Why is a Bare `except` Clause in Python Considered Bad Practice?

The Perils of a Bare 'except'

In designing a function to verify the presence of an image on-screen using PyAutoGui, you employed the following approach:

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

While this function appears to function as intended, PyCharm flags the bare 'except' as unadvisable. This raises concerns about the implications of leaving an 'except' unadorned.

The Problem with Bare 'except'

A bare 'except' indiscriminately intercepts all exceptions, even those that are not anticipated or desirable. Exceptions such as KeyboardInterrupt (Ctrl C) and Python-generated errors like SystemExit are among those that should not be handled in this manner.

It is preferable to explicitly specify the type of exception that your code is prepared to handle, at the very least 'Exception', the foundation class for all "regular" exceptions.

A More Appropriate Approach

An error 'except' block is intended for use in recovering from predefined failure scenarios. However, it is generally impossible to fully recover from unknown failure scenarios. In such cases, it is preferable to terminate the program rather than attempting to continue. This is the default behavior of the Python interpreter when an exception goes unhandled.

It is prudent to only trap exceptions that you know how to address. The remaining exceptions should be allowed to propagate up the call stack in hopes that another component can handle them. In the case of verifying image presence, the anticipated error (per documentation) is pyautogui.ImageNotFoundException.

Revised Code

Based on the aforementioned insights, your function can be refactored to address the concerns raised by the bare 'except':

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

By specifying the specific exception that the function is intended to handle, you improve the reliability and maintainability of your code.

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