Exception Handling

One of the most important features of Python is its ability to handle exceptions gracefully. Exception handling is the process of detecting and dealing with errors that occur during the execution of a program.

In Python, exception handling is implemented using a combination of try, except, else, and finally blocks.

Exception Handling

The try block

The try block is the section of code where you put the code that may raise an exception. It is essentially a “sandbox” where you can test your code for potential errors. If an exception occurs in the try block, the code execution jumps to the corresponding except block.

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try:
    # Code that may raise an exception
    x = int(input("Enter a number: "))
except ValueError:
    # We were expecting an integer, but user entered a string
    print("Invalid input. Please enter a valid number.")

The except block

The except block is where you handle the exception that was raised in the try block. You can have multiple except blocks to handle different types of exceptions. The except block is optional, but it is highly recommended to include it in your code to handle errors gracefully.

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try:
    # Code that may raise an exception
    x = 10 / 0
except ZeroDivisionError:
    # User tried to divide by 0, so ZeroDivisionError was raised
    print("Cannot divide by zero.")

The else block

The else block is executed if no exceptions are raised in the try block. It is optional, and you can omit it if you don’t need to execute any code when no exceptions occur.

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try:
    # Code that may raise an exception
    x = int(input("Enter a number: "))
except ValueError:
    # We were expecting an integer, but user entered a string
    print("Invalid input. Please enter a valid number.")
else:
    # Code to execute when no exceptions occur
    print("You entered a valid number.")

The finally block

The finally block is always executed, regardless of whether an exception was raised or not. It is often used to perform cleanup operations, such as closing files or database connections.

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try:
    # Code that may raise an exception
    file = open("data.txt")
    data = file.read()
except FileNotFoundError:
    # Code to handle the exception
    print("File not found.")
finally:
    # Code to execute regardless of whether an exception was raised or not
    file.close()

Benefits of Python Exception Handling

Python’s exception handling mechanism provides several benefits to programmers, including:

1. Improved Code Readability: Exception handling allows programmers to separate error-handling logic from regular code, which improves code readability and makes it easier to understand.

2. Robustness: Exception handling helps to make Python programs more robust by allowing them to recover from unexpected errors and continue executing.

3. Better User Experience: Exception handling makes it possible to provide more meaningful error messages to users, which can help to improve the overall user experience.

4. Easier Debugging: Exception handling makes it easier to identify and debug errors by providing detailed information about the error that occurred.

Exception Handling in Selenium

I was recently working on an automation of a complex web-application that would render its elements differently very often. This is when exception handling becomes very important.

Selenium comes with many built-in exceptions and user can even build his own exceptions.

For example, one of such exceptions is NoSuchElementException, which, as the name suggests (and this is why good naming is important), is raised when the element was not found on the webpage. It usually happens when the web element’s locator is incorrect, or the element isn’t loaded yet.

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from selenium import webdriver
from seleneium.webdriver.common.by import By
from selenium.common.exceptions import NoSuchElementException

driver = webdriver.Chrome()
try:
    driver.find_element(By.ID, 'non-existent-element')
except NoSuchElementException:
    print('Element not found')
finally:
    driver.quit()

Other common exceptions are:

1. StaleElementReferenceException
2. ElementNotVisibleExceptio
3. TimeoutException
4. ElementClickInterceptedException
5. WebDriverException
6. NoSuchFrameException
7. InvalidArgumentException
8. UnexpectedAlertPresentException
9. MoveTargetOutOfBoundsException