Exception Handling

Code sample link: https://replit.com/@jjoco/python-exceptions

Let's say you wanted to execute some block of code, that could potentially throw an error (specifically called Exceptions). Normally, if a block of code throws an error, the currently executing program would crash. To prevent this from happening, one can wrap such block of code using try-except blocks, which are try-catch blocks in many other languages.

Basic try-except

The syntax for try-except is the following:

try:
    #Do stuff
except:
    #Do when an exception is thrown

The idea is that if a block of code within the try block throws an Exception, then the program will execute an except block to handle the Exception.

If the dev wants to do something with the Exception caught, then they can use the following syntax:

try:
    #Do stuff
except Exception as err: #Creates alias for Exception object, which dev can do stuff with
    #Do when an exception is thrown

If it's possible that the try block can throw different types of Exceptions, then the try-except can have multiple except blocks, which execute when a specific Exception type is thrown:

Try-except with muiltiple excepts

try:
    #Do stuff
except ValueError:
    #Do when ValueError exception is thrown
except KeyError:
    #Do when KeyError exception is thrown

The dev can also handle different Exceptions in the same except block like the following:

Try-except with multiple exception types on one line

try:
    #Do stuff
except ValueError, KeyError:
    #Do when ValueError or a KeyError exception is thrown
except ZeroDivisionError:
    #Do when ZeroDivisionError exception is thrown
except Exception:
    #Do when any other exception is thrown

Finally

Finally, there's the finally block, which generally executes right before the try statement completes:

try:
    #Do stuff
except ValueError, KeyError:
    #Do when ValueError or a KeyError exception is thrown
except ZeroDivisionError:
    #Do when ZeroDivisionError exception is thrown
finally:
    #Do stuff before try finishes

So if a try-finally block looks like this:

try:
    raise Exception
finally:
    print("Finally clause here")

Then, standard output would show Finally clause here followed by the Exception.

There are other complexities involving this finally block, but that's out of the scope of this tutorial.