>>> def calc(chars, n, pad=0):
	"""Convert 'n' to base defined by string 'chars', with padding 'pad'"""
	output = ''
	while n != 0:
		output = chars[n % len(chars)] + output
		n -= n % len(chars)
		n /= len(chars)
		if len(output) < pad:
			return ('0' * (pad - len(output))) + output
		else:
			return output

		
>>> calc('0123456789abcdef', 42, 4)
'000a'
>>> calc('0123456789abcdef', 42, pad=4)
'000a'