Python Pillow库thumbnail方法返回None的问题求助

Hey there! I totally get where you're coming from—switching from resize() to thumbnail() to preserve aspect ratio makes perfect sense, but that None return value can throw you for a loop. Let's break down what's happening and fix this quickly.

The Core Problem #

Unlike resize() (which creates and returns a new Image object with your adjusted dimensions), thumbnail() works in-place: it modifies the original Image object directly and doesn't return anything (hence the None you're seeing). That's the key difference that's tripping you up!

Correct Usage of thumbnail() #

Here's how to use it properly with your Python 3.5 + Pillow 5.0 setup:

1. Open your image as usual
2. Call thumbnail() directly on the image object (don't assign the result to a new variable)
3. Save the modified original object

Example Code

First, let's look at the mistake that causes None:

from PIL import Image

# ❌ Wrong: thumbnail() doesn't return a new image, so resized_img is None
img = Image.open("input.jpg")
resized_img = img.thumbnail((800, 600))
resized_img.save("output.jpg")  # This will throw an AttributeError!

Now the correct approach:

from PIL import Image

# ✅ Correct: thumbnail() modifies the img object in-place
img = Image.open("input.jpg")

# Set your max width/height (thumbnail will maintain aspect ratio automatically)
# Use Image.LANCZOS (or Image.ANTIALIAS, its alias in Pillow 5.0) for high-quality resizing
img.thumbnail((800, 600), resample=Image.LANCZOS)

# Save the now-resized image
img.save("output.jpg")

Bonus: Preserving the Original Image #

If you don't want to modify the original image object (maybe you need to use it later), make a copy first:

from PIL import Image

img = Image.open("input.jpg")
# Create a duplicate to resize, leaving the original intact
img_copy = img.copy()
img_copy.thumbnail((800, 600), resample=Image.LANCZOS)
img_copy.save("output.jpg")

# Original image is still unmodified
img.save("original.jpg")

Why This Works #

Pillow's thumbnail() is designed to be memory-efficient by modifying the existing image instead of creating a new one. The tradeoff is that it doesn't return a value—something easy to miss if you're used to resize()'s behavior.

内容的提问来源于stack exchange，提问作者Braden Holt