Storing Flask uploaded images and files on Amazon S3

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Flask is still a relative newcomer in the world of Python frameworks (it recently celebrated its fifth birthday); and because of this, it's still sometimes trailing behind its rivals in terms of plugins to scratch a given itch. I recently discovered that this was the case, with storing and retrieving user-uploaded files on Amazon S3.

For static files (i.e. an app's seldom-changing CSS, JS, and images), Flask-Assets and Flask-S3 work together like a charm. For more dynamic files, there exist numerous snippets of solutions, but I couldn't find anything to fill in all the gaps and tie it together nicely.

Due to a pressing itch in one of my projects, I decided to rectify this situation somewhat. Over the past few weeks, I've whipped up a bunch of Python / Flask tidbits, to handle the features that I needed:

I've also published an example app, that demonstrates how all these tools can be used together. Feel free to dive straight into the example code on GitHub; or read on for a step-by-step guide of how this Flask S3 tool suite works.

Using s3-saver

The key feature across most of this tool suite, is being able to use the same code for working with local and with S3-based files. Just change a single config option, or a single function argument, to switch from one to the other. This is critical to the way I need to work with files in my Flask projects: on my development environment, everything should be on the local filesystem; but on other environments (especially production), everything should be on S3. Others may have the same business requirements (in which case you're in luck). This is most evident with s3-saver.

Here's a sample of the typical code you might use, when working with s3-saver:

from io import BytesIO
from os import path

from flask import current_app as app
from flask import Blueprint
from flask import flash
from flask import redirect
from flask import render_template
from flask import url_for
from s3_saver import S3Saver

from project import db
from library.prefix_file_utcnow import prefix_file_utcnow
from foo.forms import ThingySaveForm
from foo.models import Thingy

mod = Blueprint('foo', __name__)

@mod.route('/', methods=['GET', 'POST'])
def home():
    """Displays the Flask S3 Save Example home page."""

    model = Thingy.query.first() or Thingy()

    form = ThingySaveForm(obj=model)

    if form.validate_on_submit():
        image_orig = model.image
        image_storage_type_orig = model.image_storage_type
        image_bucket_name_orig = model.image_storage_bucket_name

        # Initialise s3-saver.
        image_saver = S3Saver(
            storage_type=app.config['USE_S3'] and 's3' or None,
                path.join(app.config['PROJECT_ROOT'], '..')))


            filename = prefix_file_utcnow(model,

            filepath = path.abspath(

            # Best to pass in a BytesIO to S3Saver, containing the
            # contents of the file to save. A file from any source
            # (e.g. in a Flask form submission, a
            # werkzeug.datastructures.FileStorage object; or if
            # reading in a local file in a shell script, perhaps a
            # Python file object) can be easily converted to BytesIO.
            # This way, S3Saver isn't coupled to a Werkzeug POST
            # request or to anything else. It just wants the file.
            temp_file = BytesIO()

            # Save the file. Depending on how S3Saver was initialised,
            # could get saved to local filesystem or to S3.
                app.config['THINGY_IMAGE_RELATIVE_PATH'] + filename,

            # If updating an existing image,
            # delete old original and thumbnails.
            if image_orig:
                if image_orig != model.image:
                    filepath = path.join(


                glob_filepath_split = path.splitext(path.join(
                glob_filepath = glob_filepath_split[0]
                glob_matches = image_saver.find_by_path(

                for filepath in glob_matches:
            model.image = image_orig

        # Handle image deletion
        if and image_orig:
            filepath = path.join(
                app.config['UPLOADS_FOLDER'], image_orig)

            # Delete the file. In this case, we have to pass in
            # arguments specifying whether to delete locally or on
            # S3, as this should depend on where the file was
            # originally saved, rather than on how S3Saver was
            # initialised.

            # Also delete thumbnails
            glob_filepath_split = path.splitext(path.join(
            glob_filepath = glob_filepath_split[0]

            # S3Saver can search for files too. When searching locally,
            # it uses glob(); when searching on S3, it uses key
            # prefixes.
            glob_matches = image_saver.find_by_path(

            for filepath in glob_matches:

            model.image = ''
            model.image_storage_type = ''
            model.image_storage_bucket_name = ''

        if or
            flash('Thingy %s' % (
             and 'deleted' or 'saved'),
                'Please upload a new thingy or delete the ' +
                    'existing thingy',

        return redirect(url_for('foo.home'))

    return render_template('home.html',


As is hopefully evident in the sample code above, the idea with s3-saver is that as little S3-specific code as possible is needed, when performing operations on a file. Just find, save, and delete files as usual, per the user's input, without worrying about the details of that file's storage back-end.

s3-saver uses the excellent Python boto library, as well as Python's built-in file handling functions, so that you don't have to. As you can see in the sample code, you don't need to directly import either boto, or the file-handling functions such as glob or os.remove. All you need to import is io.BytesIO, and os.path, in order to be able to pass s3-saver the parameters that it needs.

Using url-for-s3

This is a simple utility function, that generates a URL to a given S3-based file. It's designed to match flask.url_for as closely as possible, so that one can be swapped out for the other with minimal fuss.

from __future__ import print_function

from flask import url_for
from url_for_s3 import url_for_s3

from project import db

class Thingy(db.Model):
    """Sample model for flask-s3-save-example."""

    id = db.Column(db.Integer(), primary_key=True)
    image = db.Column(db.String(255), default='')
    image_storage_type = db.Column(db.String(255), default='')
    image_storage_bucket_name = db.Column(db.String(255), default='')

    def __repr__(self):
        return 'A thingy'

    def image_url(self):
        from flask import current_app as app
        return (self.image
            and '%s%s' % (
            or None)

    def image_url_storageaware(self):
        if not self.image:
            return None

        if not (
                and self.image_storage_bucket_name):
            return url_for(

        if self.image_storage_type != 's3':
            raise ValueError((
                'Storage type "%s" is invalid, the only supported ' +
                'storage type (apart from default local storage) ' +
                'is s3.') % self.image_storage_type)

        return url_for_s3(


The above sample code illustrates how I typically use url_for_s3. For a given instance of a model, if that model's file is stored locally, then generate its URL using flask.url_for; otherwise, switch to url_for_s3. Only one extra parameter is needed: the S3 bucket name.

  {% if model.image %}
  <p><a href="">View original</a></p>
  {% endif %}


I can then easily show the "storage-aware URL" for this model in my front-end templates.

Using flask-thumbnails-s3

In my use case, the majority of the files being uploaded are images, and most of those images need to be resized when displayed in the front-end. Also, ideally, the dimensions for resizing shouldn't have to be pre-specified (i.e. thumbnails shouldn't only be able to get generated when the original image is first uploaded); new thumbnails of any size should get generated on-demand per the templates' needs. The front-end may change according to the design / branding whims of clients and other stakeholders, further on down the road.

flask-thumbnails handles just this workflow for local files; so, I decided to fork it and to create flask-thumbnails-s3, which works the same as flask-thumbnails when set to use local files, but which can also store and retrieve thumbnails on a S3 bucket.

    {% if image %}
    <img src="{{ image|thumbnail(size,
                                 bucket_name=bucket_name) }}"
        alt="{{ alt }}" title="{{ title }}" />
    {% endif %}


Like its parent project, flask-thumbnails-s3 is most commonly invoked by way of a template filter. If a thumbnail of the given original file exists, with the specified size and attributes, then it's returned straightaway; if not, then the original file is retrieved, a thumbnail is generated, and the thumbnail is saved to the specified storage back-end.

At the moment, flask-thumbnails-s3 blocks the running thread while it generates a thumbnail and saves it to S3. Ideally, this task would get sent to a queue, and a "dummy" thumbnail would be returned in the immediate request, until the "real" thumbnail is ready in a later request. The Sorlery plugin for Django uses the queued approach. It would be cool if flask-thumbnails-s3 (optionally) did the same. Anyway, it works without this fanciness for now; extra contributions welcome!

(By the way, in my testing, this is much less of a problem if your Flask app is deployed on an Amazon EC2 box, particularly if it's in the same region as your S3 bucket; unsurprisingly, there appears to be much less latency between an EC2 server and S3, than there is between a non-Amazon server and S3).

Using flask-admin-s3-upload

The purpose of flask-admin-s3-upload is basically to provide the same 'save' functionality as s3-saver, but automatically within Flask-Admin. It does this by providing alternatives to the flask_admin.form.upload.FileUploadField and flask_admin.form.upload.ImageUploadField classes, namely flask_admin_s3_upload.S3FileUploadField and flask_admin_s3_upload.S3ImageUploadField.

(Anecdote: I actually wrote flask-admin-s3-upload before any of the other tools in this suite, because I began by working with a part of my project that has no custom front-end, only a Flask-Admin based management console).

Using the utilities provided by flask-admin-s3-upload is fairly simple:

from os import path

from flask_admin_s3_upload import S3ImageUploadField

from project import admin, app, db
from foo.models import Thingy
from library.admin_utils import ProtectedModelView
from library.prefix_file_utcnow import prefix_file_utcnow

class ThingyView(ProtectedModelView):
    column_list = ('image',)
    form_excluded_columns = ('image_storage_type',

    form_overrides = dict(

    form_args = dict(

    def scaffold_form(self):
        form_class = super(ThingyView, self).scaffold_form()
        static_root_parent = path.abspath(
            path.join(app.config['PROJECT_ROOT'], '..'))

        if app.config['USE_S3']:
            form_class.image.kwargs['storage_type'] = 's3'

        form_class.image.kwargs['bucket_name'] = \
        form_class.image.kwargs['access_key_id'] = \
        form_class.image.kwargs['access_key_secret'] = \
        form_class.image.kwargs['static_root_parent'] = \

        return form_class

admin.add_view(ThingyView(Thingy, db.session, name='Thingies'))


Note that flask-admin-s3-upload only handles saving, not deleting (the same as the regular Flask-Admin file / image upload fields only handle saving). If you wanted to handle deleting files in the admin as well, you could (for example) use s3-saver, and hook it in to one of the Flask-Admin event callbacks.

In summary

I'd also like to mention: one thing that others have implemented in Flask, is direct JavaScript-based upload to S3. Implementing this sort of functionality in my tool suite would be a great next step; however, it would have to play nice with everything else I've built (particularly with flask-thumbnails-s3), and it would have to work for local- and for S3-based files, the same as all the other tools do. I don't have time to address those hurdles right now – another area where contributions are welcome.

I hope that this article serves as a comprehensive guide, of how to use the Flask S3 tools that I've recently built and contributed to the community. Any questions or concerns, please drop me a line.

Post a comment


Daniel Rubio

This is some good S#$t. Much appreciated for writing and demonstrating how this works.

Daniel Rubio

Hey Jaza, I wanted to note that flask_admin_s3 has all the code necessary to incorporate file editing and deletion without S3Saver. I have worked my way around the code to make it work in my example application.


Daniel: by flask_admin_s3, do you mean flask_admin_s3_upload? If so, then yes, it does indeed incorporate everything by itself, no need for S3Saver. However, it only works with Flask-Admin, whereas S3Saver is a stand-alone library (doesn't even need Flask, its only dependency is Boto). Also, if you think it would be of use, then could you post a link to your example application, or provide relevant code snippets from it?