Last updated September 29, 2023

Web applications often require the ability to allow users to upload files such as images, movies and archives. Amazon S3 is a popular and reliable storage option for these files.

This article demonstrates how to create a Python application that uploads files directly to S3 instead of via a web application, utilising S3’s Cross-Origin Resource Sharing (CORS) support. The article and companion repository consider Python 2.7, but should be mostly also compatible with Python 3.3 and above except where noted below.

Uploading directly to S3

The main advantage of direct uploading is that the load on your application’s dynos would be considerably reduced. Using server-side processes for receiving files and transferring to S3 can needlessly tie up your dynos and will mean that they will not be able to respond to simultaneous web requests as efficiently.

The application uses client-side JavaScript and Python for signing the requests. It will therefore be a suitable guide for developing applications for the Flask, Bottle and Django web frameworks. The upload is carried out asynchronously so that you can decide how to handle your application’s flow after the upload has completed (for example, a page redirect upon successful upload rather than a full page refresh).

An example simple account-editing scenario is used as a guide for completing the various steps required to accomplish the direct upload and to relate the application of this to a wider range of use-cases. More information on this scenario is provided later.

Overview

S3 is comprised of a set of buckets, each with a globally unique name, in which individual files (known as objects) and directories, can be stored.

For uploading files to S3, you will need an Access Key ID and a Secret Access Key, which act as a username and password. The access key account will need to have sufficient access privileges to the target bucket in order for the upload to be successful.

The method described in this article involves the use of client-side JavaScript and server-side Python. In general, the completed image-upload process follows these steps:

• A file is selected for upload by the user in their web browser;
• JavaScript is then responsible for making a request to your web application on Heroku, which produces a temporary signature with which to sign the upload request;
• The temporary signed request is returned to the browser in JSON format;
• JavaScript then uploads the file directly to Amazon S3 using the signed request supplied by your Python application.

This guide includes information on how to implement the client-side and server-side code to form the complete system. After following the guide, you should have a working barebones system, allowing your users to upload files to S3. However, it is usually worth adding extra functionality to help improve the security of the system and to tailor it for your own particular uses. Pointers for this are mentioned in the appropriate parts of the guide.

Prerequisites

• The Heroku CLI has been installed;
• A Heroku application has been created for the current project;
• An AWS S3 bucket has been created. For demonstration purposes we assume a bucket has been created that permits the creation of public objects. In a production environment you may want to use private objects that can be accessed via signed URLs.

Initial setup

Heroku setup

In order for your application to access the AWS credentials for signing upload requests, they will need to be added as configuration variables in Heroku:

$ heroku config:set AWS_ACCESS_KEY_ID=xxx AWS_SECRET_ACCESS_KEY=yyy
Adding config vars and restarting app... done, v21
    AWS_ACCESS_KEY_ID     => xxx
    AWS_SECRET_ACCESS_KEY => yyy

In addition to the AWS access credentials, set your target S3 bucket’s name (not the bucket’s ARN):

$ heroku config:set S3_BUCKET=zzz
Adding config vars and restarting app... done, v21
    S3_BUCKET     => zzz

S3 setup

You will now need to edit some of the permissions properties of the target S3 bucket so that the final request has sufficient privileges to write to the bucket. In a web-browser, sign in to the AWS console and select the S3 section. Select the appropriate bucket and click the Permissions tab. A few options are now provided on this page (including Block public access, Access Control List, Bucket Policy, and CORS configuration).

Firstly, ensure that “Block all public access” is turned off, and in particular turn off “Block public access to buckets and objects granted through new access control lists” and “Block public access to buckets and objects granted through any access control lists” for the purposes of this project. Setting up the bucket in this way allows us to read its contents without signed URLs, but this may not be suitable for services running in production.

Next, you will need to configure the bucket’s CORS (Cross-Origin Resource Sharing) settings, which will allow your application to access content in the S3 bucket. Each rule should specify a set of domains from which access to the bucket is granted and also the methods and headers permitted from those domains.

For this to work in your application, click Edit and enter the following JSON:

[
    {
        "AllowedHeaders": [
            "*"
        ],
        "AllowedMethods": [
            "GET",
            "HEAD",
            "POST",
            "PUT"
        ],
        "AllowedOrigins": [
            "*"
        ],
        "ExposeHeaders": []
    }
]

Click Save changes and close the editor.

This tells S3 to allow any domain access to the bucket and that requests can contain any headers. For security, you can change the ‘AllowedOrigins’ array to only accept requests from your domain.

Direct uploading

The processes and steps required to accomplish a direct upload to S3 will be demonstrated through the use of a simple profile-editing scenario for the purposes of this article. This example will involve the user being permitted to select an avatar image to upload and enter some basic information to be stored as part of their account.

In this scenario, the following procedure will take place:

• The user is presented with a web page, containing elements encouraging the user to choose an image to upload as their avatar and to enter a username and their own name.
• An element is responsible for maintaining a preview of the chosen image by the user. By default, and if no image is chosen for upload, a default avatar image is used instead (making the image-upload effectively optional to the user in this scenario).
• When a user selects an image to be uploaded, the upload to S3 is handled automatically and asynchronously with the process described earlier in this article. The image preview is then updated with the selected image once the upload is complete and successful.
• The user is then free to move on to filling in the rest of the information.
• The user then clicks the Submit button, which posts the username, name and the URL of the uploaded image to the Python application to be checked and/or stored. If no image was uploaded by the user earlier the default avatar image URL is posted instead.

Setting up the client-side code

No third-party code is required to complete the implementation on the client-side.

The HTML and JavaScript can now be created to handle the file selection, obtain the request and signature from your Python application, and then finally make the upload request.

Firstly, create a file called account.html in your application’s templates directory and populate the head and other necessary HTML tags appropriately for your application. In the body of this HTML file, include a file input and an element that will contain status updates on the upload progress. In addition to this, create a form to allow the user to enter their username and full name and a hidden input element to hold the URL of the chosen avatar image:

<input type="file" id="file_input"/>
<p id="status">Please select a file</p>
<img id="preview" src="/static/default.png" />

<form method="POST" action="/submit_form/">
  <input type="hidden" id="avatar-url" name="avatar-url" value="/static/default.png">
  <input type="text" name="username" placeholder="Username">
  <input type="text" name="full-name" placeholder="Full name">
  <input type="submit" value="Update profile">
</form>

The #preview element initially holds a default avatar image (which would become the user’s avatar if a new image is not chosen), and the #avatar-url input maintains the current URL of the user’s chosen avatar image. Both of these are updated by the JavaScript, discussed below, when the user selects a new avatar.

Thus when the user finally clicks the Submit button, the URL of the avatar is submitted, along with the username and full name of the user, to your desired endpoint for server-side handling.

The client-side code is responsible for achieving two things:

1. Retrieve a signed request from the app with which the image can be POSTed to S3
2. Actually POST the image to S3 using the signed request

JavaScript’s XMLHttpRequest objects can be created and used for making asynchronous HTTP requests.

To accomplish this, first create a <script> block and write some code that listens for changes in the file input, once the document has loaded, and starts the upload process.

(function() {
  document.getElementById("file_input").onchange = function(){
    var files = document.getElementById("file_input").files;
    var file = files[0];
    if(!file){
      return alert("No file selected.");
    }
    getSignedRequest(file);
  };
})();

The code also determines the file object itself to be uploaded. If one has been selected properly, it proceeds to call a function to obtain a signed POST request for the file. Next, therefore, write a function that accepts the file object and retrieves an appropriate signed request for it from the app.

function getSignedRequest(file){
  var xhr = new XMLHttpRequest();
  xhr.open("GET", "/sign_s3?file_name="+file.name+"&file_type="+file.type);
  xhr.onreadystatechange = function(){
    if(xhr.readyState === 4){
      if(xhr.status === 200){
        var response = JSON.parse(xhr.responseText);
        uploadFile(file, response.data, response.url);
      }
      else{
        alert("Could not get signed URL.");
      }
    }
  };
  xhr.send();
}

The above function passes the file’s name and mime type as parameters to the GET request since these are needed in the construction of the signed request, as will be covered later in this article. If the retrieval of the signed request was successful, the function continues by calling a function to upload the actual file:

function uploadFile(file, s3Data, url){
  var xhr = new XMLHttpRequest();
  xhr.open("POST", s3Data.url);

  var postData = new FormData();
  for(key in s3Data.fields){
    postData.append(key, s3Data.fields[key]);
  }
  postData.append('file', file);

  xhr.onreadystatechange = function() {
    if(xhr.readyState === 4){
      if(xhr.status === 200 || xhr.status === 204){
        document.getElementById("preview").src = url;
        document.getElementById("avatar-url").value = url;
      }
      else{
        alert("Could not upload file.");
      }
   }
  };
  xhr.send(postData);
}

This function accepts the file to be uploaded, the S3 request data, and the URL representing the eventual location of the avatar image. The latter two arguments will be returned as part of the response from the app. The function, if the request is successful, updates the preview element to the new avatar image and stores the URL in the hidden input so that it can be submitted for storage in the app.

Now, once the user has completed the rest of the form and clicked Submit, the name, username, and avatar image can all be posted to the same endpoint.

Setting up the server-side Python code

This section discusses the use of Python for generating a temporary signature with which the upload request can be signed. This temporary signature uses the AWS access key and secret access key as a basis for the signature, but users will not have direct access to this information. After the signature has expired, then upload requests with the same signature will not be successful.

As mentioned previously, this article covers the production of an application for the Flask framework, although the steps for other Python frameworks will be similar. Readers using Python 3 should consider the relevant information on Flask’s website before continuing.

Start by creating your main application file, application.py, and set up your skeleton application appropriately:

from flask import Flask, render_template, request, redirect, url_for
import os, json, boto3

app = Flask(__name__)

if __name__ == '__main__':
  port = int(os.environ.get('PORT', 5001))
  app.run(host='0.0.0.0', port = port)

The currently-unused import statements will be necessary later on. boto3 is a Python library that will generate the pre-signed POST request. This, along with Flask, can be installed simply using pip.

Next, in the same file, you will need to create the views responsible for returning the correct information back to the user’s browser when requests are made to various URLs. First define view for requests to /account to return the page account.html, which contains the form for the user to complete:

@app.route("/account/")
def account():
    return render_template('account.html')

Now create the view, in the same Python file, that is responsible for generating and returning the signature with which the client-side JavaScript can upload the image. This is the first request made by the client before attempting an upload to S3. This view responds with requests to /sign_s3/:

@app.route('/sign_s3/')
def sign_s3():
  S3_BUCKET = os.environ.get('S3_BUCKET')

  file_name = request.args.get('file_name')
  file_type = request.args.get('file_type')

  s3 = boto3.client('s3')

  presigned_post = s3.generate_presigned_post(
    Bucket = S3_BUCKET,
    Key = file_name,
    Fields = {"acl": "public-read", "Content-Type": file_type},
    Conditions = [
      {"acl": "public-read"},
      {"Content-Type": file_type}
    ],
    ExpiresIn = 3600
  )

  return json.dumps({
    'data': presigned_post,
    'url': 'https://%s.s3.amazonaws.com/%s' % (S3_BUCKET, file_name)
  })

s3 = boto3.client('s3', config = Config(signature_version = 's3v4'))

This code performs the following steps:

• The request is received to /sign_s3/ and the S3 bucket name is loaded from the environment.
• The name and mime type of the object to be uploaded are extracted from the GET parameters of the request (this stage may differ in other frameworks). The parameters are provided by the JavaScript discussed in the previous section.
• An S3 client is constructed using the boto3 library. At this stage, the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY set earlier are automatically read from the environment.
• The pre-signed POST request data is then generated using the generate_presigned_post function. To this is passed the bucket name, the name of the file, some parameters to allow the uploaded file to be publicly readable, and an expiry time of the signed request (in seconds).
• Finally, the pre-signed request data and the location of the eventual file on S3 are returned to the client as JSON.

You may wish to assign another, customised name to the object instead of using the one that the file is already named with, which is useful for preventing accidental overwrites in the S3 bucket. This name could be related to the ID of the user’s account, for example. If not, you should provide some method for properly quoting the name in case there are spaces or other awkward characters present. In addition, this is the stage at which you could provide checks on the uploaded file in order to restrict access to certain file types. For example, a simple check could be implemented to allow only .png files to proceed beyond this point.

Finally, in application.py, create the view responsible for receiving the account information after the user has uploaded an avatar, filled in the form, and clicked Submit. Since this will be a POST request, this will also need to be defined as an ‘allowed access method’. This method will respond to requests to the URL /submit_form/:

@app.route("/submit_form/", methods = ["POST"])
def submit_form():
  username = request.form["username"]
  full_name = request.form["full-name"]
  avatar_url = request.form["avatar-url"]

  update_account(username, full_name, avatar_url)

  return redirect(url_for('profile'))

In this example, an update_account() function has been called, but creation of this method is not covered in this article. In your application, you should provide some functionality, at this stage, to allow the app to store these account details in some form of database and correctly associate the information with the rest of the user’s account details.

In addition, the URL for the profile page has not been defined in this article (or companion code). Ideally, for example, after updating the account, the user would be redirected back to their own profile so that they can see the updated information.

Running the app

Everything should now be in place to perform the direct uploads to S3. To test the upload, save any changes and use heroku local to start the application:

$ heroku local
15:44:36 web.1  | started with pid 12417

Press Ctrl+C to return to the prompt. If your application is returning 500 errors (or other server-based issues), then start your server in debug mode and view the output in the Terminal emulator to help fix your problem. For example, in Flask:

...
app.debug = True
port = int(os.environ.get('PORT', 5001))
app.run(host='0.0.0.0', port=port)

Summary

This article covers uploading to Amazon S3 directly from the browser using Python to temporarily sign the upload request. Although the guide and companion code focuses on the Flask framework, the idea should easily carry over to other Python applications.