Python: Try/Except

Code Might Fail

It’s important to use try/except/finally whenever your application could fail through no fault of your own.

Why Plan for Errors?

People ask:

• Why plan for errors?
• Shouldn’t we fix all errors in our code before we release it?
• Why do we need try/except/finally?

Perfect Code Can Still Have Exceptions

We should always strive to fix all coding and logic errors. However, sometimes our code can be perfect - but exceptions can still happen. try/except/finally is a way to gracefully handle unexpected errors and prevent our program from crashing.

Example

Suppose you write a script to read baseball_game_results.csv each night at midnight.

It runs fine until someone changes the filename to rslts.csv.

Now, your code terminates with an ugly error because the necessary file can’t be found.

To code professionally, we can use try/except to handle this error gracefully.

try:
    # Attempt to open the file
    with open('baseball_game_results.csv', 'r') as f:
        # Do something with the file
        
except FileNotFoundError:
    # Handle the case where the file is not found
    print('ERROR: File not found. Please name the file to baseball_game_results.csv')
finally:
    # Clean up any resources (e.g. file handles) used by the code
     

Other Programming Languages

Other programming languages use something very similar, but might use the keywords try/catch/finally. As in “try this, and if you catch an exception, do this.”

Throwing Exceptions

Exceptions are thrown by nested functions, up, up, up, until some level “catches” the exception and deals with it, or the program terminates with an ugly error.

It’s important to handle exceptions gracefully and prevent our programs from crashing.