How Do You Get POSTed JSON in Flask?

Naveera A.

—October 21, 2022

The Problem

How can you access the data that has been sent to a Flask app through a POST request?

The Solution

A user can submit various types of data using a POST request. Depending on the type of data, Flask uses different ways to extract and parse the data before storing it in the global request object.

We’re going to take a look at two of the most common types of data. You can read more about how Flask handles other kinds of data on the official documentation.

Form Data

When a user fills out a form on the frontend of a web app and hits submit, the browser sends that data to our app. The data is submitted using a POST request with the Content-Type header set to multipart/form-data.

When Flask receives form data, it parses and stores it in the form property of the request object. The form property is an ImmutableMultiDict, which implements all standard dictionary methods.

Let’s say a user submits a form with two input fields: name and age. We can extract form data, like so:

from flask import Flask, request
# ...

@app.route('/form_example', methods=['POST'])
def handle_form():
    print(request.form.get('name'))
    print(request.form.get('age'))
    return request.form

JSON Data

The most commonly used format of the incoming data is JSON. Flask stores the JSON data in the json property of the request object. We can access the data like so:

from flask import Flask, request
# ...

@app.route('/json_example', methods=['POST'])
def handle_json():
    data = request.json
    print(data.get('name'))
    print(data.get('age'))
    return data

An important point to remember is that the json will only contain the data if the Content-Type header is set to application/json.

We can use the headers attribute of the request object to check the Content-Type, like so:

from flask import Flask, request
# ...

@app.route('/json_example', methods=['POST'])
def handle_json():
  content_type = request.headers.get('Content-Type')
  if (content_type == 'application/json'):
    ...
  else:
    return "Content type is not supported."

A better way to check is to use the is_json property of the request object:

from flask import Flask, request
# ...

@app.route('/json_example', methods=['POST'])
def handle_json():
  if request.is_json:
    ...
  else:
    ...

If the Content-Type header is not set correctly, Flask will store the data as a string (even if it is JSON) in the data property of the request object.

We can use Flask’s json module to covert the string back to JSON, like so:

from flask import Flask, request, json
# ...

@app.route('/string_example', methods=['POST'])
def handle_non_json():
    data = json.loads(request.data)
    return data

The good thing about using the json module is that it can extract the data correctly even when it receives a request with Content-Type header set to application/json.

Either of the following two requests will result in the same response:

$ curl -X POST -H "Content-type: application/json" -d "{\"name\" : \"John\", \"age\" : \"5\"}" "localhost:5000/string_example"

$ curl -X POST -H "Content-type: text/plain" -d "{\"name\" : \"John\", \"age\" : \"5\"}" "localhost:5000/string_example"

It is best to use this solution in situations where we don’t know which header will be sent to our app.

Further Reading

How do you get the Data Received in a Flask Request?