How can I use ujson as a Flask encoder/decoder? - json

Question

I have seen that once can use simplejson as a JSON encoder / decoder within a Flask application like this:
from simplejson import JSONEncoder, JSONDecoder
app.json_encoder = JSONEncoder
app.json_decoder = JSONDecoder
But ujson does not have such objects:
>>> from ujson import JSONEncoder
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: cannot import name 'JSONEncoder' from 'ujson' (/.../site-packages/ujson.cpython-38-x86_64-linux-gnu.so
What I tried
I thought of something like this:
from uuid import UUID, uuid4
import ujson as json
from flask import Flask, jsonify
from flask.json import JSONEncoder
class CustomJSONEncoder(JSONEncoder):
def default(self, obj):
if isinstance(obj, UUID):
return str(obj)
return JSONEncoder.default(self, obj)
def encode(self, o):
return json.dumps(o)
app = Flask(__name__)
app.json_encoder = CustomJSONEncoder
#app.route("/")
def index():
return jsonify({"foo": uuid4()})
app.run()
But I'm uncertain because the help to the decoder shows this:
| decode(self, s, _w=<built-in method match of re.Pattern object at 0x7f6a608404b0>, _PY3=True)
| Return the Python representation of ``s`` (a ``str`` or ``unicode``
| instance containing a JSON document)
|
| raw_decode(self, s, idx=0, _w=<built-in method match of re.Pattern object at 0x7f6a608404b0>, _PY3=True)
| Decode a JSON document from ``s`` (a ``str`` or ``unicode``
| beginning with a JSON document) and return a 2-tuple of the Python
| representation and the index in ``s`` where the document ended.
| Optionally, ``idx`` can be used to specify an offset in ``s`` where
| the JSON document begins.
|
| This can be used to decode a JSON document from a string that may
| have extraneous data at the end.
Is my implementation ok? How would I support those other parameters? When is decode and when raw_decode used?
When I run this, I get:
[2020-10-09 10:54:52,063] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
raise value
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
rv = self.dispatch_request()
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "main.py", line 28, in index
return jsonify({"foo": uuid4()})
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/json/__init__.py", line 370, in jsonify
dumps(data, indent=indent, separators=separators) + "\n",
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/flask/json/__init__.py", line 211, in dumps
rv = _json.dumps(obj, **kwargs)
File "/home/moose/.pyenv/versions/3.8.3/lib/python3.8/site-packages/simplejson/__init__.py", line 398, in dumps
return cls(
File "main.py", line 14, in encode
return json.dumps(o)
TypeError: UUID('1f45a2bc-c964-48f0-b2f5-9ef7a2557966') is not JSON serializable

Answer 1

You can use a try block like that:
import ujson as json
from flask.json import JSONEncoder
class CustomJSONEncoder(JSONEncoder):
def default(self, obj):
try:
return json.dumps(obj)
except TypeError:
return JSONEncoder.default(self, obj)
from flask import Flask
app = Flask(__name__)
app.json_encoder = CustomJSONEncoder