PIL - can't change pixel values after converting type of image from L to F - AttributeError: 'int' object has no attribute 'data' - python-imaging-library

Question

I have an "L" type image that I converted to float "F". I get an error when trying to change its pixel values after conversion. Below is a snippet.
a = mnist_images[i].convert(mode="F")
a = a.point(lambda j: 0)
And the error message
<ipython-input-45-f89cda3cf30e> in generate_moving_mnist(shape, seq_len, seqs, num_sz, nums_per_image)
86 b=mnist_images[i].point(lambda j: 0)
87 a=mnist_images[i].convert(mode="F")#.point(lambda j: 0)
---> 88 a = a.point(lambda j: 0)
89
90 # paste the mnist_images[i] at position[i] on the current canvas
~\anaconda3\envs\tf\lib\site-packages\PIL\Image.py in point(self, lut, mode)
1572 # UNDONE wiredfool -- I think this prevents us from ever doing
1573 # a gamma function point transform on > 8bit images.
-> 1574 scale, offset = _getscaleoffset(lut)
1575 return self._new(self.im.point_transform(scale, offset))
1576 # for other modes, convert the function to a table
~\anaconda3\envs\tf\lib\site-packages\PIL\Image.py in _getscaleoffset(expr)
483 def _getscaleoffset(expr):
484 stub = ["stub"]
--> 485 data = expr(_E(stub)).data
486 try:
487 (a, b, c) = data # simplified syntax
AttributeError: 'int' object has no attribute 'data'
I am able to change the pixel values if I do not convert the image type i.e. the line below on its own works just fine.
b = mnist_images[i].point(lambda j: 0)
I am currently using a workaround but I think its quite slow
a=mnist_images[i].copy()
a=a.convert(mode="F")
for row in range(a.size[0]):
for col in range(a.size[1]):
a.putpixel((row, col), new_pixel_value)

Answer 1

It appears from line 1581 of PIL/Image.py that the point() function is not supported on type "F" float images:
if self.mode == "F":
# FIXME: _imaging returns a confusing error message for this case
raise ValueError("point operation not supported for this mode")
If you would care to say what you are actually trying to do, I can maybe suggest a workaround.