Exception Handling

It will be terrible to have a live program that does not have any form of error recovery. Thankfully, Python provides us with a suite of tools to gracefully handle errors without crashing the entire program.

Keywords

Before we can jump into the examples, we will need to understand the Python keywords that perform exception handling.

1. try

   Statements that are likely to result in an exception are placed between try and except blocks.
   
   

2. except

   If any exception occurs in the try block, code in the except block will be executed. There can be multiple except blocks for each try block - this enables the programmer to have specific handlers for different exceptions.
   
   

3. else

   Code in this block will get executed if no exceptions occur. This block is not mandatory.
   
   

4. finally

   Like the keyword suggests, code in this block gets executed post execution of the code in the try ... except blocks. It is key to note that this execution always happens. Due to this property, the finally block is often used for clean-up operations. This block is not mandatory.

The following is a code representation of the text above.

try:
    # code that is likely to result in exceptions
    pass
except:
    # code that handles the exceptions raised
    print("exception!")
else:
    # code that is executed is no exception is raised
    pass
finally:
    # code that is executed regardless of what happens in prior blocks
    pass

Common Exceptions

We will next cover common issues and the in-built exceptions that Python provides to handle them.

ValueError

A ValueError occurs when a function is applied to an object of an appropriate type, but inappropriate value. For instance, attempting to convert a non-integer string to an integer will throw a ValueError.

try:
    int('string')
except ValueError as e:
    print(e)

invalid literal for int() with base 10: 'string'

TypeError

A TypeError occurs when a function is applied to an object of an inappropriate type. For instance, trying to sum a string and an integer will throw a TypeError.

try:
    '2' + 2
except TypeError as e:
    print(e)

must be str, not int

KeyError

A KeyError occurs when the program attempts to access a key that does not exist in the dictionary.

dict = {}

try:
    dict['key']
except KeyError as e:
    print(e)

'key'

IndexError

An IndexError occurs when the program attempts to access an item in an iterable that is out of range. For instance, attempting to access index 3 in a list of 3 items (Python is zero-based indexing, therefore index 3 does not exist) results in an IndexError.

my_list = ['a', 'b', 'c']

try:
    my_list[3]
except IndexError as e:
    print(e)

list index out of range