Saving images to stdout using some file formats does not write the image correctly

[infmagic2047]

What did you do?

I created a image and saved it to stdout using PPM format.

What did you expect to happen?

The image is written correctly.

What actually happened?

Only the PPM header is written, and the image data is not.

What are your OS, Python and Pillow versions?

• OS: Gentoo Linux
• Python: Tested with both 3.8.11 and 3.9.6
• Pillow: 8.3.1

import sys
from PIL import Image

im = Image.frombytes('RGB', (4, 4), b'\0' * 48)
im.save(sys.stdout, 'ppm')

[infmagic2047]

Additional note: This bug only happens when saving to stdout. It does not seem to happen when saving to a named file, file object, or other file-like objects like BytesIO.

Interestingly, some other file formats are affected as well: when saving the image as JPEG, no data is written at all. With GIF the image is corrupted (see below). PNG seems to be unaffected, though.

GIF example:

im = Image.frombytes('RGB', (4, 4), b'\xff\0\0' * 16)
im.save(sys.stdout, 'gif')

The output has two black pixels (unexpected) in lower left.

[wiredfool]

sys.stdout is effectively a text file, the equivalent of io.StringIO, likely to be in utf-8 or your specific locale. (https://docs.python.org/3/library/sys.html#sys.stdout) Pillow has never been able to save to a io.StringIO instance on python 3, it has always required a io.BytesIO.

Most image formats have invalid utf-8 sequences in them, so they're not generally going to work with sys.stdout, unless you explicitly write the binary data to the underlying stream as mentioned in the link.

[infmagic2047]

Well, Pillow should be able to convert sys.stdout to sys.stdout.buffer according to #5437. Testing with sys.stdout.buffer returns the same erroneous results as well.

[wiredfool]

Missed that change.

[mclegrand]

(note: this bug has implications "downstream" in Inkscape where we use pillow to convert to different formats on export : https://gitlab.com/inkscape/inkscape/-/issues/2860 - Proposed fix ("remove lines 496 to 502 in ImageFile.py") appears to solve it )

[radarhere]

PNG is unaffected because it wraps fp in _idat, and so does appear to be stdout as far as ImageFile can tell.