Exceptions

Here is a simple code that requires a user to type in a number:

while True:
   age = int(input("Type in your guess : At what age Alexander the great died? : " ))
   print(age)

The code expects an integer input from the user.

Syntax errors are parsing errors that we've had lots of times while we're coding. For example, suppose we missed the colon(':') from the code above, we get the following complain:

  File "e.py", line 1
    while True 
              ^
SyntaxError: invalid syntax

Note that the code was not executed at all. We got the syntax error while PYthon interpreter was doing parsing.

What if a user type in "dumb" instead of a number string?

Type in your guess : At what age Alexander the great died? : dumb
Traceback (most recent call last):
  File "e.py", line 2, in <module>
    age = int(input("Type in your guess : At what age Alexander the great died? : " ))
ValueError: invalid literal for int() with base 10: 'dumb'

The "ValueError" is one of the exceptions.

Even if a statement or expression is syntactically correct, it may cause an error when we execute it. Errors detected during execution are called exceptions and we want to handle (capture) them in our code.

Here are examples of exceptions:

>>> 1/0
Traceback (most recent call last):
  File "", line 1, in 
ZeroDivisionError: division by zero
>>> a + 10
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'a' is not defined
>>> '10' + 10
Traceback (most recent call last):
  File "", line 1, in 
TypeError: Can't convert 'int' object to str implicitly
>>> 

How can we use exceptions?

1. Error Handling:
   Python raises exceptions at runtime. We can catch (or ignore) and respond to the errors in our code. If we opt to ignore the raised exception, Python's default exception-handling kicks in and stops our code with an error message. So, if we don't like the default way of handling exception, we may want to catch it using "try".
2. Closing time operation:
   We can use "try/finally" to guarantee the execution of a certain operation regardless of the presence or absence of exceptions in our code.

Let's put "try" and "finally" into the code:

# e.py
while True:
   try:
      age = int(input("Type in your guess : Age of the Universe : " ))
      print(age)
      break
   except ValueError:
      print("Please make sure you type in an integer")
   except:
      break
   finally:
      print("age loop" )

Run it:

$ python e.py
Type in your guess : Age of the Universe : dumb
Please make sure you type in an integer
age loop
Type in your guess : Age of the Universe : 45,500
Please make sure you type in an integer
age loop
Type in your guess : Age of the Universe : 10
10
age loop

Note the line "except:" catches all other exceptions not caught in "except ValueError:".

Usually, Python raises exceptions for us but our code can raise exceptions as well:

>>> try:
...    raise IndexError
... except IndexError:
...    print('got exception raised by our code')
... 
got exception raised by our code

So far, the exceptions have been raised for the built-in exceptions.

We can define our own exceptions as shown below:

# e2.py
class Negative(Exception):
   pass
def oops():
   raise Negative()

try:
   age = int(input("Type in your guess : Age of the Universe : " ))
   print(age)
   if age <= 0:
      print('calling ooops')
      oops()
except ValueError:
   print("Please make sure you type in an integer")
except Negative:
   print("Please make sure you type in a positive integer")
except:
   print("Somethings wrong!")
finally:
   print("finally!" )

Run the code:

$ python e2.py
Type in your guess : Age of the Universe : -10
-10
calling ooops
Please make sure you type in a positive integer
finally!