How to encode custom python objects as BSON with Pymongo?

To encode custom python objects as BSON with Pymongo, you have to write a SONManipulator. From the docs:

SONManipulator instances allow you to specify transformations to be applied automatically by PyMongo.

from pymongo.son_manipulator import SONManipulator
class Transform(SONManipulator):
  def transform_incoming(self, son, collection):
    for (key, value) in son.items():
      if isinstance(value, Custom):
        son[key] = encode_custom(value)
      elif isinstance(value, dict): # Make sure we recurse into sub-docs
        son[key] = self.transform_incoming(value, collection)
    return son
  def transform_outgoing(self, son, collection):
    for (key, value) in son.items():
      if isinstance(value, dict):
        if "_type" in value and value["_type"] == "custom":
          son[key] = decode_custom(value)
        else: # Again, make sure to recurse into sub-docs
          son[key] = self.transform_outgoing(value, collection)
    return son

then add it to your pymongo database object −

db.add_son_manipulator(Transform())

Make a note that you don't have to add the _type field if you want to silently cast a numpy array to a python array.