Saving image to buffer does not overwrite?

[Spartanjackwar]

What did you do?

https://upload.wikimedia.org/wikipedia/commons/3/39/Oatmeal.jpg
Using the above linked image, I did the following within an IDLE session:

>>> from PIL import Image, ImageOps
>>> import io, os
>>> size = (100,100)
>>> img = Image.open("C:/Oatmeal.jpg")
>>> temp = ImageOps.contain(img, size)
>>> temp.show()
>>> buff = io.BytesIO()
>>> temp.save(buff, format='jpeg', quality=100, subsampling=0)
>>> wranglebuff = Image.open(buff)
>>> wranglebuff.show()
>>> temp.save(buff, format='png')
>>> wranglebuff = Image.open(buff)
>>> wranglebuff.show()

What did you expect to happen?

I expected the second show() in the buffer to be different (higher quality) than the first show() of the buffer.

What actually happened?

Both buffer show() invocations show the exact same image. It is possible that calling save() to the same in-memory object twice will continuously append to the stream as opposed to overwriting.

What are your OS, Python and Pillow versions?

• OS: Windows 10
• Python: 3.9.0
• Pillow: 11.3.0

[radarhere]

It is possible that calling save() to the same in-memory object twice will continuously append to the stream as opposed to overwriting.

From my testing, Pillow is writing to the current position of the stream. Pillow is freely seeking around the buffer that you have handed to it, and after wranglebuff.show(), the stream is at the end.

I expect you would like Pillow to seek to the start of the stream when saving. I'm not convinced however, and think that users passing in streams would prefer manual control. If I were using Pillow to write an image to a stream as part of a custom file format, I'd want Pillow to start writing at the position the stream was at, and changing that behaviour now sounds like it could break existing code.

Wouldn't it be very simple for you to insert buff.seek(0) before your last save operation?

[radarhere]

@Spartanjackwar did that answer your question?