Generating Presigned Posts - https://boto3.readthedocs.io/en/latest/guide/s3.html#generating-presigned-posts

resource.py

from flask_restful import Resource


class UserProfileImageResource(Resource):

    @jwt_required()
    def post(self, user_id):
        """
        /v1/users/<user_id>/signed_post
        """
        args = request.json
        if 'Content-Type' not in args:
            raise APIException("Content-Type must be specified")

        content_type = args.get('Content-Type')
        ext = mimetypes.guess_extension(content_type)
        if not ext:
            raise APIException("Unable to get extension for Content-Type")

        user = User.get_by_id(user_id)
        if not user:
            raise APIException("User not found or unauthorized")

        s3 = S3(current_app)  # S3 helper functions
        identity = user.identity.hex
        key = identity + ext

        # check if it exists
        keys = s3.list_keys(prefix=key)
        if keys:
            s3.delete(key)

        post_info = get_upload_info(key)
        return flask.jsonify(method='POST', url=post_info['url'], fields=post_info['fields'])

serverside.py

import mimetypes
import boto3


def get_upload_info(key):
    conditions = []
    fields = {'acl': "private"}

    content_type = mimetypes.guess_type(key)[0]
    if content_type:
        fields['Content-Type'] = content_type

    # Boto doesn't automatically add fields to conditions.
    for field_key, field_value in fields.items():
        conditions.append({field_key: field_value})

    client = boto3.client('s3', aws_access_key_id=AWS_ACCESS_KEY, aws_secret_access_key=AWS_SECRET_ACCESS_KEY)
    
    post_info = client.generate_presigned_post(
        Bucket=S3_BUCKET,
        Key=key,
        Fields=fields,
        Conditions=conditions,
        ExpiresIn=600
    )
    return post_info

test_as_client.py

import json
from pprint import pprint

import logging
import requests

session = requests.Session()
session.headers.update({'Content-Type': 'application/json'})


def login():
    r = session.post(api_url + '/auth', data=json.dumps(auth_data))
    r.raise_for_status()

    json_args = r.json()
    token = json_args['data']['token']
    session.headers.update({'Authorization': 'JWT {}'.format(token)})
    return json_args['data']['user']['identity']


def get_upload_info(user_id):
    # Get the permissions and url
    resp = session.post(
        api_url + "/v0/users/{}/signed_post".format(user_id),
        data=json.dumps({'Content-Type': 'image/png'})
    )
    resp.raise_for_status()
    return resp.json()


def upload_image_to_s3(data):
    url = data['url']
    fields = data['fields']
    filename = fields['key']
    image_data = open("/Users/xxxxx/Pictures/avatar/overlord59-magic-tux.png", 'rb').read()

    resp = requests.post(url, data=fields, files={'file': image_data})
    resp.raise_for_status()  # BOOM
    pprint(resp.text)

identity = login()

data = get_upload_info(identity)
pprint(data)

upload_image_to_s3(data)