FlaskBasics.md

Flask Basics Warm-up Activity

Flask is a Python web framework - a tool for creating web applications with Python. Read through some of the basic concepts to become familiar with them.

Flask Boiler-Plate and Replit

When creating a new Python project on Replit, it is possible to load some Flask boiler-plate code. Running that code will spin up a web server. Replit hosts the server and navigates to the website. It looks like this:

Routing

When building a web app, it is important to determine which things happen at which URLs. This is called routing.

One of the most important parts of the boiler-plate code is the following decorator:

@app.route('/')

Adding that line above a function will tell the Flask app that when the user goes to the root URL, the function below it should run.

To create another page, a developer would use a different @app.route value. For example, take a look at the following code:

@app.route('/about')
def about_page():
    return "Here is some information about the site"

That decorator means that if someone went to the base URL with /about at the end, it would run the about_page function and display the text returned.

There are some interesting ways to extend the routing capabilities, but for now, keep it simple: @app.route tells the app which functions map to which pages.

HTML Templates

In the boiler-plate code, the app only displays raw text. This may be all that's necessary, but typically, it would make sense to render some HTML. That's where templates come in.

A template is a file that contains static data (like HTML) as well as placeholders for dynamic data (more on that later). For now, focus on the static data part.

In a Repl project for a full-fledged Flask app, there will likely be a folder named templates. The name is very important, because that specific folder is where Flask looks for template files. For example, if there were a file named home.html in the templates folder, it could be rendered by the app.

render_template

In order to use templates, it is necessary to import the render_template function from Flask. Like this:

from flask import Flask, render_template

Then, that function can be used to render some HTML! Instead of returning regular text like "Hello world," the render_template function takes in a filename and returns the rendered HTML. For example:

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

The code above would cause the homepage (root url) to display the rendered contents of the home.html file (assuming it is in the templates folder)!

Templates make it much easier to work with HTML in Flask; instead of having to put a bunch of HTML within Python strings, it can be contained in HTML files.

Passing Variables

One of the main capabilities of templates in Flask is the ability to dynamically render templates. This is possible by passing variables as keyword arguments. This way, instead of creating many different functions and templates, it can be possible to create one of each and simply change up the variables passed.

In the Python Code

On the Python side, passing a variable to a template looks like this:

render_template("home.html", name="Elaine")

That second name argument in the call to the render_template function means that the value will be accessible from the template! This won't matter too much if it's a static value, but there is potential to make it more dynamic.

In the HTML Template

On the HTML side, using a variable in a template looks like this:

{% raw %}

<p>Hello, {{ name }}!</p>

{% endraw %}

Enclosing something in double curly brackets, {% raw %}{{{% endraw %} and {% raw %}}}{% endraw %}, means that Flask will look for a dynamic value by that name. In the example from above, the finally-rendered HTML would look like this:

<p>Hello, Elaine!</p>

The render_template function goes in and replaces the {% raw %}{{ name }}{% endraw %} in the template with the value of the name variable. That way, the same page can say hello to any number of different names!

Summary

Here are some important notes to remember:

• Flask is a web development framework for Python
• Different pages can be routed to Python functions using the @app.route decorator
• HTML can be rendered from Python using the render_template function
• Variables can be passed to the template using keyword arguments
• Variables can be referenced within a template between {% raw %}{{{% endraw %} and {% raw %}}}{% endraw %}

Don't worry if this is a little overwhelming - there will be plenty of time to practice!