For those of us more dictionary oriented, there is https://pypi.python.org/pypi/voluptuous (which is OK for the most part, as long as you are only trying to do validation, and nothing too crazy)
+1 for marshmallow - most of the serialization libraries are 80% there, but marshmallow has it 95% down - all the weird corner cases about nested models and lists of nested models and all that. Plus the dev is very helpful and courteous on github.
I've tried truckloads of Python serialisation libs over the last few years and marshmallow is the one that finally makes me feel like I don't need to look for another one.
Looks extremely similar to https://github.com/schematics/schematics . Always good to see two projects approach the problem the same way independently - higher chance that the solution is right :)
Schematics has a very useful feature in "roles" - i.e. a good way of hiding certain fields in certain situations (e.g. admin views vs self vs other vs anonymous). Does marshmallow have something similar?