4. Exception Syntax¶

Catch All Exceptions¶

Catch all exceptions, regardless of their type. This will prevent your program from crashing, but this type of exception handling is rarely useful because you can’t do anything meaningful to recover from the abnormal condition. In fact, you don’t even know what the abnormal condition is since it could be any exception.

try:
    # Your normal code goes here.
    # Your code should include function calls which might raise exceptions.
except:
    # If ANY exception was raised, then execute this code block.

Catch A Specific Exception¶

This is perhaps the most often used syntax. It catches one specific condition and tries to recover from the condition.

try:
    # Your normal code goes here.
    # Your code should include function calls which might raise exceptions.
except ExceptionName:
    # If ExceptionName was raised, then execute this block.

Catch Multiple Specific Exceptions¶

If you are writing a code block that contains calls to functions that may raise multiple different exceptions, then you can write separate except clauses to handle each. You may also include an else clause after your except clauses to contain any code that you want to run in case the try clause does not raise an exception.

try:
    # Your normal code goes here.
    # Your code should include function calls which might raise exceptions.
except ExceptionOne:
    # If ExceptionOne was raised, then execute this block.
except ExceptionTwo:
    # If ExceptionTwo was raised, then execute this block.
else:
    # If there was no exception then execute this block.

Be aware that when you have more than one except clause in a try: except; block, it is only the first matching exception that will be triggered and have its code block executed. Therefore, you want to list the exceptions in the order of more specific to less specific. For example, you saw in the lesson on Standard Exceptions that the ZeroDivisionError is a “child” of the ArithmeticError. This means that the former is more specific than the latter. So if you want to catch both errors because you want to do different things based on which error it is, then you would want to list the except clause for the ZeroDivisionError first.

Clean-up After Exceptions¶

If you have code that you want to be executed even if exceptions occur, you can include a finally code block:

try:
    # Your normal code goes here.
    # Your code might include function calls which might raise exceptions.
    # If an exception is raised, some of these statements might not be executed.
finally:
    # This block of code will always execute, even if there are exceptions raised

It’s a good idea to review the Python Tutorial since there are even more syntactical variations for exception handling than those covered above.

An Example of File I/O¶

Although we will not be covering file input and output in the main text of this course (there is a Hacker Chapter you can work through if you are interested), it is worth noting that one place where you will always want to include exception handling is when you read or write to a file.

Here is a typical example of file processing. Note that the outer try: except: block takes care of a missing file or the fact that the existing file can’t be opened for writing. The inner try: except: block protects against output errors, such as trying to write to a device that is full. The finally code guarantees that the file is closed properly, even if there are errors during writing.

try:
    f = open("my_file.txt", "w")
    try:
        f.write("Writing some data to the file")
    finally:
        f.close()
except IOError:
    print("Error: my_file.txt does not exist or it can't be opened for output.")