Message Encoding – JSON

fedmsg messages are encoded as JSON.

Use the functions fedmsg.encoding.loads(), fedmsg.encoding.dumps(), and fedmsg.encoding.pretty_dumps() to encode/decode.

When serializing objects (usually python dicts) with fedmsg.encoding.dumps() and fedmsg.encoding.pretty_dumps(), the following exceptions to normal JSON serialization are observed.

  • datetime.datetime objects are correctly converted to seconds since the epoch.
  • For objects that are not JSON serializable, if they have a .__json__() method, that will be used instead.
  • SQLAlchemy models that do not specify a .__json__() method will be run through fedmsg.encoding.sqla.to_json() which recursively produces a dict of all attributes and relations of the object(!) Be careful using this, as you might expose information to the bus that you do not want to. See Cryptography and Message Signing for considerations.
fedmsg.encoding.loads(s, encoding=None, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

Deserialize s (a str or unicode instance containing a JSON document) to a Python object.

If s is a str instance and is encoded with an ASCII based encoding other than utf-8 (e.g. latin-1) then an appropriate encoding name must be specified. Encodings that are not ASCII based (such as UCS-2) are not allowed and should be decoded to unicode first.

object_hook is an optional function that will be called with the result of any object literal decode (a dict). The return value of object_hook will be used instead of the dict. This feature can be used to implement custom decoders (e.g. JSON-RPC class hinting).

object_pairs_hook is an optional function that will be called with the result of any object literal decoded with an ordered list of pairs. The return value of object_pairs_hook will be used instead of the dict. This feature can be used to implement custom decoders that rely on the order that the key and value pairs are decoded (for example, collections.OrderedDict will remember the order of insertion). If object_hook is also defined, the object_pairs_hook takes priority.

parse_float, if specified, will be called with the string of every JSON float to be decoded. By default this is equivalent to float(num_str). This can be used to use another datatype or parser for JSON floats (e.g. decimal.Decimal).

parse_int, if specified, will be called with the string of every JSON int to be decoded. By default this is equivalent to int(num_str). This can be used to use another datatype or parser for JSON integers (e.g. float).

parse_constant, if specified, will be called with one of the following strings: -Infinity, Infinity, NaN, null, true, false. This can be used to raise an exception if invalid JSON numbers are encountered.

To use a custom JSONDecoder subclass, specify it with the cls kwarg; otherwise JSONDecoder is used.

fedmsg.encoding.dumps(self, o)

Return a JSON string representation of a Python data structure.

>>> JSONEncoder().encode({"foo": ["bar", "baz"]})
'{"foo": ["bar", "baz"]}'
fedmsg.encoding.pretty_dumps(self, o)

Return a JSON string representation of a Python data structure.

>>> JSONEncoder().encode({"foo": ["bar", "baz"]})
'{"foo": ["bar", "baz"]}'

SQLAlchemy Utilities

fedmsg.encoding.sqla houses utility functions for JSONifying sqlalchemy models that do not define their own .__json__() methods.

Use at your own risk. fedmsg.encoding.sqla.to_json() will expose all attributes and relations of your sqlalchemy object and may expose information you not want it to. See Cryptography and Message Signing for considerations.

fedmsg.encoding.sqla.expand(obj, relation, seen)

Return the to_json or id of a sqlalchemy relationship.

fedmsg.encoding.sqla.to_json(obj, seen=None)

Returns a dict representation of the object.

Recursively evaluates to_json(...) on its relationships.