Python REST API with Flask – Part 2 – File upload and static file serve

In my previous article, REST API  with Python Flask gives an introduction to what REST architecture means and how Python framework can be used to create APIs and easily expose them to the web for others to use.  It is the beginning. If you have read it, then you would have known how to create a sample Flask application and expose a few APIs as POST and GET calls. It’s pretty simple.

In this article, we proceed further with exploring more functionality of Flask. The functionalities we are going to look at is serving static content to the web and how to accept files, or how to give an upload functionality with Flask.

We will build on the code that we have written in the previous article. So, let’s begin.

Serving Static Files

When you are building APIs, there might arise situations where you need to send files to the UI. For example, say, you want to send an Image or an HTML file or any other file. Usually, the user would get a link to a file like for example “http://sourcedexter.com/wp-content/uploads/2017/07/typing_python-course.png”. As you can see, this image is a static content, which you can view if you click the link. In other words, it can be called as a download link.

To achieve this in flask it is pretty simple.

Flask has an inbuilt method “send_from_directory” that you can use to send files. Firstly, create a folder called “static” in the root folder of your flask application.

Once that is complete, you can use the following code. (See the code comments to understand what the code is doing)

from flask import Flask, request, send_from_directory

# set the "static" directory as the static folder.
# this will ensure that all the static files are under one folder
app = Flask(__name__, static_url_path='/static')

# serving some static html files
@app.route('/html/path:path')
def send_html(path):
return send_from_directory('static', path)

if __name__ == "__main__":
app.run()

In the above code, if you created another folder within a static folder called “html” and put some files in it,
you can access the files using the URL “http://localhost:5000/html/test.html”. Of course, replace test.html with the file name that you have created.

So, you can replicate this by creating any number of folders within a static folder and also writing a method to handle that URL pattern.  It is also possible to have files directly in the static folder itself without having to create more internal folders.

Uploading files with Flask

Just like many other functionalities, providing an upload feature in Flask is very easy. There are two components involved here. First is the UI which will send the files to the flask backend. Second is the Flask application that accepts the files and stores it or does something else with it.

Sending files from the UI

The UI should send the files as a multipart-form-data. This feature allows a user to click a button and select a file which will then be sent to the backend.

If you are interested in implementing this functionality in the UI  you can refer to this guide. It shows how to implement the upload functionality using html5.

Upload file handler in Flask

Assuming that the UI is present and can send files to the backend, you can have a handler in Flask to accept the file and perform operations on it.

@app.route('/upload', methods=['POST'])
def upload_file():

print request.files
# checking if the file is present or not.
if 'file' not in request.files:
return "No file found"

file = request.files['file']
file.save("static/test.jpg")
return "file successfully saved"

If you are interested in learning about Flask in depth, then this course REST APIs with Flask and Python (if you plan to enroll, log in/ sign up to get 90% discount on this course) is the best one for you. It takes you through the basic as well as all the advanced topics in Flask in a structured project oriented approach.