UnboundLocalError: local variable 'image' referenced before assignment

Question

I'm running this code,

def extract_features(filename, model):
    try:
        image = Image.open(filename)

    except:
        print("ERROR: Couldn't open image! Make sure the image path and extension is correct")
        
    image = image.resize((299,299))
    image = np.array(image)
    # for images that has 4 channels, we convert them into 3 channels
    if image.shape[2] == 4: 
        image = image[..., :3]
    image = np.expand_dims(image, axis=0)
    image = image/127.5
    image = image - 1.0
    feature = model.predict(image)
    return feature

It gives me this error:

UnboundLocalError: local variable 'image' referenced before assignment

This line

print("ERROR: Couldn't open image! Make sure the image path and extension is correct")

is printed by the interpreter.

Can someone explain why this error occurred?

Because since the code inside the try-block has thrown, the variable image was not initialized inside it. To avoid this error, you can return inside your except-block. — enzo
– enzo, Commented Jun 10, 2021 at 4:12
It means Image.open failed, so image is never defined. You should abort rest of code since it's useless without image. You should also print out the raw/actual error message instead of defining your own print(...), to get a better understanding of why you are getting an error. There are other errors, not just about paths. — Gino Mempin
– Gino Mempin, Commented Jun 10, 2021 at 4:15

Gino Mempin · Accepted Answer · 2021-06-10 12:13:38Z

UnboundLocalError: local variable 'image' referenced before assignment

The issue is here:

def extract_features(filename, model):
    try:
        image = Image.open(filename)
    except:
        print("ERROR: Couldn't open image! Make sure the image path and extension is correct")

    image = image.resize((299, 299))
    ...

When an Exception occurs, it goes to the except block, and then image is never assigned a proper value. Then, the code continued to run outside the try-except block, where you tried to reference image in image.resize(...). But, since it's still undefined, you get the mentioned error.

The other answer suggests a solution, but I don't recommend the solution to put all the code inside the try-except block. I would say the code you have now is "better", in that it's more readable which particular lines/operations can raise an Exception, and then handle those Exceptions appropriately.

In your case, you just need to skip the rest of the code when an Exception occurs:

def extract_features(filename, model):
    try:
        image = Image.open(filename)
    except:
        print("ERROR: Couldn't open image! Make sure the image path and extension is correct")
        return None 
   
    # If the code gets here, image is now *assigned* and can be *referenced* 
    image = image.resize((299, 299))
    ...

Here, you can return immediately when something has gone wrong. You can return something to indicate an error state, like a None here, then handle it in some other part of your program. Or, if you just wanted to print a friendlier error message, then print it out then just re-raise the same Exception:

except:
    print("ERROR: Couldn't open image! Make sure the image path and extension is correct")
    raise

Either you then handle the re-raised Exception somewhere else, or it will stop your program completely (which makes sense if it will not work without a working image).

Also, as I suggested in the comments, it's better to not hide the Exception. Get it and log it using the built-in logging module. It can provide more information or there might be other errors other than wrong paths:

import logging

def extract_features(filename, model):
    try:
        image = Image.open(filename)
    except Exception as exc:
        logging.error("Couldn't open image! Make sure the image path and extension is correct", exc_info=exc)
        raise

    # If the code gets here, image is now *assigned* and can be *referenced*
    image = image.resize((299, 299))
    ...

ERROR:root:Couldn't open image! Make sure the image path and extension is correct
Traceback (most recent call last):
  File "test.py", line 8, in extract_features
    image = Image.open(filename)
  File "/path/to/PIL/Image.py", line 2967, in open
    raise UnidentifiedImageError(
PIL.UnidentifiedImageError: cannot identify image file 'test.png'

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