The documentation states that the bson.binary.Binary class is instantiated with a default subtype of BINARY_SUBTYPE if not otherwise defined.

Relevant documentation URL: http://api.mongodb.org/python/1.9%2B/api/bson/binary.html?highlight=binary#bson.binary.Binary

The source, on the other hand, shows that the default subtype for this class is OLD_BINARY_SUBTYPE:

def _new_(cls, data, subtype=OLD_BINARY_SUBTYPE):
if not isinstance(data, str):
raise TypeError("data must be an instance of str")
if not isinstance(subtype, int):
raise TypeError("subtype must be an instance of int")
if subtype >= 256 or subtype < 0:
raise ValueError("subtype must be contained in [0, 256)")
self = str._new_(cls, data)
self.__subtype = subtype
return self

Based upon a statement in the documentation (bson.binary.BINARY_SUBTYPE - "This is becomming the default subtype and should be the most commonly used."), it seems the source should be changed to default to BINARY_SUBTYPE instead of OLD_BINARY_SUBTYPE.