Implement str.zfill method as function

zero_fill.py

def is_integer(n):
    """Return True if given n is integer.
    
        Difference with isinstance(n, int) is that
        this function returns True for string instance '123'.
    """
    try:
        int(n)
    except:
        return False
    return True
    

def zero_fill(number, zeros=4):
    """Fill up zeros for the number if number is short"""
    if not is_integer(number):
        raise ValueError("Only Integer is accepted")
    number = str(number)
    if len(number) >= zeroes:
        return
    return '0' * (zeroes - len(number)) + number
    
    

oh, you made a function to figure out whether the input is integer or not. Great Job!