How to catch CV2 errors with try - except statements in Python

Question

so, I have a few png files that are saved following the pattern

name_number1_section_number2

The two fields number1 and number 2 are taken up by integers, so a file would be named

name_1_section_23

for example.

Now, I want to load these with cv2 to read out the share of black pixels in each of them. I use two nested loops for this. The issue is, that some number1-number2 combinations do not exist. "name_4_section_27" might not exist, while "name_1_section27" does. This would not be an issue if this were a regular Python command, then I would just use except "FileNotFoundError".

I have tried:

for i in range(n, k):
    for j in range(n, z):
        try: img = cv2.imread(fp + r"name_" + str(i) + "_section_" + str(j) + ".png")
        except (cv2.error, cv2.Error.StsAssert): print(fp + r"name_" + str(i) + "_section_" + str(j) + ".png does not exist")
        some Code
        [...]

Yet, cv2 still throws me an error message. The except statement does not seem to work. I used the error code given here, see -215, but it does not help with the issue. I have tried most combinations of cv, cv2, opencv, opencv-python.error, yet I have had no success. I tried using except error as e which did not help. Apart from that I tried to get around this by using img = open(file, "b") to catch the FileNotFoundError there, but this cannot be used with cv2.imread (as least as far as I know).

Since there are a lot of files, I do not want to have to open them manually. Any way to catch CV2 errors with try - except or are there other workarounds?

KR

Why not glob the files that do exist, rather than guessing their names and finding they don't exist. for f in glob.glob('*.png'): — Mark Setchell
– Mark Setchell, Commented Oct 26, 2023 at 14:08
The exception wasn't cv2.error or cv2.Error.StsAssert. It was something else. — John Gordon
– John Gordon, Commented Oct 26, 2023 at 22:53

LeM0 · Accepted Answer · 2023-10-26 22:53:39Z

1

You can just use the Exception class to catch errors you are meant to catch.

except Exception: print(fp + r"name_" + str(i) + "_section_" + str(j) + ".png does not exist")

Edit.: As mentioned in the comments using Exception is not the most optimal solution. The documentation you linked is primarily a C++ documentation.

What should work in python is the following which seems to be correct in your code:

except cv2.error as e: <handle a cv2 error>

however according to a older post related to this topic: #8873657

Some OpenCV functions don't throw errors, they print out a message and exit the program (either with exit or abort, I can't remember). In other words, there are some errors that can't be caught.

However as mentioned in the comments, you should use something like

for f in glob.glob('*.png'):

to loop through the files, rather then guessing wheatear the file exists. Or you can use the OS library to check if the file exists first:

if os.path.exists(file_path):
    #load image

edited Oct 26, 2023 at 22:53

answered Oct 26, 2023 at 14:12

LeM0

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7 Comments

Dan Mašek Over a year ago

Exception is not a Python keyword.

Péter Szilvási Over a year ago

You should not use the base Exception class when catching errors because it can catch and handle all exceptions, making it difficult to differentiate between different types of errors and potentially hiding unexpected issues in your code.

LeM0 Over a year ago

@PéterSzilvási you are right. The actual best solution is probably what Mark Setchell Suggested in the comments under the OG post

Dan Mašek Over a year ago

Yes, I'm aware of that class. What I'm pointing out is that the term "keyword" has a very specific meaning when talking about a language, and you've used it incorrectly.

LeM0 Over a year ago

@DanMašek you are right, that wasnt the correct term. I have approved the edit.

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