Hash algorithm for dynamic growing/streaming data?

This is theoretically possible (the md5 so far should contain all the state you need to continue) but it looks like the normal APIs don't provide what you need. If you can suffice with a CRC instead, this will probably be a lot easier, since those are more commonly used for the "streaming" cases like you need. See here:

binascii.crc32(data[, crc])

crc32() accepts an optional crc input which is the checksum to continue from.

Hope that helps.

Not from the known digest, but from the known state. You can use a pure python MD5 implementation and save its state. Here is an example using _md5.py from from PyPy:

import _md5

def md5_getstate(md):
    return (md.A, md.B, md.C, md.D, md.count + [], md.input + [], md.length)

def md5_continue(state):
    md = _md5.new()
    (md.A, md.B, md.C, md.D, md.count, md.input, md.length) = state
    return md

m1 = _md5.new()
m1.update("hello, ")
state = md5_getstate(m1)
m2 = md5_continue(state)
m2.update("world!")
print m2.hexdigest()

m = _md5.new()
m.update("hello, world!")
print m.hexdigest()

As e.dan noted, you can also use almost any checksuming algorithm (CRC, Adler, Fletcher), but they do not protect you well from the intentional data modification, only from the random errors.

EDIT: of course, you can also re-implement the serialization method using ctypes from the thread you referenced in a more portable way (without magic constants). I believe this should be version/architecture independent (tested on python 2.4-2.7, both i386 and x86_64):

# based on idea from http://groups.google.com/group/comp.lang.python/msg/b1c5bb87a3ff5e34

try:
    import _md5 as md5
except ImportError:
    # python 2.4
    import md5
import ctypes

def md5_getstate(md):
    if type(md) is not md5.MD5Type:
        raise TypeError, 'not an MD5Type instance'
    return ctypes.string_at(id(md) + object.__basicsize__,
                            md5.MD5Type.__basicsize__ - object.__basicsize__)

def md5_continue(state):
    md = md5.new()
    assert len(state) == md5.MD5Type.__basicsize__ - object.__basicsize__, \
           'invalid state'    
    ctypes.memmove(id(md) + object.__basicsize__,
                   ctypes.c_char_p(state),
                   len(state))
    return md

m1 = md5.new()
m1.update("hello, ")
state = md5_getstate(m1)
m2 = md5_continue(state)
m2.update("world!")
print m2.hexdigest()

m = md5.new()
m.update("hello, world!")
print m.hexdigest()

It is not Python 3 compatible, since it does not have an _md5/md5 module.

Unfortunately hashlib's openssl_md5 implementation is not suitable for such hacks, since OpenSSL EVP API does not provide any calls/methods to reliably serialize EVP_MD_CTX objects.