TypeError: Object of type 'datetime' is not JSON serializable (with serialize function)

Question

I'm I getting this TypeError: Object of type 'datetime' is not JSON serializable error, even though I have a specific serialize function described in my model.

This is my code:

Flask route (rendered by React):

menus.py

@menus_bp.route('/menus', methods=['GET', 'POST'])
def menus():

    response_object = {
        'status': 'fail',
        'message': 'Invalid payload.'
        }
    try:
        user = User.query.filter_by(id=1).first()
        if user.menu == []:
            return edit_menu()
        else:
            template = render_template('menus.html')
            response_object = {
                'status': 'success',
                'message': 'success',
                'data': [{"restaurant": user.restaurant,
                          "menu": menu,
                          "content": template}] # template passed to React
                }

            # db method
            Create_Menu(user=user)

        return jsonify(response_object), 200
    except (exc.IntegrityError, ValueError):
        db.session.rollback()
        return jsonify(response_object), 400

methods.py

def Create_Menu(user):
    menu = Menu(user=user)
    db.session.add(menu)
    db.session.commit()

    return {"status": True,
            "menu": menu}

and finally the Menu model, which has a serialize() function:

class Menu(db.Model):
    __tablename__='menu'
    """
    Model for storing menus. 
    """   
    id = db.Column(db.Integer, primary_key=True)
    created = db.Column(db.DateTime, default=func.now(), nullable=False)       
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))

    def __init__(self, user):
        self.user = user

    def serialize(self):
       return { 'id' : self.id,
                'created': self.created,
                'coffees' : [ item.serialize() for item in self.coffees]}

But I'm getting the following traceback:

TypeError: Object of type 'datetime' is not JSON serializable

File "/usr/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
return self.wsgi_app(environ, start_response)
File "/usr/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
response = self.handle_exception(e)
File "/usr/lib/python3.6/site-packages/flask_restful/__init__.py", line 269, in error_router
return original_handler(e)
File "/usr/lib/python3.6/site-packages/flask_cors/extension.py", line 161, in wrapped_function
return cors_after_request(app.make_response(f(*args, **kwargs)))
File "/usr/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib/python3.6/site-packages/flask/_compat.py", line 34, in reraise
raise value.with_traceback(tb)
File "/usr/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
response = self.full_dispatch_request()
File "/usr/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/lib/python3.6/site-packages/flask_restful/__init__.py", line 269, in error_router
return original_handler(e)
File "/usr/lib/python3.6/site-packages/flask_cors/extension.py", line 161, in wrapped_function
return cors_after_request(app.make_response(f(*args, **kwargs)))
File "/usr/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib/python3.6/site-packages/flask/_compat.py", line 34, in reraise
raise value.with_traceback(tb)
File "/usr/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/lib/python3.6/site-packages/flask_debugtoolbar/__init__.py", line 125, in dispatch_request
return view_func(**req.view_args)
File "/usr/lib/python3.6/site-packages/flask_restful/__init__.py", line 462, in wrapper
return self.make_response(data, code, headers=headers)
File "/usr/lib/python3.6/site-packages/flask_restful/__init__.py", line 491, in make_response
resp = self.representations[mediatype](data, *args, **kwargs)
File "/usr/lib/python3.6/site-packages/flask_restful/representations/json.py", line 21, in output_json
dumped = dumps(data, **settings) + "\n"
File "/usr/lib/python3.6/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/usr/lib/python3.6/json/encoder.py", line 201, in encode
chunks = list(chunks)
File "/usr/lib/python3.6/json/encoder.py", line 430, in _iterencode
yield from _iterencode_dict(o, _current_indent_level)
File "/usr/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 325, in _iterencode_list
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 325, in _iterencode_list
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
yield from chunks
File "/usr/lib/python3.6/json/encoder.py", line 437, in _iterencode
o = _default(o)
File "/usr/lib/python3.6/json/encoder.py", line 180, in default
o.__class__.__name__)
TypeError: Object of type 'datetime' is not JSON serializable

It used to work when I was redering templates with Flask at backend, but now with frontend requets, it breaks with the erro above.

what is wrong now? why does not my serialize function work anymore?

Answer 1

See below (using custom JSON encoder)

See http://flask.pocoo.org/snippets/119/ as well

import datetime
import json


class DateTimeEncoder(json.JSONEncoder):
    def default(self, z):
        if isinstance(z, datetime.datetime):
            return (str(z))
        else:
            return super().default(z)


my_dict = {'date': datetime.datetime.now()}

print(json.dumps(my_dict,cls=DateTimeEncoder))

output

{"date": "2019-06-12 15:44:14.978766"}

Answer 2

As pointed out in the comments in the accepted answer, you can simply use:

json.dumps(my_dict, default=str)

Unless you want to make it more complicated for whatever reason.