Exception and Error Handling in Python

Exception handling in Python

Exception happens when procedure execution hits an unexpected condition. Bellows are some common samples:

• Visit the index beyond the len of list/tuple
• Referring an non-existing variable
• Convert an inappropriate type
• Mixing data types without appropriate coercion

Traditional ways we handle the exception:

• Keep silent, substitute a default value and continuous (not good. caller not aware)
• Return an ‘error’ value (not perfect. caller has to deal with the return code)
• Stop execution and siginal error condition (raise an exception)

common errors

Here are the common errors we will see in Python programing

• SyntaxError: Python can’t parse program
• NameError: local or global name not found
• AttributeError: attribute reference fails
• TypeError: operand doesn’t have correct type
• ValueError: operand type okay, but value is illegal
• IOError: IO system reports malfunction (e.g. file not found)

Python Exception handling block:

Try:
    # main program which may cause exception
    except “exception type1”, “exception value 1”:
        exception handle block 1
    except “exception type2”, “exception value 2”:
        exception handle block 2
    … …
    except “exception typen”, “exception value n”:
        exception handle block n
    else:
        #program block if none of the exceptions are raised
    finally:
        #the block which will be executed no matter if there is any exception.