try except else blocks

Topics

• idiomatic python

Putting too much code inside of a try catch exceptions you didn’t intend to handle. Instead of expanding a try block, put additional code into the else block following the associated except block, or in a totally separate try statement:

try:
    with open('source.json', 'r') as f:
        data = json.load(f)
except FileNotFoundError:
    print("Error: Source file not found.")
except json.JSONDecodeError:
    print("Error: Could not decode JSON from source file.")
else:
    # This code runs only if the try block succeeds without exceptions
    # Process the data (e.g., extract a specific value)
    processed_data = data.get('important_key')
 
    # Now, attempt to write to a destination file in a separate try block
    try:
        with open('destination.txt', 'w') as f:
            f.write(str(processed_data))
    except IOError:
        # This exception is clearly from the write operation
        print("Error: Failed to write to destination file.")

In above code, if you place both read and write operations in a single try block and then catch IOError, you know an IOError occurred. You do not know which operation failed. Was it the read from source.json or the write to destination.txt? You would need to add more code inside the except block to figure that out, which complicates the error handling logic itself.

This approach isolates the source of the error. An exception in the try block means the input failed. An exception in the else block means the output failed. This separation makes the code’s intent clear and debugging far simpler.

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