Handling errors in python, invalid characters?

I can do this easily using an if/else statement but how can I do it using exception handling?

No one would do this in real life:

try:
    assert c in ('n', 'y')
except AssertionError:
    print("Invalid Input")

I seriously don't see the reason to do so.

ans = ''
while ans not in ['y', 'n']:
    ans = input('please enter y or n: ').lower()

Is a perfectly easy way to implement this. I guess you could try something like this:

while True:
    ans = input('please enter y or n: ').lower()
    try:
        assert(ans in ['y', 'n'])
        break
    except:
        print('invalid answer')

But why?

As others are saying this NOT a conventional case for using Exception handling.

However if it will help you understand how exception handling works then consider this code:

#!python

def validate_input(response):
    if response not in ('y', 'n'):
        raise ValueError, 'Invalid input'
    return True

some_input = None
while not some_input:
    some_input = raw_input('Please enter "y" or "n"')
    try:
        validate_input(some_input)
    except ValueError:
        print >> sys.stderr, "I'm not letting you out 'til you give a valid answer"
        some_input = False

This should work (though I haven't tested it). But it's a rather ungainly way to perform simple input validation.

(I could have used while True: and perform a break in an else: clause for my try: block ... but I think this way is a tiny bit more readable since the loop clearly will continue until some_input has any (Python) "non-false" value).

Without any further information, suppose that you have a function that you want to execute depending on the provided answer. The answers can then be stored in a dictionary as keys and these functions as values. The try/except is then very suitable. Example:

answer_functions = {
    'y': yes_function,
    'n': no_function
}

while True:

    answer = input('y/n? ')

    try:
        answer_function = answer_functions[answer]
    except KeyError:
        print('Invalid input.')
    else:
        answer_function()
        break

In the try-block you try to get the correct handler-function using the input as key. If the key isn't in the dictionary a KeyError is raised which you can catch with the except-clause. I like this approach in the case when there are many different options which need to be handled differently, since the resulting code is really clean.