Python - 5.Exception Handling

From:http://interactivepython.org/courselib/static/pythonds/Introduction/ExceptionHandling.html

Exception Handling

There are two types of errors that typically occur when writing programs.

• syntax error - simply means that the programmer has made a mistake in the structure of a statement or expression.

For example:

>>> for i in range(10)
SyntaxError: invalid syntax (<pyshell#61>, line 1)

• logic error - , denotes a situation where the program executes but gives the wrong result.

In some cases, logic errors lead to very bad situations such as trying to divide by zero or trying to access an item in a list where the index of the item is outside the bounds of the list. In this case, the logic error leads to a runtime error that causes the program to terminate. These types of runtime errors are typically called exceptions.

>>> anumber = int(input("Please enter an integer "))
Please enter an integer -23
>>> print(math.sqrt(anumber))
Traceback (most recent call last):
  File "<pyshell#102>", line 1, in <module>
    print(math.sqrt(anumber))
ValueError: math domain error
>>>

• • 　　We can handle this exception by calling the print function from within a try block.

>>> try:
       print(math.sqrt(anumber))
    except:
       print("Bad Value for square root")
       print("Using absolute value instead")
       print(math.sqrt(abs(anumber)))

Bad Value for square root
Using absolute value instead
4.79583152331
>>>

• • 　　It is also possible for a programmer to cause a runtime exception by using the raise statement. For example, instead of calling the square root function with a negative number, we could have checked the value first and then raised our own exception. The code fragment below shows the result of creating a new RuntimeError exception. Note that the program would still terminate but now the exception that caused the termination is something explicitly created by the programmer.

>>> if anumber < 0:
...    raise RuntimeError("You can't use a negative number")
... else:
...    print(math.sqrt(anumber))
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
RuntimeError: You can't use a negative number
>>>

There are many kinds of exceptions that can be raised in addition to the RuntimeError shown above. See the Python reference manual for a list of all the available exception types and for how to create your own.