid Fill object to the cell's fill attribute:


from openpyxl.styles import PatternFill

# Correct assignment
ws['B1'].fill = PatternFill(fill_type="solid", fgColor="00FF00")  # Green fill

Errors like "TypeError: Expected <class 'openpyxl.styles.fills.Fill'>" can be confusing at first, but they are often caused by simple mistakes or version incompatibilities. By following the solutions above, you should be able to resolve the issue and continue working with Excel files using openpyxl without trouble.

Remember, always double-check your PatternFill arguments and make sure your libraries are up to date!

References

  • https://stackoverflow.com/questions/57759046/how-to-solve-the-problem-expected-class-openpyxl-styles-fills-fill-when
  • https://github.com/pandas-dev/pandas/issues/40499
  • https://openpyxl.readthedocs.io/en/stable/api/openpyxl.styles.fills.html

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