Why is using a bare `except` clause dangerous in Python error handling?

Dangers of Using Bare 'except'

While attempting to create a function that detects image presence on the screen, a developer encountered a warning from PyCharm to avoid leaving 'except' bare. To understand the issue, let's examine the provided function:

def check_image_on_screen(image):
    try:
        pyautogui.locateCenterOnScreen(image)
        return True
    except:
        return False

The problem arises because the bare 'except' block catches all exceptions, including those that should not be suppressed. This includes the 'KeyboardInterrupt' exception (when users press Ctrl C to interrupt execution) and Python-raised errors like 'SystemExit'.

Appropriate Exception Handling

In the absence of a specific expected exception, it's advisable to at least catch the 'Exception' base type, which covers all "Regular" exceptions. However, in this scenario, the expected error is explicitly stated in the 'pyautogui' documentation as 'ImageNotFoundException'. A more appropriate function would utilize this specific exception:

def check_image_on_screen(image):
    try:
        pyautogui.locateCenterOnScreen(image)
        return True
    except pyautogui.ImageNotFoundException:
        return False

Exceptional Failure Handling

Remember, 'except' blocks are intended to recover from known failure states. Unknown failures are often unrecoverable, and it's best to allow such exceptions to propagate up the call stack to seek handling elsewhere. In this case, the Python interpreter by default handles uncaught exceptions by terminating the program gracefully.

Conclusion: To ensure robust exception handling, use 'except' blocks to catch known failure states and handle them appropriately. Let unknown exceptions propagate, allowing the interpreter's default behavior to act accordingly.

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