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Decorator for REST endpoints in flask. Validate JSON request data.

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Decorator for REST endpoints in flask. Validate JSON request data.

When building json REST services I find myself already specifying json-schema for POST data while defining swagger spec. This package brings json validation to flask. It omits the need to validate the data yourself while profiting from an already established standard ( Defining the schema right before the route helps the self-documentation of an endpoint (see usage).

This package uses jsonschema to for validation:


This package provides a flask route decorator to validate json payload.

from flask import Flask, jsonify, g, url_for
from flask_expects_json import expects_json
# example imports
from models import User
from orm import NotUniqueError

app = Flask(__name__)

schema = {
    'type': 'object',
    'properties': {
        'name': {'type': 'string'},
        'email': {'type': 'string'},
        'password': {'type': 'string'}
    'required': ['email', 'password']

@app.route('/register', methods=['POST'])
def register():
    # if payload is invalid, request will be aborted with error code 400
    # if payload is valid it is stored in

    # do something with your data
    user = User().from_dict(
    except NotUniqueError as e:
        # exception path: duplicate database entry
        return jsonify(dict(message=e.message)), 409

    # happy path: json response
    resp = jsonify(dict(auth_token=user.encode_auth_token(), user=user.to_dict()})
    resp.headers['Location'] = url_for('users.get_user',
    return resp, 201

The expected json payload is recognizable through “schema”. If schema is not met the requests aborts (400) with a hinting error message.

Mimetype checking

As of 1.2.0 this decorator uses flask.request.get_json(force=False) to get the data. This means the mimetype of the request has to be ‘application/json’. Can be disabled by setting force=False. Be aware that this creates a major security vulnerability to CSRF since CORS is not enforced for certain mimetypes. Thanks to Argishti Rostamian for noticing.

def strict():
    return 'This view will return 400 if mimetype is not \'application/json\'

@expects_json({}, force=False)
def insecure():
    return 'This view will validate the data no matter the mimetype.'

Default values

Normally validators wont touch the data. By default this package will not fill in missing default values provided in the schema. If you want to you can set fill_default=True explicitly. The validation will be performed after this action, so default values can lead to invalid data.


python test




  • Changed error message when get_json() fails.


  • Bugfix in DefaultValidatingDraft4Validator when trying to set a default value on strings.

1.3.0 - 2018-02-16


  • Defaults wont be filled in request data by default. Set fill_defaults=True explicitly.

1.2.0 - 2018-02-15


  • Security: set force=False as default argument. Before: force=True

1.1.0 - 2018-02-03


  • missing default values will be filled into the request data
  • can be turned off via fill_defaults=False

1.0.6 - 2018-01-29

  • Code-style/readme changes.
  • Add tests for Python 3.4, 3.5, 3.6
  • Changes made for proper CI and automatic release
  • Add code coverage

1.0.0 - 2018-01-21


  • Initial version of expects_json() decorator
  • Simple validation of request data
  • Store data in

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