A Custom SqlAlchemy JSON encoder class to encode models instances in json format which supports major object instances useful in developing api's in flask.

json_encoder.py

# To use this custom JsonEncoder class, you have to create a __json__() named 
# function in each sqlalchemy model class, which basically return a list of model
# class attributes to be parsed, otherwise all attributes are processed present
# in `dir(class_name)`.
# 
# Model Example:
#         class User(db.Model):
#           id = db.Column(db.Integer, primary_key=True)
#           name = db.Column(db.String(100))
#           email = db.Column(db.String(100))
# 
#         def __init__(self, name, email):
#           self.name = name
#           self.email = email
# 
#         def __json__(self):
#           return ['name', 'email']
# 
# 
# And, also add this custom json encoder class (i.e AlchemyJsonEncoder) in your
# flask application instance (i.e app) 
# 
#  app = Flask(__name__)
#  app.json_encoder = AlchemyJSONEncoder
# 
# Note:: This json encoder class supports various objects
#       1) Datetime
#       2) UUID
#       3) date
#       4) Decimal (returned by SqlAlchemy db.Numeric class)
#       5) Markup
# 
from sqlalchemy.ext.declarative import DeclarativeMeta
from flask import json
from decimal import Decimal
from datetime import date


class AlchemyJSONEncoder(json.JSONEncoder):
    def default(self, o):
        # check if object `o` is of custom declared model instance
        if isinstance(o.__class__, DeclarativeMeta):
            data = {}
            fields = o.__json__() if hasattr(o, '__json__') else dir(o)
            for field in [f for f in fields if not f.startswith('_') and
                          f not in ['metadata', 'query', 'query_class']]:
                value = o.__getattribute__(field)
                try:
                    if json.dumps(value):
                        data[field] = value
                except TypeError:
                    data[field] = None
            return data
        # check if object `o` is of Decimal instance
        elif isinstance(o, Decimal):
            return o.to_eng_string()
        # check if object `o` is of date instance
        elif isinstance(o, date):
            return o.isoformat()
        # rest of objects are handled by default JSONEncoder like 'Datetime', 
        # 'UUID', 'Markdown' and various others
        return json.JSONEncoder.default(self, o)