[New-bugs-announce] [issue30343] Subclassed json.JSONEncoder does not respect default method for supported types

Christopher Harrison report at bugs.python.org
Thu May 11 09:58:26 EDT 2017

New submission from Christopher Harrison:

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

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.

components: Library (Lib)
messages: 293494
nosy: Christopher Harrison
priority: normal
severity: normal
status: open
title: Subclassed json.JSONEncoder does not respect default method for supported types
type: behavior
versions: Python 3.6

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