Does Python PIL resize maintain the aspect ratio?

Yes it will keep aspect ratio using thumbnail method:

image = Image.open(source_path)
image.thumbnail(size, Image.ANTIALIAS)
image.save(dest_path, "JPEG")

Yes. the thumbnail() method is what is needed here… One thing that hasn’t been mentioned in this or other posts on the topic is that ‘size’ must be either a list or tuple. So, to resize to a maximum dimension of 500 pixels, you would call:
image.thumbnail((500,500), Image.ANTIALIAS)

See also this post on the subject:
How do I resize an image using PIL and maintain its aspect ratio?

No, it does not. But you can do something like the following:

im = PIL.Image.open("email.jpg"))
width, height = im.size
im = im.resize((width//2, height//2))

Here the height and width are divided by the same number, keeping the same aspect ratio.

Okay, so this requires a couple of lines of code to get what you want.

First you find the ratio, then you fix a dimension of the image that you want (height or width). In my case, I wanted height as 100px and let the width adjust accordingly, and finally use that ratio to find new height or new width.

So first find the dimensions:

width, height = logo_img.size
ratio = width / height
      

Then, fix one of the dimension:

new_height = 100

Then find new width for that height:

new_width = int(ratio * new_height)
            
logo_img = logo_img.resize((new_width, new_height))

Recent Pillow version (since 8.3) have the following method to have an image resized to fit in given box with keeping aspect ratio.

image = ImageOps.contain(image, (2048,2048))

I think following is the easiest way to do:

Img1 = img.resize((img.size),PIL.Image.ANTIALIAS)

It depends on your demand, if you want you can set a fixed height and width or if you want you can resize it with aspect-ratio.

For the aspect-ratio-wise resize you can try with the below codes :

To make the new image half the width and half the height of the original image:

from PIL import Image
im = Image.open("image.jpg")
resized_im = im.resize((round(im.size[0]*0.5), round(im.size[1]*0.5)))
        
#Save the cropped image
resized_im.save('resizedimage.jpg')

To resize with fixed width and ratio wise dynamic height :

from PIL import Image
new_width = 300
im = Image.open("img/7.jpeg")
concat = int(new_width/float(im.size[0]))
size = int((float(im.size[1])*float(concat)))
resized_im = im.resize((new_width,size), Image.ANTIALIAS)
#Save the cropped image
resized_im.save('resizedimage.jpg')

Recent versions of Pillow have some useful methods in PIL.ImageOps.

Depending on exactly what you’re trying to achieve, you may be looking for one of these:

• ImageOps.fit(image, size [, …]) (docs) – resizes your image, maintaining its aspect ratio, and crops it to fit the given size

• ImageOps.contain(image, size [, …]) (docs) – resizes your image, maintaining its aspect ratio, so that it fits within the given size

One complete example:

import PIL.Image


class ImageUtils(object):
    @classmethod
    def resize_image(cls, image: PIL.Image.Image, width=None, height=None) -> PIL.Image.Image:
        if height is None and width is not None:
            height = image.height * width // image.width
        elif width is None and height is not None:
            width = image.width * height // image.height
        elif height is None and width is None:
            raise RuntimeError("At lease one of width and height must be present")
        return image.resize((width, height))


def main():
    ImageUtils.resize_image(PIL.Image.open("old.png"), width=100).save("new.png")


if __name__ == '__main__':
    main()