Handling exceptions

An expression may be syntactically correct but still generate an error at runtime. Such errors are called exceptions. Unless they are "handled", exceptions cause the program to be interrupted and error messages to be printed out.

>>> chars = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> chars[8]
...
IndexError: list index out of range
>>> 10 * (1/0)
...
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
...
NameError: name 'spam' is not defined
>>> '2' + 2
...
TypeError: cannot concatenate 'str' and 'int' objects
>>> open('data')
...
IOError: [Errno 2] No such file or directory: 'data'

Exceptions are objects with types like TypeError and ZeroDivisionError.
A program can handle exceptions by specifying what to do if one occurs. To do this, you use a try statement, which has this form (among others):
```
try:
   statements
except exception_type:
   statements
```
A try statement works as follows.
- First the try clause (the statements between the try and except keywords) is executed.
- If no exception occurs, the except clause (the statements after except) is skipped, and execution of the try statement is finished.
- If an exception occurs during the execution of the try clause, the rest of the clause is skipped, and if the exception's type matches the exception type after except, the except clause is executed.
- If the exception doesn't match the exception type, it is passed on to any outer try statements there might be. If no handler is found for the exception, execution is interrupted and an error message is printed out.

An example (from the Python tutorial):

>>> while True:
...     try:
...             x = int(input("Please enter a number: "))
...             break
...     except ValueError:
...             print("Oops! That wasn't a valid number.  Try again...")
... 
Please enter a number: abc
Oops! That wasn't a valid number.  Try again...
Please enter a number: 8*8
Oops! That wasn't a valid number.  Try again...
Please enter a number: 88

There can be multiple except clauses, for different types of exceptions, or the types can all be included in a tuple after the except.
```
...
except (ValueError, NameError, ZeroDivisionError):
    pass
```
Practice
- Define a function that takes two numbers and divides the first number by the second. If the second number is 0, it prints a warning and returns 10³⁰. (Use ZeroDivisionError as the exception type.)
- Define a function that takes a sequence and an index and returns the item in the sequence in the index position or the last item in the sequence if the index is too large. (Use IndexError as the exception type.)
- Define a function that takes a filename and returns the contents of the file as a list of strings, one for each line, if the file exists and the empty string if the file does not exist. (Use IOError as the exception type.)

Raising exceptions

You can also force an exception to occur by using a raise statement. A raise statement looks like this:
```
raise exception_instance
```
or
raise exception_type
. A simple way to make an exception instance is like this:
exception_type(string)
. The string prints out when the exception is raised.

An example of raising an exception in a method.
class Word(object): def add_pos(self, pos): if pos not in ['n', 'v', 'a', 'adv', 'prep', 'conj']: raise Exception(pos + ' is not a valid POS') self.pos = pos >>> ========================================================= >>> w = Word() >>> w.add_pos('postp') Traceback (most recent call last): File "", line 1, in w.add_pos('part') File "", line 5, in add_pos raise Exception(pos + ' is not a valid POS') Exception: postp is not a valid POS