Flask Tutorial!

This tutorial will cover a lot about Flask and its functions. But first of all...

What is Flask?

Well, Flask is a micro web framework written in Python. It allows you to build web applications.

This tutorial will teach you the following:

Routing a function
Returning HTML code
Templating
Handling GET/POST requests
Redirecting
Sending a file

Now we know what Flask is, let's create a Python repl, and get started!

To use Flask, we have to import the Flask class and create an instance. Then we'll route and run a function, and run the application:

from flask import Flask

app = Flask("app",
  template_folder="templates", # name of folder containing html templates
  static_folder="static" # name of folder for static files
)

app.run(host="0.0.0.0", port=8080) # run the application

After you run the code (and wait for the packaging), you will see a page showing a page not found error (because we haven't added any pages yet).

To add a page, we will use the route decorator, and create a function that will return some HTML to the page.

@app.route("/")
def index():
  return "hello world"

When you run the code, you will see a page that says hello world. To add a page in a different path name, you can use @app.route("/PATHNAME"), then create the function.

You can also have variables to route the function by using the less/greater than signs:

@app.route("/<string>")
def index(string):
  return "hello world"

This will make it so that if you enter anything as the path name, it will return hello world. Remember to include the variable in the parameter! To use the variable in the function, we can simply just type the variable name:

@app.route("/<string>")
def index(string):
  return string

Now this code will make it so if you enter /python as the path name, it will return python, and if you enter /replit as the path name, it will return replit.

To have static files (such as CSS or JavaScript), create a folder called static, and files will be kept in that folder. The path name for the static files will be /static/FILENAME.

Templating

Now this part will get a bit more interesting. So instead of returning a string in the function, we can render an HTML template. To do this, create a folder called templates, and create a file called index.html. We will have to import the render_template function from Flask:

from flask import Flask, render_template

The following code will render an HTML template:

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

In the index.html file we have:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <button>Click me!</button>
  </body>
</html>

When you run your code, you should see a button that says Click me!.

Variables

To use variables in your template, use curly brackets in the HTML code: {{VARIABLENAME}}. In the Python code, we will need to create the variable and inject it into the template. In the HTML we have:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <h1>I like {{favfood}}.</h1>
  </body>
</html>

And in the Python code we have:

@app.route("/")
def index():
  food = "pizza"
  return render_template("index.html", favfood=food)

You should see I like pizza when you run the code. You can add however many variables you would like, but be sure to add parameters containing the variables into the render_template function.

If/Else

Flask allows you to use if and else statements in your HTML template. To create if and else statements in the template, use this syntax:

{% if STATEMENT %}
<!-- insert html here -->
{% elif STATEMENT %}
<!-- insert html here -->
{% else %}
<!-- insert html here -->
{% endif %} <!-- ends the if/else statement -->

So let's create an if/else statement. In the Python code we have:

@app.route("/")
def index():
  num = 5
  return render_template("index.html", number=num)

In the index.html, we use the variable number in the if/else statement:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    {% if number == 10 %}
    <p>The number is 10.</p>
    {% elif number == 5 %}
    <p>The number is 5.</p>
    {% else %}
    <p>The number is neither 10 nor 5.</p>
    {% endif %}
  </body>
</html>

When you run this, you should see The number is 5..

For Loops

Flask also allows you to use for loops in HTML templates. This can save a lot of code. To create for loops, use this syntax like if/else:

{% for STATEMENT %}
<!-- insert html here -->
{% endfor %} <!-- ends the for loop -->

Let's try it. In the Python code we have:

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

And in the HTML template we have:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    {% for i in range(10) %}
    {{i}} <!-- shows the number "i" -->
    {% endfor %}
  </body>
</html>

The page should now show 0 1 2 3 4 5 6 7 8 9.

You can also use lists in a for loop. To do this, first create the list in the Python code:

@app.route("/")
def index():
  foods = ["pizza", "burger", "hot dog"]
  return render_template("index.html", foods=foods)

In index.html we have:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    {% for food in foods %} <!-- use the "foods" list -->
    <p>{{food}} is a food</p>
    {% endfor %}
  </body>
</html>

You should get this when you run it:

pizza is a food
burger is a food
hot dog is a food

Handling GET/POST requests

To recieve GET/POST requests, we will need to import request from Flask, and in the route decorator, we'll have to add the methods:

from flask import Flask, render_template, request

app = Flask("app")

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

@app.route("/submit", methods=["GET", "POST"])
def submit():
  if request.method == "GET": # if form method is GET
    text = request.args["text"]
    return text
  elif request.method == "POST": # if form method is POST
    text = request.form["text"]
    return text

app.run(host="0.0.0.0", port=8080)

In index.html:

<form action="/submit" method="GET">
  <input name="text">
  <input type="submit">
</form>

Once you enter a value in the input, you should see the text you entered. You can also change the form method to POST.

Redirecting

Redirecting is easy with Flask. Just import the redirect function and use it:

from flask import Flask, render_template, request, redirect

app = Flask("app")

@app.route("/")
def index():
  return redirect("URL")

app.run(host="0.0.0.0", port=8080)

So if you wanted to redirect to the Replit website, you would do:

def index():
  return redirect("https://replit.com")

Sending Files

Sending files is also easy with Flask. You will need to import send_file from Flask:

from flask import Flask, render_template, request, redirect, send_file

app = Flask("app")

@app.route("/")
def index():
  return send_file("image.png")

app.run(host="0.0.0.0", port=8080)

So you would need to have a file called image.png. The output would show the image file. If you send files, you must have access to them.

That's the end of this tutorial, and I hope you liked it and found it helpful!

Voters

Comments

hot new top

codingjlu

Alright THIS IS A GOOD TUTORIAL THAT DESERVES an upvote (or two) :)

@codingjlu tysm!

Woah nice!

thanks @VulcanWM

i just dont get how send files work @ruiwenge2

@VulcanWM for example you uploaded a file called file.png in the main directory. then you'll have to import send_file from Flask, and the file that you use will be file.png

see here:

oh makes sense, thanks! @ruiwenge2

np! @VulcanWM

the cool thing is you can use the img tag to embed that image, like <img src="https://illfatedunfitpercent.ruiwenge2.repl.co">
@VulcanWM

yeah @ruiwenge2

thank you! this just helped me in something i was trying to do right now! @ruiwenge2

i'm just curious, what were you trying to do? @VulcanWM

oh! i think it was the pfp @VulcanWM

Yep the way you actually went on the repl to see the code @ruiwenge2

lol @VulcanWM

@ruiwenge2 What will send_file do ?

It will send an image file or something to the person opening the website. An example is here: https://replit.com/@VulcanWM/OvalEyes
The output will be shown here: https://ovaleyes.vulcanwm.repl.co/pfps/vulcanwm @Yoplayer1py

i see you're on the website lmfao @ruiwenge2

@VulcanWM OvalEyes is a creative name

ty (i only made it that bc i have oval eyes) @ruiwenge2

@VulcanWM lol

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PeterMoss

Flask is a good choice if you want to create a basic and lightweight web application. Although it is not as strong and broad as Django, it does provide capabilities like as support for unit testing and the creation of REST APIs. https://domyonlineexam.com/online-course-help.php

Willie7488

Thanks for sharing this info this is useful keep it up.
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Sanchez25

Flask is one of the easiest microframework to learn in Python. If you wish to develop a simple and lightweight web application then Flask is suitable for that. It is not as powerful and extensive as Django still provides features such as support for unit testing and building REST APIs.

CodingElf66

Really nicely written tutorial, I'm reading it right now and I have a clear view of Flask, thank you @ruiwenge2! Tysm! This surely needs more upvotes to match your hard work and everything! Here's an upvote!!!

@JeffreyChen13 thanks!!

good

so cool :)

@JWZ6 thanks

@ruiwenge2 np

@JWZ6 its my birthday today :)
and i made a multiplayer pong game. do you want to play against me?
https://replit.com/talk/share/MultiPong-A-multiplayer-pong-game-and-its-my-birthday-today-D/143432

@ruiwenge2 sure

@JWZ6 can you give me feedback on my socket.io tutorial? https://replit.com/talk/learn/SocketIO-Tutorial-What-its-for-and-how-to-use/143781

A036Nikhil

can you please create a tutorial on Creating Login form using flask and replit database?

@A036Nikhil here’s a template: https://replit.com/talk/templates/Website-with-Login-System-made-with-Flask/138102

QuantumCodes

Wow! The best tutorial I've seen till now! Thanks for making this! For the get or post method, html gives data to py? Can we also send data from py to html?

@QuantumCodes you can receive data in html with a get request, but not post