Easy binary2decimal and decimal2binary (Python recipe) by Guyon Morée

ActiveState Code (http://code.activestate.com/recipes/425080/)

problems. 1) does not properly handle negative numbers or floats

2) Leaves a leading '0' on the string it returns

To fix these, you could do:

def D(n):
    '''undefined on negative numbers, non-integers'''
    if n <= 0: return ''
    return D(n>>1) + str(n%2)

still has problem. The modified version can not handle 0 correctly.

>>> D(0)
''

Not '0' as expected. Is there a better solution?

sure.

def E(n):
    '''undefined on negative numbers, non-integers'''
    s = ''
    if n < 0: return ''
    if n == 0: return '0'
    while n > 0:
        s = str(n%2) + s
        n = n >> 1
    return s

Opportunity to use divmod().

def Decimal2Binary(dec_num):
    """ Return the binary representation of dec_num """
    if dec_num == 0: return '0'
    head, tail = divmod(dec_num, 2)
    return Decimal2Binary(head) + str(tail)

control leading zeros.

def Decimal2BinaryBits(dec_num, bits):
    res = Decimal2Binary(dec_num)[:bits].zfill(bits)
    return res


print Decimal2BinaryBits(0x1234, 16)
>0001001000110100
print Decimal2Binary(0x1234)
>01001000110100

a bit faster. binary operators should be fastest when it comes to dealing with binary values

def bit_is_set(n, num): return (1 << n) & num != 0

def int_to_bin(num):
    out, n = '', 0
    while (1 << n) <= num:
        out += ((1 << n) & num != 0)  and '1' or '0'  # bit_is_set()
        n += 1
    return out and out[-1::-1] or '0'