How to fix "must be passed as a "sequence"" warning for PIL in Python?

Question

I am using PIL in python to combine images vertically and horizontally from dermen's answer in here. However, I am getting a warning:

/home/ceren/Documents/Python/combine_image.py:17: FutureWarning: arrays to stack must be passed as a "sequence" type such as list or tuple. Support for non-sequence iterables such as generators is deprecated as of NumPy 1.16 and will raise an error in the future.
  imgs_comb = np.hstack( (np.asarray( i.resize(min_shape) ) for i in imgs ) )

I am a python beginner, so I cannot quite understand what to do to resolve this issue. I thought [f2, f4, f5] in the code is already a list. However, the warning says it is not?? I know it is not an essential thing to fix, but learning how to fix it would be nice.

What should I change in my code so that I do not receive that warning any further?

My code is below:

Inside main:

fcom1 = 'Image.png'
ci.combine_horizontal([f2, f4, f5], fcom1, date)

In image combining function:

def combine_horizontal(f, fcom, date):
    list_im = f
    imgs    = [ PIL.Image.open(i) for i in list_im ]
    min_shape = sorted( [(np.sum(i.size), i.size ) for i in imgs])[0][1]
    imgs_comb = np.hstack( (np.asarray( i.resize(min_shape) ) for i in imgs )    
    imgs_comb = PIL.Image.fromarray( imgs_comb)
    imgs_comb.save(fcom)

Please edit the __full__ error message into your question (yes, including the stack trace) — DisappointedByUnaccountableMod, May 20 '21 at 17:56
I edited the question text to include the full warning message. @barny — Ceren, May 20 '21 at 18:14
This code has indentation and parentheses issues. Please copy and paste the actual code you ran! — Mad Physicist, May 20 '21 at 18:25
I am sorry, I am new here. I guess I made a mistake when copy-pasting to the post. Thank you for your help. It is now working flawlessly after modifying my codes according to your comment. @MadPhysicist — Ceren, May 20 '21 at 18:49

score 2 · Accepted Answer · answered May 20 '21 at 18:24

The warning message tells you exactly which line is causing the problem, and why.

np.hstack does not (or won't) accept generator expressions as an input. The following line contains a generator expression:

np.hstack(np.asarray( i.resize(min_shape)) for i in imgs)

You can make the problem go away by consuming the generator in a list comprehension:

np.hstack([np.asarray( i.resize(min_shape)) for i in imgs])

Or wrap it in a tuple (which would be my personal aesthetic preference:

np.hstack(tuple(np.asarray( i.resize(min_shape)) for i in imgs))

Ah, I thought that the problem came from the way I call the function. I understand now. I modified my code as you said and the issue has been solved. Thank you very much! — Ceren, May 20 '21 at 18:46

How to fix "must be passed as a "sequence"" warning for PIL in Python?

1 Answers1