Python Programming/Errors

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In python there are two types of errors; syntax errors and exceptions.

[edit] Syntax errors

Syntax errors are the most basic type of error. They arise when the Python parser is unable to understand a line of code. Syntax errors are always fatal, i.e. there is no way to successfully execute a piece of code containing syntax errors.

[edit] Exceptions

Exceptions arise when the python parser knows what to do with a piece of code but is unable to perform the action. An example would be trying to access the internet with python without an internet connection; the python interpreter knows what to do with that command but is unable to perform it.

[edit] Dealing with exceptions

Unlike syntax errors, exceptions are not always fatal. Exceptions can be handled with the use of a try statement.

Consider the following code to display the HTML of the website 'goat.com'. When the execution of the program reaches the try statement it will attempt to perform the indented code following, if for some reason there is an error (the computer is not connected to the internet or something) the python interpreter will jump to the indented code below the 'except:' command.

import urllib2
url = 'http://www.goat.com'
try:
    req = urllib2.Request(url)
    response = urllib2.urlopen(req)
    the_page = response.read()
    print the_page
except:
    print "We have a problem."

Another way to handle an error is to except a specific error.

try:
    age = int(raw_input("Enter your age: "))
    print "You must be {0} years old.".format(age)
except ValueError:
    print "Your age must be numeric."

If the user enters a numeric value as his/her age, the output should look like this:

Enter your age: 5
You must be 5 years old.

However, if the user enters a non-numeric value as his/her age, a ValueError is thrown when trying to execute the int() method on a non-numeric string, and the code under the except clause is executed:

Enter your age: five
Your age must be a number.

You can also use a try block with a while loop to validate input:

valid = False
while valid == False:
    try:
        age = int(raw_input("Enter your age: "))
        valid = True     # This statement will only execute if the above statement executes without error.
        print "You must be {0} years old.".format(age)
    except ValueError:
        print "Your age must be numeric."

The program will prompt you for your age until you enter a valid age:

Enter your age: five
Your age must be numeric.
Enter your age: abc10
Your age must be numeric.
Enter your age: 15
You must be 15 years old.