Convert string to list of bits and viceversa

not sure why, but here are two ugly oneliners using only builtins:

s = "Hi"
l = map(int, ''.join([bin(ord(i)).lstrip('0b').rjust(8,'0') for i in s]))
s = "".join(chr(int("".join(map(str,l[i:i+8])),2)) for i in range(0,len(l),8))

yields:

>>> l
[0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1]
>>> s
'Hi'

In real world code, use the struct or the bitarray module.

There are many ways to do this with library functions. But I am partial to the third-party bitarray module.

>>> import bitarray
>>> ba = bitarray.bitarray()

Conversion from strings requires a bit of ceremony. Once upon a time, you could just use fromstring, but that method is now deprecated, since it has to implicitly encode the string into bytes. To avoid the inevitable encoding errors, it's better to pass a bytes object to frombytes. When starting from a string, that means you have to specify an encoding explicitly -- which is good practice anyway.

>>> ba.frombytes('Hi'.encode('utf-8'))
>>> ba
bitarray('0100100001101001')

Conversion to a list is easy. (Also, bitstring objects have a lot of list-like functions already.)

>>> l = ba.tolist()
>>> l
[False, True, False, False, True, False, False, False, 
 False, True, True, False, True, False, False, True]

bitstrings can be created from any iterable:

>>> bitarray.bitarray(l)
bitarray('0100100001101001')

Conversion back to bytes or strings is relatively easy too:

>>> bitarray.bitarray(l).tobytes().decode('utf-8')
'Hi'

And for the sake of sheer entertainment:

>>> def s_to_bitlist(s):
...     ords = (ord(c) for c in s)
...     shifts = (7, 6, 5, 4, 3, 2, 1, 0)
...     return [(o >> shift) & 1 for o in ords for shift in shifts]
... 
>>> def bitlist_to_chars(bl):
...     bi = iter(bl)
...     bytes = zip(*(bi,) * 8)
...     shifts = (7, 6, 5, 4, 3, 2, 1, 0)
...     for byte in bytes:
...         yield chr(sum(bit << s for bit, s in zip(byte, shifts)))
... 
>>> def bitlist_to_s(bl):
...     return ''.join(bitlist_to_chars(bl))
... 
>>> s_to_bitlist('Hi')
[0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1]
>>> bitlist_to_s(s_to_bitlist('Hi'))
'Hi'

You could use the built-in bytearray:

>>> for i in bytearray('Hi', 'ascii'):
...     print(i)
... 
72
105

>>> bytearray([72, 105]).decode('ascii')
'Hi'

And bin() to convert to binary.

There are probably faster ways to do this, but using no extra modules:

def tobits(s):
    result = []
    for c in s:
        bits = bin(ord(c))[2:]
        bits = '00000000'[len(bits):] + bits
        result.extend([int(b) for b in bits])
    return result

def frombits(bits):
    chars = []
    for b in range(len(bits) / 8):
        byte = bits[b*8:(b+1)*8]
        chars.append(chr(int(''.join([str(bit) for bit in byte]), 2)))
    return ''.join(chars)