gh-158270: json: make dumps() skip JSONEncoder

[methane]

• Issue: json: Speed up json.dump() with non-default options. #158270

[eendebakpt]

There is a behaviour change when users override the JSONEncoder.default. This is not very clean programming, but it is used (e.g. https://stackoverflow.com/a/18561055, accepted answer to "Making object JSON serializable with regular encoder"): JSONEncoder.default = _default) and JSONEncoder.default is a public attribute.

Reproducer:

import io
import json


class Foo:
    pass


_orig_default = json.JSONEncoder.default

def _patched_default(self, o):
    if isinstance(o, Foo):
        return "FOO"
    return _orig_default(self, o)

json.JSONEncoder.default = _patched_default

obj = {"a": Foo()}

print("dumps(obj)          ->", json.dumps(obj))
sio = io.StringIO()
json.dump(obj, sio, indent=2)
print("dump(obj, indent=2) ->", sio.getvalue().replace("\n", ""))
try:
    print("dumps(obj, indent=2)->", json.dumps(obj, indent=2).replace("\n", ""))
except TypeError as e:
    print("dumps(obj, indent=2)-> TypeError:", e)

main:

dumps(obj)          -> {"a": "FOO"}
dump(obj, indent=2) -> {  "a": "FOO"}
dumps(obj, indent=2)-> {  "a": "FOO"}

this PR:

dumps(obj)          -> {"a": "FOO"}
dump(obj, indent=2) -> {  "a": "FOO"}
dumps(obj, indent=2)-> TypeError: Object of type Foo is not JSON serializable

[hugovk]

This tests dump() but only dumps() is now changed.

[methane]

I was not expecting monkey patching JSONEncoder to be used in practice rather than creating a subclass.
Given that this optimization is not particularly critical, I don't think it is worth implementing at the expense of compatibility.