[Tutor] n.isalnum() is failing

[Terry]

Hi!

I am running Python 2.5, and I have an IF statement in the below program
that does not seem 
to be doing it's job. The program simply acquires a range of years from
the user and prints out 
all the leap years between the two dates. I am having trouble in
trapping for possible user 
errors. I am using x.isalnum() to check that I have a number for each
year entered, and it 
doesn't look to me like it is functioning properly.

When I purposely enter bad data, such as 'aaaa' for one of the a year
entries, the IF statement:

        if start.isalnum() == 1 and end.isalnum() == 1:

-Fails to detect that 'aaaa' is not a number and lets the program then
die tragically a horrible 
sudden awkward death on the very next line:

        start = int(start); end = int(end)

Traceback (most recent call last):
  File "/home/lucky/Documents/Python_Programs/leap_years.py", line 119,
in <module>
    start = int(start); end = int(end)
ValueError: invalid literal for int() with base 10: 'aaaa'

If I say on the commandline:

>>> n = 'aaaa'
>>> n.isalnum()
True                     <------------It seems to me it should be saying
False!!!
>>> 


My program starts here:

def leap(yyyy):
    answer = 0
    t1 = yyyy / 4
    if t1 == int(t1):
        t2 = yyyy / 100
        t3 = yyyy / 400
        if t2 <> int(t2) or t3 == int(t3):
            answer = "-- leap year!"
    return answer

print "This program finds all leap years between two dates.";print;print

start = raw_input("Enter yyyy for beginning year : ")
end = raw_input("Enter yyyy for ending year : ")

if len(start) == 4 and len(end) == 4:
    if start.isalnum() == 1 and end.isalnum() == 1:
#<----------fails to detect 'aaaa' as not a number
        start = int(start); end = int(end)
#<----------commits suicide here
        if start > 0 and end > start:
            print; print
            for i in range(start, end + 1):
                answer = leap(i)
                if answer != 0:
                    print i, answer
print "Done!"
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