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Build Web API with Flask --- Work with JSON-like Dict

··435 words·3 mins·
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This is a simple post about how to send JSON-like Dict data to a Flask server via requests package.

Post dict to Flask server

To post Python dict using requests package, we can use the data parameter:

import requests

payload = {'foo': 1, 'bar': 2}, data=payload)

In the above example, the Content-Type for posted data is application/x-www-form-urlencoded, you can get the posted data in the Flask server side via request.form:

from flask import request

data = request.form.to_dict()

Post and receive dict with value of list type

One caveat when we use request.form.to_dict() is that we only get the first element for a dict value of list type. For example, if payload is the following dict:

payload = {'id': '123', 'type': 'jpg', 'box': [0, 0, 100, 100]}

On the Flask server side, after request.form.to_dict(), you only get the following:

{'id': '123', 'type': 'jpg', 'box': '0'}

which is completely non-obvious for Flask beginners. To get the full list instead, we have two ways:

  • Use requests.form.to_dict(flat=False) (see here on the description about to_dict()). One drawback is that values that are non-list originally are converted to list. So now you get:
    {'id': ['123'], 'type': ['jpg'], 'box': ['0', '0', '100', '100']}
  • Use request.form.getlist('key') to get the list corresponding to key, more about this here.

Directly posting JSON data

Another way to post Python dict is to directly post and receive JSON data. When making requests, we can use the json parameter of method:

r =, json=payload)

In this way, requests package will serialize your dict into JSON format. The Content-Type in HTTP header will be set to application/json. In the Flask side, we need to use request.get_json() to get the posted JSON data.

Post or return base64 encoded image in dict

When you post base64 encoded image in dict via requests package, you may see the following error:

TypeError: Object of type bytes is not JSON serializable

This is because the Python JSON library cannot serialize byte type. You can convert the base64 encoded image as string via decode() method:

import base64
import requests

with open('test.jpg', 'rb') as f:
    im_b64 = base64.b64encode(

payload = {'image': im_b64.decode()}
r =, json=payload)

Similarly, when you want to return base64 encoded image in the Flask server side via jsonify() method, you need also to convert bytes type to str before JSON library can serialize it.



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