binary to octal conversion

bin_to_oct.py

# binary to oct
def bin_to_oct(num):
    total = ""
    subtotal = 0
    numstr = str(num)
    while(numstr):
        # foreach digit of the nibble (3 binary digits) translate into single oct digit
        # start from least sig digit
        subtotal = 0
        # pad front of numbers with 0's to fill to 4 bits
        if len(numstr) < 3:
            numstr = "0"*(3-len(numstr))+numstr
        for i in range(1,4):
            dig = numstr[-i]
            subtotal += int(dig) * int(pow(2,i-1))
        total = str(subtotal) + total
        # slice off last 3 bits and loop again
        numstr = numstr[:-3]    
    return total