Flask tutorial

This tutorial shows how to build a Flask application following the dependency injection principle.

Start from the scratch or jump to the section:

You can find complete project on the Github.

What are we going to build?

We will build a web application that helps to search for repositories on the Github. Let’s call it Github Navigator.

How does Github Navigator work?

  • User opens a web page that asks to provide a search query.

  • User types the query and hits Enter.

  • Github Navigator takes that and searches through the Github for matching repositories.

  • When search is done Github Navigator returns user a web page with the result.

  • The results page shows all matching repositories and the provided search query.

  • For any matching repository user sees:
    • the repository name

    • the owner of the repository

    • the last commit to the repository

  • User can click on the repository, the repository owner or the last commit to open its web page on the Github.

../_images/screen-02.png

Prepare the environment

Let’s create the environment for the project.

First we need to create a project folder:

mkdir ghnav-flask-tutorial
cd ghnav-flask-tutorial

Now let’s create and activate virtual environment:

python3 -m venv venv
. venv/bin/activate

Project layout

Environment is ready and now we’re going to create the layout of the project.

Create next structure in the current directory. All files should be empty. That’s ok for now.

Initial project layout:

./
├── githubnavigator/
│   ├── __init__.py
│   ├── application.py
│   ├── containers.py
│   └── views.py
├── venv/
└── requirements.txt

Now it’s time to install Flask and Dependency Injector.

Put next lines into the requirements.txt file:

dependency-injector
flask

Now let’s install it:

pip install -r requirements.txt

And check that installation is successful:

python -c "import dependency_injector; print(dependency_injector.__version__)"
python -c "import flask; print(flask.__version__)"

You should see something like:

(venv) $ python -c "import dependency_injector; print(dependency_injector.__version__)"
4.0.0
(venv) $ python -c "import flask; print(flask.__version__)"
1.1.2

Versions can be different. That’s fine.

Hello World!

Let’s create minimal application.

Put next into the views.py:

"""Views module."""


def index():
    return 'Hello, World!'

Ok, we have the view.

Now let’s create a container. Container will keep all of the application components and their dependencies.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers


class Container(containers.DeclarativeContainer):
    ...

Container is empty for now. We will add the providers in the following sections.

Finally we need to create Flask application factory. It will create and configure container and Flask application. It is traditionally called create_app(). We will assign index view to handle user requests to the root / of our web application.

Put next into the application.py:

"""Application module."""

from flask import Flask

from .containers import Container
from . import views


def create_app() -> Flask:
    container = Container()

    app = Flask(__name__)
    app.container = container
    app.add_url_rule('/', 'index', views.index)

    return app

Ok. Now we’re ready to say “Hello, World!”.

Do next in the terminal:

export FLASK_APP=githubnavigator.application
export FLASK_ENV=development
flask run

The output should be something like:

* Serving Flask app "githubnavigator.application" (lazy loading)
* Environment: development
* Debug mode: on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
* Restarting with fsevents reloader
* Debugger is active!
* Debugger PIN: 473-587-859

Open your browser and go to the http://127.0.0.1:5000/.

You should see Hello, World!.

That’s it. Our minimal application is up and running.

Make it pretty

Now let’s make it look pretty. We will use Bootstrap 4. For adding it to our application we will get Bootstrap-Flask extension. It will help us to add all needed static files in few clicks.

Add bootstrap-flask to the requirements.txt:

dependency-injector
flask
bootstrap-flask

and run in the terminal:

pip install -r requirements.txt

Let’s initialize bootstrap-flask extension. We will need to modify create_app().

Edit application.py:

"""Application module."""

from flask import Flask
from flask_bootstrap import Bootstrap

from .containers import Container
from . import views


def create_app() -> Flask:
    container = Container()

    app = Flask(__name__)
    app.container = container
    app.add_url_rule('/', 'index', views.index)

    bootstrap = Bootstrap()
    bootstrap.init_app(app)

    return app

Now we need to add the templates. For doing this we will need to add the folder templates/ to the githubnavigator package. We also will need two files there:

  • base.html - the layout

  • index.html - the main page

Create templates folder and put two empty files into it base.html and index.html:

./
├── githubnavigator/
│   ├── templates/
│   │   ├── base.html
│   │   └── index.html
│   ├── __init__.py
│   ├── application.py
│   ├── containers.py
│   └── views.py
├── venv/
└── requirements.txt

Now let’s fill in the layout.

Put next into the base.html:

<!doctype html>
<html lang="en">
    <head>
        {% block head %}
        <!-- Required meta tags -->
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

        {% block styles %}
            <!-- Bootstrap CSS -->
            {{ bootstrap.load_css() }}
        {% endblock %}

        <title>{% block title %}{% endblock %}</title>
        {% endblock %}
    </head>
    <body>
        <!-- Your page content -->
        {% block content %}{% endblock %}

        {% block scripts %}
            <!-- Optional JavaScript -->
            {{ bootstrap.load_js() }}
        {% endblock %}
    </body>
</html>

And put something to the index page.

Put next into the index.html:

{% extends "base.html" %}

{% block title %}Github Navigator{% endblock %}

{% block content %}
<div class="container">
    <h1 class="mb-4">Github Navigator</h1>

    <form>
        <div class="form-group form-row">
            <div class="col-10">
                <label for="search_query" class="col-form-label">
                    Search for:
                </label>
                <input class="form-control" type="text" id="search_query"
                       placeholder="Type something to search on the GitHub"
                       name="query"
                       value="{{ query if query }}">
            </div>
            <div class="col">
                <label for="search_limit" class="col-form-label">
                    Limit:
                </label>
                <select class="form-control" id="search_limit" name="limit">
                    {% for value in [5, 10, 20] %}
                    <option {% if value == limit %}selected{% endif %}>
                        {{ value }}
                    </option>
                    {% endfor %}
                </select>
            </div>
        </div>
    </form>

    <p><small>Results found: {{ repositories|length }}</small></p>

    <table class="table table-striped">
        <thead>
            <tr>
                <th>#</th>
                <th>Repository</th>
                <th class="text-nowrap">Repository owner</th>
                <th class="text-nowrap">Last commit</th>
            </tr>
        </thead>
        <tbody>
        {% for repository in repositories %} {{n}}
            <tr>
              <th>{{ loop.index }}</th>
              <td><a href="{{ repository.url }}">
                  {{ repository.name }}</a>
              </td>
              <td><a href="{{ repository.owner.url }}">
                  <img src="{{ repository.owner.avatar_url }}"
                       alt="avatar" height="24" width="24"/></a>
                  <a href="{{ repository.owner.url }}">
                      {{ repository.owner.login }}</a>
              </td>
              <td><a href="{{ repository.latest_commit.url }}">
                  {{ repository.latest_commit.sha }}</a>
                  {{ repository.latest_commit.message }}
                  {{ repository.latest_commit.author_name }}
              </td>
            </tr>
        {% endfor %}
        </tbody>
    </table>
</div>

{% endblock %}

Ok, almost there. The last step is to make index view to render the index.html template.

Edit views.py:

"""Views module."""

from flask import request, render_template


def index():
    query = request.args.get('query', 'Dependency Injector')
    limit = request.args.get('limit', 10, int)

    repositories = []

    return render_template(
        'index.html',
        query=query,
        limit=limit,
        repositories=repositories,
    )

That’s it.

Make sure the app is running or use flask run and open http://127.0.0.1:5000/.

You should see:

../_images/screen-01.png

Connect to the GitHub

In this section we will integrate our application with Github API.

We will use PyGithub library for working with Github API.

Let’s add it to the requirements.txt:

dependency-injector
flask
bootstrap-flask
pygithub

and run in the terminal:

pip install -r requirements.txt

Now we need to add Github API client the container. We will need to add two more providers from the dependency_injector.providers module:

  • Factory provider that will create Github client.

  • Configuration provider that will be used for providing the API token and the request timeout for the Github client.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers
from github import Github


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    github_client = providers.Factory(
        Github,
        login_or_token=config.github.auth_token,
        timeout=config.github.request_timeout,
    )

Note

We have used the configuration value before it was defined. That’s the principle how Configuration provider works.

Use first, define later.

Note

Don’t forget to remove the Ellipsis ... from the container. We don’t need it anymore since we container is not empty.

Now let’s add the configuration file.

We will use YAML.

Create an empty file config.yml in the root of the project:

./
├── githubnavigator/
│   ├── templates/
│   │   ├── base.html
│   │   └── index.html
│   ├── __init__.py
│   ├── application.py
│   ├── containers.py
│   └── views.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

github:
  request_timeout: 10

We will use PyYAML library for parsing the configuration file. Let’s add it to the requirements file.

Edit requirements.txt:

dependency-injector
flask
bootstrap-flask
pygithub
pyyaml

and install it:

pip install -r requirements.txt

We will use environment variable GITHUB_TOKEN to provide the API token.

Now we need to edit create_app() to make two things when application starts:

  • Load the configuration file the config.yml.

  • Load the API token from the GITHUB_TOKEN environment variable.

Edit application.py:

"""Application module."""

from flask import Flask
from flask_bootstrap import Bootstrap

from .containers import Container
from . import views


def create_app() -> Flask:
    container = Container()
    container.config.from_yaml('config.yml')
    container.config.github.auth_token.from_env('GITHUB_TOKEN')

    app = Flask(__name__)
    app.container = container
    app.add_url_rule('/', 'index', views.index)

    bootstrap = Bootstrap()
    bootstrap.init_app(app)

    return app

Now we need create an API token.

As for now, don’t worry, just take this one:

export GITHUB_TOKEN=cbde697a6e01424856fde2b7f94a88d1b501320e

Note

To create your own token:

  • Follow the Github guide.

  • Set the token to the environment variable:

export GITHUB_TOKEN=<your token>

That’s it.

Github API client setup is done.

Search service

Now it’s time to add SearchService. It will:

  • Perform the search.

  • Fetch commit extra data for each result.

  • Format result data.

SearchService will use Github API client.

Create empty file services.py in the githubnavigator package:

./
├── githubnavigator/
│   ├── templates/
│   │   ├── base.html
│   │   └── index.html
│   ├── __init__.py
│   ├── application.py
│   ├── containers.py
│   ├── services.py
│   └── views.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Services module."""

from github import Github
from github.Repository import Repository
from github.Commit import Commit


class SearchService:
    """Search service performs search on Github."""

    def __init__(self, github_client: Github):
        self._github_client = github_client

    def search_repositories(self, query, limit):
        """Search for repositories and return formatted data."""
        repositories = self._github_client.search_repositories(
            query=query,
            **{'in': 'name'},
        )
        return [
            self._format_repo(repository)
            for repository in repositories[:limit]
        ]

    def _format_repo(self, repository: Repository):
        commits = repository.get_commits()
        return {
            'url': repository.html_url,
            'name': repository.name,
            'owner': {
                'login': repository.owner.login,
                'url': repository.owner.html_url,
                'avatar_url': repository.owner.avatar_url,
            },
            'latest_commit': self._format_commit(commits[0]) if commits else {},
        }

    def _format_commit(self, commit: Commit):
        return {
            'sha': commit.sha,
            'url': commit.html_url,
            'message': commit.commit.message,
            'author_name': commit.commit.author.name,
        }

Now let’s add SearchService to the container.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers
from github import Github

from . import services


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    github_client = providers.Factory(
        Github,
        login_or_token=config.github.auth_token,
        timeout=config.github.request_timeout,
    )

    search_service = providers.Factory(
        services.SearchService,
        github_client=github_client,
    )

Inject search service into view

Now we are ready to make the search work.

Let’s inject SearchService into the index view. We will use Wiring feature.

Edit views.py:

"""Views module."""

from flask import request, render_template
from dependency_injector.wiring import inject, Provide

from .services import SearchService
from .containers import Container


@inject
def index(search_service: SearchService = Provide[Container.search_service]):
    query = request.args.get('query', 'Dependency Injector')
    limit = request.args.get('limit', 10, int)

    repositories = search_service.search_repositories(query, limit)

    return render_template(
        'index.html',
        query=query,
        limit=limit,
        repositories=repositories,
    )

To make the injection work we need to wire the container instance with the views module. This needs to be done once. After it’s done we can use Provide markers to specify as many injections as needed for any view.

Edit application.py:

"""Application module."""

from flask import Flask
from flask_bootstrap import Bootstrap

from .containers import Container
from . import views


def create_app() -> Flask:
    container = Container()
    container.config.from_yaml('config.yml')
    container.config.github.auth_token.from_env('GITHUB_TOKEN')
    container.wire(modules=[views])

    app = Flask(__name__)
    app.container = container
    app.add_url_rule('/', 'index', views.index)

    bootstrap = Bootstrap()
    bootstrap.init_app(app)

    return app

Make sure the app is running or use flask run and open http://127.0.0.1:5000/.

You should see:

../_images/screen-02.png

Make some refactoring

Our index view has two hardcoded config values:

  • Default search query

  • Default results limit

Let’s make some refactoring. We will move these values to the config.

Edit views.py:

"""Views module."""

from flask import request, render_template
from dependency_injector.wiring import inject, Provide

from .services import SearchService
from .containers import Container


@inject
def index(
        search_service: SearchService = Provide[Container.search_service],
        default_query: str = Provide[Container.config.default.query],
        default_limit: int = Provide[Container.config.default.limit.as_int()],
):
    query = request.args.get('query', default_query)
    limit = request.args.get('limit', default_limit, int)

    repositories = search_service.search_repositories(query, limit)

    return render_template(
        'index.html',
        query=query,
        limit=limit,
        repositories=repositories,
    )

Edit config.yml:

github:
  request_timeout: 10
default:
  query: "Dependency Injector"
  limit: 10

That’s it. The refactoring is done. We’ve made it cleaner.

Tests

In this section we will add some tests.

We will use pytest with its Flask extension and coverage.

Edit requirements.txt:

dependency-injector
flask
bootstrap-flask
pygithub
pyyaml
pytest-flask
pytest-cov

And install added packages:

pip install -r requirements.txt

Create empty file tests.py in the githubnavigator package:

./
├── githubnavigator/
│   ├── templates/
│   │   ├── base.html
│   │   └── index.html
│   ├── __init__.py
│   ├── application.py
│   ├── containers.py
│   ├── services.py
│   ├── tests.py
│   └── views.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Tests module."""

from unittest import mock

import pytest
from github import Github
from flask import url_for

from .application import create_app


@pytest.fixture
def app():
    app = create_app()
    yield app
    app.container.unwire()


def test_index(client, app):
    github_client_mock = mock.Mock(spec=Github)
    github_client_mock.search_repositories.return_value = [
        mock.Mock(
            html_url='repo1-url',
            name='repo1-name',
            owner=mock.Mock(
                login='owner1-login',
                html_url='owner1-url',
                avatar_url='owner1-avatar-url',
            ),
            get_commits=mock.Mock(return_value=[mock.Mock()]),
        ),
        mock.Mock(
            html_url='repo2-url',
            name='repo2-name',
            owner=mock.Mock(
                login='owner2-login',
                html_url='owner2-url',
                avatar_url='owner2-avatar-url',
            ),
            get_commits=mock.Mock(return_value=[mock.Mock()]),
        ),
    ]

    with app.container.github_client.override(github_client_mock):
        response = client.get(url_for('index'))

    assert response.status_code == 200
    assert b'Results found: 2' in response.data

    assert b'repo1-url' in response.data
    assert b'repo1-name' in response.data
    assert b'owner1-login' in response.data
    assert b'owner1-url' in response.data
    assert b'owner1-avatar-url' in response.data

    assert b'repo2-url' in response.data
    assert b'repo2-name' in response.data
    assert b'owner2-login' in response.data
    assert b'owner2-url' in response.data
    assert b'owner2-avatar-url' in response.data


def test_index_no_results(client, app):
    github_client_mock = mock.Mock(spec=Github)
    github_client_mock.search_repositories.return_value = []

    with app.container.github_client.override(github_client_mock):
        response = client.get(url_for('index'))

    assert response.status_code == 200
    assert b'Results found: 0' in response.data

Now let’s run it and check the coverage:

py.test githubnavigator/tests.py --cov=githubnavigator

You should see:

platform darwin -- Python 3.8.3, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
plugins: flask-1.0.0, cov-2.10.0
collected 2 items

githubnavigator/tests.py ..                                     [100%]

---------- coverage: platform darwin, python 3.8.3-final-0 -----------
Name                             Stmts   Miss  Cover
----------------------------------------------------
githubnavigator/__init__.py          0      0   100%
githubnavigator/application.py      15      0   100%
githubnavigator/containers.py        7      0   100%
githubnavigator/services.py         14      0   100%
githubnavigator/tests.py            34      0   100%
githubnavigator/views.py            10      0   100%
----------------------------------------------------
TOTAL                               80      0   100%

Note

Take a look at the highlights in the tests.py.

It emphasizes the overriding of the Github API client.

Conclusion

In this tutorial we’ve built a Flask application following the dependency injection principle. We’ve used the Dependency Injector as a dependency injection framework.

Containers and Providers helped to specify how to assemble search service and integrate it with a 3rd-party library.

Configuration provider helped to deal with reading YAML file and environment variable.

We used Wiring feature to inject the dependencies into the index() view. Provider overriding feature helped in testing.

We kept all the dependencies injected explicitly. This will help when you need to add or change something in future.

You can find complete project on the Github.

What’s next?