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Flask Basics

Flask Basics#

The app receives all the requests. The requests are sent to the correct function or view. The function/view is found through the route.

Installing Flask#

    pip install flask

Simple App#

    from flask import Flask

Now we need an app instance and we want it named the same as the current namespace

    app = Flask(__name__)

This means use whatever our current namespace is. Run the app and set debug=True so it auto refreshes

You can run the site now with


But you will get a 404 as there is no route

Routes should be defined before

Creating a Route#

A route is a function and a routing decorator directive

    def index():
        return "Hello from treehouse"
URL Parameters#

If you want to add parameters to the route you need the request object

    from flask import request

It is a global object, so it is always available but python neckbeards hate it. The function is given a default and the name is acquired with request.args.get('name', name)

    def index(name="surfer190"):
        name = request.args.get('name', name)
        return f'Hello from { name }'

Neat Routes#

Views can have more than 1 route


This acquired the name parameter and the request.args.get() is no longer required

You can even specify the data type in the route. If a different data type is given the route will 404:

    def add(num1, num2):
        return '{} + {} = {}'.format(num1, num2, num1 + num2)

You need to return a stirng, you cannot return a string.

If you want to accept both int and float use multiple routes:

    def add(num1, num2):
        return '{} + {} = {}'.format(num1, num2, num1 + num2)


You can render html with a multiline string:

    def add(num1, num2):
        return '''
        <!doctype html>
        <h1>{} + {} = {}</h1>
        '''.format(num1, num2, num1 + num2)

But the better way is use templates

  1. Create a templates folder

  2. Add the template file base.html:

    <!doctype html>
            <h1>Num1 + Num2 = Sum-thing</h1>
  3. Import render_template from flask

    from flask import render_template
  4. In your view function

    return render_template('base.html')

Flask uses jinja2 to template and in jinja2 you print variables with {{ }}.Ensure that the view is passed the variable in render_template.

    def add(num1, num2):
        return render_template('add.html', num1=num1, num2=num2)

We can also use ** to unpack a context dictionary

    def add(num1, num2):
        context = {'num1': num1, 'num2': num2}
        return render_template('add.html', **context)

Template Inheritance#

Removes duplicate html and makes use of blocks that can be swapped out

Create a layout.html file in templates, copy generic code and add blocks for changable context:

            {% block content %}
    {% endblock %}

To use it in other template files you need to extend:

            {% extends 'layout.html' %}

To add the content from the parent block use the super() function in the block

Static Files#

Static files like images, js and css are stored in the static directory

You can then reference the file with /static/*

    <link rel="stylesheet" href="/static/styles.css">


There are many libraries available for making forms.

You can set a route only applicable to certain methods

    @app.route('/save', methods=['POST'])

Then set the url of the form:

    <form action="{{ url_for('save') }}" method="POST">


To redirect import url_for and redirect from flask

    return redirect(url_for('index'))

Form Data#

When posting to a url the request has a form attribute


Cookies are good for when you don’t have a database and don’t want to use javascript local storage.

In flask cookies are set on the response, which you don’t have access to until after the return. So you have to fake the response with make_response

    response = make_response(redirect(url_for('index')))
        return response

To set the cookie as a json object of the form data use:

    response.set_cookie('character', json.dumps(dict(request.form.items())))

Remember json.dumps() stands for dump string


Flash Messages#

    from flask import flash

Use the user’s session, for that you need a secret_key

    app.secret_key = 'KJHSADOABSLKDNknkjsfhoi768HDIu76'

Setting flash message inthe view

    flash("Saved changes!")

Can set a category:

    flash("Yay! you registered", "success")

Displaying messags in the template

            <div class="wrap no-bottom messages">
        {% with messages = get_flashed_messages() %}
            {% if messages %}
            <ul class="flashes">
                {% for message in messages %}
                <li>{{ message }}</li>
                {% endfor %}
            {% endif %}
        {% endwith %}

Remember flashes only happen once (persist till next time page is viewed)


Connecting to and disconnecting from the db responsibility.

We can run certain things before and after a request. With decorators @app.before_request

Remember the function for @app.after_request takes in a response object

The Global object#

g is a global object that gets passed around. We can use to set up things we want available everywhere.

    from flask import g

Then you can set the database:

def before_request():
    '''Connect to the database before each request'''
    g.db = models.DATABASE

and close the database after:

def after_request(response):
    '''Close the database connection after each request'''
    return response

Run the app#

if __name__ == '__main__':


To get the login sorted you need the LoginManager

    from flask_login import LoginManager

You also need to enable sessions so you need the secret_key

    app.secret_key = 'djkh7dhYYGSHhhsbjdyd'

You need to create the login mnager and initilaise the app and give it the view t redirect anonymous users to:

    login_manager = LoginManager()
    login_manager.login_view = 'login'

Add the user_loader method:

    def load_user(userid):
            return models.User.get( == userid)
        except models.DoesNotExist:
            return None


Forms are all about validation and a bit about display

The defactor package is flask_wtf build on wtforms

pip install flask-wtf

Example form with validation

    from flask_wtf import Form
    from wtforms import StringField, PasswordField
    from wtforms.validators import (
        DataRequired, Email, Regexp, ValidationError
        Length, EqualTo)

    from models import User

    def name_exists(form, field):
        if ==
            raise ValidationError('Username already exists')

    def email_exists(form, field):
        if ==
            raise ValidationError('Email already exists')

    class RegisterForm(Form):
        username = StringField(
                    messsage=("Username should be one word; letters, "
                            "numbers or underscores only")
        email = StringField(
        password = PasswordField(
                EqualTo('password2', message='Passwords must match'),
        password2 = PasswordField(
            'Confirm Password',

Handling forms in the view#

    @app.route('/register', methods=('GET', 'POST',))
    def register():
        form = forms.RegisterForm()
        if form.validate_on_submit():
            flash("Yay! you registered", "success")
            return redirect(url_for('index'))
        return render_template('register.html', form=form)

Showing the view#

            <form method="POST" action="" class="form">
        {{ form.hidden_tag() }}
        {% for field in form %}
            <div class="field">
                {% if field.errors %}
                    {% for error in field.errors %}
                        <div class="notification error">{{ error }}</div>
                    {% endfor %}
                {% endif %}
                {{ field(placeholder=field.label.text) }}


Macros are pieces of reusabel tremplate code

Create the macro in templates/macros.html:

            {% macro render_field(field) %}
    <div class="field">
        {% if field.errors %}
            {% for error in field.errors %}
                <div class="notification error">{{ error }}</div>
            {% endfor %}
        {% endif %}
        {{ field(placeholder=field.label.text) }}
    {% endmacro %}

Then use the macro with:

            {% from 'macros.html' import render_field %}

    <form method="POST" action="" class="form">
    {{ form.hidden_tag() }}
    {% for field in form %}
            {{ render_field(field) }}
    {% endfor %}

Template Layout#

Setting variables for flash messages:

            {% with messages = get_flashed_messages(with_categories=True) %}
      {% if messages %}
        {% for category, message in messages %}
          <div class="notification {{ category }}">{{ message }}</div>
        {% endfor %}
      {% endif %}
    {% endwith %}

Checking logged in:

            <!-- Log in/Log out -->
    {% if current_user.is_authenticated() %}
    <a href="{{ url_for('logout') }}" class="icon-power" title="Log out"></a>
    {% else %}
    <a href="{{ url_for('login') }}" class="icon-power" title="Log in"></a>
    <a href="{{ url_for('register') }}" class="icon-profile" title="Register"></a>
    {% endif %}

Returning a 404#

To return a 404 we need abort

    from flask import abort



Setting a custom404 page#

    def not_found(error):
        return render_template('404.html'), 404