GifWriter is failing for Pillow 7.1.0+

[ehpor]

This is visible in the standard tests. Error does not occur for Pillow 7.0.0. This may have to do with the rewrite of png files in Pillow. The error that occurred with Pillow 7.1.0 was python-pillow/Pillow#4518, for which a fix was released with Pillow 7.1.1. In the new version, the error message has changed to:

    def test_gif_writer():
        grid = make_pupil_grid(256)
    
        mw = GifWriter('test.gif')
    
        for i in range(25):
                field = Field(np.random.randn(grid.size), grid)
    
                plt.clf()
                imshow_field(field)
    
                mw.add_frame()
    
>       mw.close()

tests\test_plotting.py:23:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
hcipy\plotting\animation.py:139: in close
    self.convert()
hcipy\plotting\animation.py:132: in convert
    return self.convert_to_gif(self.filename, self.path_to_frames, self.framerate, num_files_to_convert=self.num_frames)
hcipy\plotting\animation.py:125: in convert_to_gif
    loop=0)
C:\ProgramData\Anaconda3\lib\site-packages\PIL\Image.py:2134: in save
    save_handler(self, fp, filename)
C:\ProgramData\Anaconda3\lib\site-packages\PIL\GifImagePlugin.py:499: in _save_all
    _save(im, fp, filename, save_all=True)
C:\ProgramData\Anaconda3\lib\site-packages\PIL\GifImagePlugin.py:510: in _save
    if not save_all or not _write_multiple_frames(im, fp, palette):
C:\ProgramData\Anaconda3\lib\site-packages\PIL\GifImagePlugin.py:430: in _write_multiple_frames
    for im_frame in ImageSequence.Iterator(imSequence):
C:\ProgramData\Anaconda3\lib\site-packages\PIL\ImageSequence.py:49: in __next__
    self.im.seek(self.position)
C:\ProgramData\Anaconda3\lib\site-packages\PIL\PngImagePlugin.py:748: in seek
    self._seek(f)
C:\ProgramData\Anaconda3\lib\site-packages\PIL\PngImagePlugin.py:784: in _seek
    ImageFile._safe_read(self.fp, self.__prepare_idat)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  

fp = <_io.BufferedReader name='test.gif_frames\\00000.png'>, size = 8192

    def _safe_read(fp, size):
        """
        Reads large blocks in a safe way.  Unlike fp.read(n), this function
        doesn't trust the user.  If the requested size is larger than
        SAFEBLOCK, the file is read block by block.
    
        :param fp: File handle.  Must implement a <b>read</b> method.
        :param size: Number of bytes to read.
        :returns: A string containing up to <i>size</i> bytes of data.
        """
        if size <= 0:
            return b""
        if size <= SAFEBLOCK:
>           return fp.read(size)
E           ValueError: read of closed file

C:\ProgramData\Anaconda3\lib\site-packages\PIL\ImageFile.py:549: ValueError

I'm not sure yet whether this is a bug in HCIPy or in Pillow.

[ehpor]

Bug was in Pillow, and was fixed already but not yet released. python-pillow/Pillow#4543 suggested a workaround that seems to work fine for all platforms.