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
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.
----------
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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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30343>
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