Subclassed json.JSONEncoder does not respect default method for supported types #74528

Activity

1. Xophmeister commented on May 11, 2017

   

   Xophmeistermannequin

   on May 11, 2017

   MannequinAuthor

   More actions

   If you subclass json.JSONEncoder to enable serialisation of custom types beyond those supported, you are meant to transform values of said type into a serialisable version within an overridden default method. For example:

       class MyJSONEncoder(json.JSONEncoder):
           def default(self, o):
               if isinstance(o, MyType):
                   return str(o)

           # Raise TypeError when we have a type we can't serialise
           super().default(o)
   

   This, however, won't work if your custom type is an instance of one of the supported types. This is because, in Lib/json/encoder.py, the _iterencode function (defined in _make_iterencode) type checks against supported types before it delegates to the default method.

   The reason this came up is because I wanted to serialise a named tuple into a JSON object, with keys corresponding to the named tuple's field names. Ignoring the merits (or otherwise) of this desired outcome, this can't work because a named tuple is still a tuple and thus the default method is never called.

   In Python 2.7, the _iterencode method was part of the JSONEncoder class, so you could override it in a subclass; even if doing so is somewhat brittle. In Python 3, this method has been moved out into the module namespace (tested in 3.6; looking through the repo, it looks like this change was made in Python 3.1), so it can't easily be monkey-patched without it affecting other things.

   I believe this to be a bug. It seems reasonable to subclass at least named tuples, dictionaries and lists in such a way that you'd want a different JSON serialisation to their defaults.

   👍React with 👍1Reacted by Dominic Davis-Foster
2. Xophmeister

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on May 11, 2017
3. Xophmeister commented on May 12, 2017

   

   Xophmeistermannequin

   on May 12, 2017

   MannequinAuthor

   More actions

   I have written a proof-of-concept implementation and submitted a pull request. See the PR for details: #1558

   (My CLA is pending; submitted around 2017-05-12T10:30:00Z+0100)
4. merwok commented on Oct 19, 2020

   

   merwok

   on Oct 19, 2020

   Member

   More actions

   See also https://bugs.python.org/issue34858

   This needs a discussion on python-ideas or -dev.
5. ezio-melotti

   transferred this issue fromon Apr 10, 2022
6. serhiy-storchaka

   added this to @serhiy-storchaka's project python-jsonon May 8, 2022
7. serhiy-storchaka

   added this to JSON issueson May 19, 2022