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Intro to Flask

Intro to Flask

Objectives

  • Use the Flask microframework to create a Python web server
  • Configure a GET route to send raw data
  • Configure a GET route accept query parameters
  • Configure the server to serve static content

Flask Docs Mongo Tutorial

What is Flask?

Flask describes itself as a microframework. It comes with a small set of utilities that will help you define routes on a server, and send files back to a browser.

Flask acts a lot like Express in that it imports lots of little single-use packages. We describe this as a "microframework." The framework relies on all of the little packages developers choose to pull in themselves and the framework just deals with allowing those packages to be used in a server environment easily.

Install Flask and Create a Minimal Server

Use your terminal's pip3 command to install Flask for Python3.

pip3 install Flask

Now that Flask is install, create a file server.py that imports Flask and configures a simple GET route at the server's root. You'll notice the way Flask defines routes is very similar to how routes are defined in Express!

It's this simple:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
  return "Hello World!"

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

Configure Route Variables

Like Express, Flask allows us to bake valuable information into our URL structure. Embedding information into our URLs allows our web applications to have beautiful URLs that users can remember easily.

Flask extracts the string in the part of the URL and passes it to the route function as a parameter. It will automatically keep the value as a string unless you specify another type, like int. If you specify a type Flask runs the value through what it calls a "convertor". Flask comes with a few convertors out of the box.

IMPORTANT: Flask expects every route to eventually return a String. Look at the multiply route example. Although it specifies that it accepts two ints as route variables it still must manually convert the mathematical result into a String before the server sends a response.

Convertor Name Description
string accepts any text without a slash (the default)
int accepts integers
float like int but for floating point values
path like the default but also accepts slashes
any matches one of the items provided
uuid accepts UUID strings 
from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
  return "Hello World!"

@app.route("/profile/<username>")
def profile(username):
  return "You're viewing {}'s profile.".format(username)

@app.route("/r/<subreddit>")
def profile(subreddit):
  return "Welcome to the {} subreddit!".format(subreddit)


@app.route("/multiply/<int:num1>/<int:num2>")
def multiply(num1, num2):
  result = num1 * num2
  return str(result)

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

Serve Static Content

The app = Flask() function includes useful parameters to customize how static content is served. These two parameters define where your static directory is located, and where that directory is made available as a URL on your server.

  • static_folder defines the name of the directory containing static content.

  • static_url_path defines what URL you have to direct your browser to to access the static content.

  • if static_url_path is not defined then it defaults to 'static' and an index.html in the static directory would be accessible at http://127.0.0.1:5000/static/index.html

  • if static_url_path is set to 'public' then your index page would be

    available at http://127.0.0.1:5000/public/index.html

  • if static_url_path is set to an empty string '' then your index page

    is available at the server root: http://127.0.0.1:5000/ (this is most

    likely what you want.)

You can serve a simple index.html file that includes a script tag link to Angular and now you've got a simple server serving an Angular app!

Application directory structure:

~/Code/my_flask_server/
--server.py
--static/
----index.html

server.py

from flask import Flask
app = Flask(__name__, static_folder='static', static_url_path='')

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

Last Remarks

There you go! That's the basics of using Python together with Flask. Remember, Flask calls itself a "microframework". It tries to do provide developers with just what they need to create a simple web server and nothing more. It stays out of the way of making major development decisions for you and relies on developers creating powerful web servers by choosing their own components to deal with things like Database management and whatever else you'd need.

Flask adheres to the Unix Philosphy where they "strive to do one thing and do it well."

Here's a summary of the Unix Philosophy:

  • Write programs that do one thing and do it well.
  • Write programs to work together.
  • Write programs to handle text streams, because that is a universal interface.

Microframework vs Monolithic Framework

In contrast, we'll also learn about another Python web framework: Django. Django is a "monolithic" framework that involves itself in much more in the configuration and operation of a web server. Whereas Flask makes zero decisions for you and relies on developers importing lots of Third-party libraries, Django comes with LOTS of tools in it's box. Django uses it's own template system, has it's own Object Relation Model (ORM) that it uses to interact with databases and even includes it's own administrative tools, that are actually really awesome and allow you to easily build websites where you get an administrative panel where you can edit entries in your database within the website.

Both frameworks have their advantages. If you're looking to pop something up quick and light, then use Flask and follow their motto of bringing in dependencies, "drop by drop." If you're looking to build a site that would benefit from already-integrated user accounts, and batteries-included administrative tools, then Django might be a better choice.

Happy developing.