two functions to make sure your string only contains digits [0-9]

make_sure_digits.py

def make_sure_digits(string):
    import re
    regex = u"(^\d+$)"
    match = re.match(regex, string)
    if match:
        return match.group()
    return ""

def make_sure_int(string):
    try:
        return int(string)
    except ValueError:
        return ""