Uploading a file to S3 while using Flask with Flask-Restful to create a REST API.

Flask-Restful_S3_File_Upload.py

# -*- coding: utf-8 -*-
"""
An example flask application showing how to upload a file to S3
while creating a REST API using Flask-Restful.

Note: This method of uploading files is fine for smaller file sizes,
      but uploads should be queued using something like celery for
      larger ones.
"""
from cStringIO import StringIO

from boto.s3.connection import S3Connection
from boto.s3.key import Key as S3Key
from flask import Flask
from flask.ext.restful import Api as FlaskRestfulAPI, Resource, reqparse, abort
from werkzeug.datastructures import FileStorage

## config
ALLOWED_EXTENSIONS = ['jpg', 'jpeg', 'png']
FILE_CONTENT_TYPES = { # these will be used to set the content type of S3 object. It is binary by default.
    'jpg': 'image/jpeg',
    'jpeg': 'image/jpeg',
    'png': 'image/png'
}
AWS_ACCESS_KEY_ID = 'aws-access-key-id'
AWS_SECRET_ACCESS_KEY = 'aws-secret-access-key'

## app initilization
app = Flask(__name__)
app.config.from_object(__name__)

## extensions
api = FlaskRestfulAPI(app)

## Helper Methods
def upload_s3(file, key_name, content_type, bucket_name):
    """Uploads a given StringIO object to S3. Closes the file after upload.

    Returns the URL for the object uploaded.

    Note: The acl for the file is set as 'public-acl' for the file uploaded.

    Keyword Arguments:
    file -- StringIO object which needs to be uploaded.
    key_name -- key name to be kept in S3.
    content_type -- content type that needs to be set for the S3 object.
    bucket_name -- name of the bucket where file needs to be uploaded.
    """
    # create connection
    conn = S3Connection(app.config['AWS_ACCESS_KEY_ID'], app.config['AWS_SECRET_ACCESS_KEY'])

    # upload the file after getting the right bucket
    bucket = conn.get_bucket(bucket_name)
    obj = S3Key(bucket)
    obj.name = key_name
    obj.content_type = content_type
    obj.set_contents_from_string(file.getvalue())
    obj.set_acl('public-read')

    # close stringio object
    file.close()

    return obj.generate_url(expires_in=0, query_auth=False)


class FileStorageArgument(reqparse.Argument):
    """This argument class for flask-restful will be used in
    all cases where file uploads need to be handled."""
    
    def convert(self, value, op):
        if self.type is FileStorage:  # only in the case of files
            # this is done as self.type(value) makes the name attribute of the
            # FileStorage object same as argument name and value is a FileStorage
            # object itself anyways
            return value

        # called so that this argument class will also be useful in
        # cases when argument type is not a file.
        super(FileStorageArgument, self).convert(*args, **kwargs)


## API Endpoints

class UploadImage(Resource):

    put_parser = reqparse.RequestParser(argument_class=FileStorageArgument)
    put_parser.add_argument('image', required=True, type=FileStorage, location='files')

    def put(self):
        #TODO: a check on file size needs to be there.

        args = self.put_parser.parse_args()
        image = args['image']

        # check logo extension
        extension = image.filename.rsplit('.', 1)[1].lower()
        if '.' in image.filename and not extension in app.config['ALLOWED_EXTENSIONS']:
            abort(400, message="File extension is not one of our supported types.")

        # create a file object of the image
        image_file = StringIO()
        image.save(image_file)

        # upload to s3
        key_name = '{0}.{1}'.format('some-name', extension)
        content_type = app.config['FILE_CONTENT_TYPES'][extension]
        bucket_name = 'bucket-is-me'
        logo_url = upload_s3(image_file, key_name, content_type, bucket_name)
        
        return {'logo_url': logo_url}


api.add_resource(UploadImage, '/upload_image')


if __name__ == '__main__':
    app.run(debug=True)