Keep a datetime.date in 'yyyy-mm-dd' format when using Flask's jsonify

Question

For some reason, the jsonify function is converting my datetime.date to what appears to be an HTTP date. How can I keep the date in yyyy-mm-dd format when using jsonify?

test_date = datetime.date(2017, 4, 27)
print(test_date)  # 2017-04-27
test_date_jsonify = jsonify(test_date)
print(test_date_jsonify.get_data(as_text=True))  # Thu, 27 Apr 2017 00:00:00 GMT

As suggested in the comments, using jsonify(str(test_date)) returns the desired format. However, consider the following case:

test_dict = {"name": "name1", "date":datetime.date(2017, 4, 27)}
print(test_dict) # {"name": "name1", "date":datetime.date(2017, 4, 27)}

test_dict_jsonify = jsonify(test_dict)
print(test_dict_jsonify.get_data(as_text=True)) # {"date": "Thu, 27 Apr 2017 00:00:00 GMT", "name": "name1"}

test_dict_jsonify = jsonify(str(test_dict))
print(test_dict_jsonify.get_data(as_text=True)) # "{"date": datetime.date(2017, 4, 27), "name": "name1"}"

In this case, the str() solution does not work.

Answer 1

edit: this answer is now too old for Flask versions 2.3+.

for those newer versions, instead customize json_provider_class; reference: https://flask.palletsprojects.com/en/2.2.x/api/?highlight=json_encoder#flask.Flask.json_provider_class

---

Following this snippet you can do this:

from flask.json import JSONEncoder
from datetime import date


class CustomJSONEncoder(JSONEncoder):
    def default(self, obj):
        try:
            if isinstance(obj, date):
                return obj.isoformat()
            iterable = iter(obj)
        except TypeError:
            pass
        else:
            return list(iterable)
        return JSONEncoder.default(self, obj)

app = Flask(__name__)
app.json_encoder = CustomJSONEncoder

Route:

import datetime as dt

@app.route('/', methods=['GET'])
def index():
    now = dt.datetime.now()
    return jsonify({'now': now})