def a(b):
    try:
        b = int(b) # or b = float(b) if needed
    except:
        print "please input a number~~"
        return 

    if b>1:
        return 'A'
    elif b==1:
        return 'B'
    else:
        return 'C'


print a('0')
print a('1')
print a('2')
print a('hello world')

C
B
A
please input a number~~
None