Unclear error when displaying images from arrays of float dtype in notebook

[thomasaarholt]

Floating point images from numpy arrays via the PIL.Image.fromarray() raise an unclear error when accidentally returned in jupyter notebook as the final line in a cell. This triggers IPython.display.display() on the image object, which in turn calls the image object _repr_png_() method, which raises the error below.

What did you do?

Called

import numpy as np
from PIL.Image import fromarray
fromarray(np.random.random((30, 30)))

What did you expect to happen?

Either:
A) Attempt to show the image given some defaults (like showing the image on a 8bit depth (resulting in a black image or very dark image)
B) More likely: Raise a clear error saying that you can't create images out of floating point values, and should instead (probably rescale to an appropriate bitdepth range and) change the dtype to be uint8 or some other accepted dtype.

What actually happened?

I get an error about how one cannot write mode F as png:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~\Miniconda3\envs\py\lib\site-packages\PIL\PngImagePlugin.py in _save(im, fp, filename, chunk, save_all)
   1219     try:
-> 1220         rawmode, mode = _OUTMODES[mode]
   1221     except KeyError as e:

KeyError: 'F'

The above exception was the direct cause of the following exception:

OSError                                   Traceback (most recent call last)
~\Miniconda3\envs\py\lib\site-packages\IPython\core\formatters.py in __call__(self, obj)
    343             method = get_real_method(obj, self.print_method)
    344             if method is not None:
--> 345                 return method()
    346             return None
    347         else:

~\Miniconda3\envs\py\lib\site-packages\PIL\Image.py in _repr_png_(self)
    671         """
    672         b = io.BytesIO()
--> 673         self.save(b, "PNG")
    674         return b.getvalue()
    675 

~\Miniconda3\envs\py\lib\site-packages\PIL\Image.py in save(self, fp, format, **params)
   2149 
   2150         try:
-> 2151             save_handler(self, fp, filename)
   2152         finally:
   2153             # do what we can to clean up

~\Miniconda3\envs\py\lib\site-packages\PIL\PngImagePlugin.py in _save(im, fp, filename, chunk, save_all)
   1220         rawmode, mode = _OUTMODES[mode]
   1221     except KeyError as e:
-> 1222         raise OSError(f"cannot write mode {mode} as PNG") from e
   1223 
   1224     #

OSError: cannot write mode F as PNG

What are your OS, Python and Pillow versions?

• OS: Win 10
• Python: 3.9
• Pillow: 8.0.1

[radarhere]

For A, you may be interested in #2663 and the suggested PR #3172. The idea is that you could append convert_mode=True and it would then convert automatically if saving wasn't possible.

[radarhere]

For B, you're asking for specific error messages for IPython, which I don't think can be neatly added in while keeping the other uses of Pillow in mind - in any other situation, the user might see this error message and just pick a format other than PNG.

#5139 is the best I personally have to offer, adding an error explaining that Pillow was trying to saving to PNG for display.

[thomasaarholt]

Thanks :)

[radarhere]

Be aware that #7266 is now proposing an alternative to this - capturing any errors and returning None instead.