Python: Choose number of bits to represent binary number

Use the Format String Syntax:

>>> format(1, '#04b')
'0b01'
>>> format(1, '#05b')
'0b001'
>>> format(1, '#06b')
'0b0001'

You can use str.zfill to pad the binary part:

def padded_bin(i, width):
    s = bin(i)
    return s[:2] + s[2:].zfill(width)