Function vs Class Method in Python: Explained Using an Image Processing Example

[Rajeev Bagra]

Context

While working on a Python image-processing assignment, we were asked to write a small helper function called img_to_pix(filename).

The goal of this function is simple:

• Take an image file name as input
• Open the image using Python’s PIL (Python Imaging Library)
• Return a list of pixel values from that image

A common point of confusion is whether this function should be written as a class method or a normal function.
Below is a clear explanation in Q&A form.

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Q: What does img_to_pix(filename) do?

Answer:
It opens an image file and returns a list of its pixel values.
For color images, the list contains RGB tuples.
For black-and-white images, the list contains integers representing brightness.

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Q: Why is img_to_pix(filename) written as a normal function and not a class method?

Answer:
Because it does not belong to a class and does not depend on any stored object data.
It takes an input, does its work, and returns a result immediately.

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Q: What exactly is a class (or instance) method in Python?

Answer:
A class or instance method:

• Is defined inside a class
• Uses self as its first parameter
• Works with data stored in the object

If a function does not meet these conditions, it should not be a class method.

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Q: Why doesn’t img_to_pix(filename) use self?

Answer:
Because it does not store anything and does not reuse any internal data.
Everything the function needs is already provided by the filename argument.

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Q: Why is writing self.filename = filename incorrect in this case?

Answer:
self only exists inside a class.
Since img_to_pix is not defined inside a class, Python does not know what self refers to, and the code will fail.

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Q: When would it make sense to turn this into a class method?

Answer:
It would make sense if:

• You were working with the same image repeatedly
• You wanted to store the image or filename inside an object
• Multiple image-processing functions needed access to the same data

In that case, using a class would be appropriate.

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Q: Why does the assignment expect a simple function instead of a class?

Answer:
The assignment is designed to:

• Teach how to use existing libraries
• Minimize the amount of code written
• Focus on function usage rather than object-oriented design

Using a class would add unnecessary complexity.

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Final Takeaway

If a function does not need to remember anything or manage internal state, it should be written as a simple function, not a class method.

That is exactly why img_to_pix(filename) is implemented as a normal helper function.

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