❓ Q: What happens if I write pixels = img.getdata() in Pillow (Python)?

[Rajeev Bagra]

❓ Q: What happens if I write pixels = img.getdata() in Pillow (Python)?

Question:
In Python’s Pillow library, instead of writing

pixels = list(img.getdata())

what if I simply write:

pixels = img.getdata()

What exactly is stored in pixels?

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✅ A: pixels becomes a read-only pixel sequence, not a list

When you write:

pixels = img.getdata()

pixels is NOT a Python list.

It is a Pillow internal sequence object (type: ImagingCore) that provides read-only access to the image’s pixel data.

You can think of it as:

a lightweight view of the image’s pixels, not a full copy

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❓ Q: Can I loop through pixels?

Answer: Yes.

for p in pixels:
    print(p)

Each p is:

• (R, G, B) for RGB images
• (R, G, B, A) for RGBA images
• an integer 0–255 for grayscale images

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❓ Q: Can I access pixels by index?

Answer: Yes.

print(pixels[0])     # first pixel
print(pixels[10])    # 11th pixel

Indexing works because ImagingCore behaves like a sequence.

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❓ Q: Can I modify pixel values using pixels?

Answer: No ❌

pixels[0] = (255, 0, 0)

This will raise a TypeError because the object is read-only.

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❓ Q: Can I use list methods like append()?

Answer: No ❌

pixels.append((0, 0, 0))

This fails because pixels is not a list.

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❓ Q: Why do many examples use list(img.getdata())?

Answer:
Because converting to a list gives you a modifiable copy of pixel data.

pixels = list(img.getdata())
pixels[0] = (255, 0, 0)  # ✅ works

This allows:

• modification
• slicing
• storage independent of the image

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❓ Q: Is there any memory difference?

Answer: Yes — this is important.

Code	Memory Usage
img.getdata()	Low (no copy)
list(img.getdata())	High (full copy)

For large images, keeping it as img.getdata() is more memory-efficient.

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❓ Q: How can I check what type img.getdata() returns?

Answer:

print(type(img.getdata()))

Output:

<class 'ImagingCore'>

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🔍 Summary

• img.getdata() → read-only iterable pixel sequence
• list(img.getdata()) → modifiable list of pixels
• Use getdata() when reading only
• Use list(getdata()) when editing or storing pixels
• For heavy processing, use NumPy arrays

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