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Error Handling

  • As we saw earlier Python can give error messages under certain conditions. This is known as raising an exception and Python’s default behaviour is to halt execution. Exceptions are raised using the raise keyword:

# ...

# Execution stopped:
raise RuntimeError

  • In some cases we may be able to manage the error and still continue. This is known as exception handling and is achieved using try ... except clauses,

arr = [1,2,3,4,5]

for i in range(6):
    try:
        val = arr[i]
        print(str(val))
    except IndexError:
        print('Error: List index out of range, leaving loop')
        break

  • If an exception is raised then the code immediately jumps to the nearest except block. The output of the above code block is:

1
2
3
4
5
Error: List index out of range, leaving loop

  • As with other control structures there is an extra else clause that can be added which will only be executed if no error was raised,

arr = [1,2,3,4,5]
value = 0
try:
   value = arr[5]
except IndexError:
   print('5 is not a valid array index')
else:
   print('6th element is ' + str(value))

gives the output

5 is not a valid array index

  • With a try...except...else structure only one of the except/else clauses will ever be executed. In some circumstances however it is necessary to perform some operation, maybe a clean up, regardless of whether an exception was raised. This is done with a try...except...finally structure,

value = 0
arr = [1,2,3,4]
element = 6
try:
    value = arr[element]
except IndexError:
    print(str(element) + ' is not a valid array index')
else:
    print(str(element + 1 ) + 'th element is ' + str(value))
finally:
    print('Entered finally clause, do cleanup ...')

gives the output

6 is not a valid array index
Entered finally clause, do cleanup ...

  • Changing the value of the element variable between valid/invalid values will show that one of the except/else clauses gets executed and then the finally clause always gets executed.

  • It is also possible to catch exceptions of any type by leaving off the specific error that is to be caught. This is however not recommended as then it is not possible to say exactly what error occurred

value = 0
arr = [1,2,3,4]
element = 6
try:
    value = arr[element]
except:     # Catch everything
    print("Something went wrong but I don't know what")

gives the output

Something went wrong but I don't know what

Category:Tested Examples