long <-> str binary conversion

Is there any lib that convert very long numbers to string just copying the data?

These one-liners are too slow:

def xlong(s):
    return sum([ord(c) << e*8 for e,c in enumerate(s)])

def xstr(x):
    return chr(x&255) + xstr(x >> 8) if x else ''

print xlong('abcd'*1024) % 666
print xstr(13**666)

From stackoverflow

• You want the struct module.

  packed = struct.pack('l', 123456)
  assert struct.unpack('l', packed)[0] == 123456
  

  J.F. Sebastian : It will not work for large numbers e.g., 13**666

• How about

  from binascii import hexlify, unhexlify
  
  def xstr(x):
      hex = '%x' % x
      return unhexlify('0'*(len(hex)%2) + hex)[::-1]
  
  def xlong(s):
      return int(hexlify(s[::-1]), 16)
  

  I didn't time it but it should be faster and also work on larger numbers, since it doesn't use recursion.

• I'm guessing you don't care about the string format, you just want a serialization? If so, why not use Python's built-in serializer, the cPickle module? The dumps function will convert any python object including a long integer to a string, and the loads function is its inverse. If you're doing this for saving out to a file, check out the dump and load functions, too.

  >>> import cPickle
  >>> print cPickle.loads(cPickle.dumps(13**666)) % 666
  73
  >>> print (13**666) % 666
  73
  

• If you need fast serialization use marshal module. It's around 400x faster than your methods.

• Performance of cPickle vs. marshal (Python 2.5.2, Windows):

  python -mtimeit -s"from cPickle import loads,dumps;d=13**666" "loads(dumps(d))"
  1000 loops, best of 3: 600 usec per loop
  
  python -mtimeit -s"from marshal import loads,dumps;d=13**666" "loads(dumps(d))"
  100000 loops, best of 3: 7.79 usec per loop
  
  python -mtimeit -s"from pickle import loads,dumps;d= 13**666" "loads(dumps(d))"
  1000 loops, best of 3: 644 usec per loop
  

  marshal is much faster.

• In fact, I have a lack of long(s,256) . I lurk more and see that there are 2 function in Python CAPI file "longobject.h":

  PyObject * _PyLong_FromByteArray( const unsigned char* bytes, size_t n, int little_endian, int is_signed);
  int _PyLong_AsByteArray(PyLongObject* v, unsigned char* bytes, size_t n, int little_endian, int is_signed);
  

  They do the job. I don't know why there are not included in some python module, or correct me if I'am wrong.