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Serialize/deserialize md5 context. Pseudocode:
import md5
# Start hash generation
m = md5.new()
m.update("Content")
# Serialize m
serialized_m = serialize(m)
# In another function/machine, deserialize m
# and continue hash generation
m2 = deserialize(serialized_m)
m2.update("More content")
m2.digest() 
There are C++ lib but no Python one.

In the python context I believe what you are asking for is to make hashlib hash objects pickleable.

I don't think it's possible to serialize an crypto hash object like MD5. The internal states of those objects are opaque data structures. There is no way official way to access the data structures in a platform independent way.
It *might* be possible to hack something for our own implementation of md5 but definitely not for OpenSSL's implementation.

Unless someone supplies a patch that works inter-operably across all platforms using public APIs in openssl and with the internals of all of python's builtin non-openssl hash functions including unitests for all of the above... This isn't going to be implemented in the stdlib.