.. _flask-tutorial: Flask tutorial ============== .. meta:: :keywords: Python,Flask,Tutorial,Education,Web,Example,DI,Dependency injection,IoC, Inversion of control,Refactoring,Tests,Unit tests,Pytest,py.test,Bootstrap, HTML,CSS :description: This tutorial shows how to build a Flask application following the dependency injection principle. You will create the web application, connect to the Github API, cover it with unit the test and make some refactoring. This tutorial shows how to build a ``Flask`` application following the dependency injection principle. Start from the scratch or jump to the section: .. contents:: :local: :backlinks: none 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. .. image:: flask-images/screen-02.png Prepare the environment ----------------------- Let's create the environment for the project. First we need to create a project folder: .. code-block:: bash mkdir ghnav-flask-tutorial cd ghnav-flask-tutorial Now let's create and activate virtual environment: .. code-block:: bash 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: .. code-block:: bash dependency-injector flask Now let's install it: .. code-block:: bash pip install -r requirements.txt And check that installation is successful: .. code-block:: bash python -c "import dependency_injector; print(dependency_injector.__version__)" python -c "import flask; print(flask.__version__)" You should see something like: .. code-block:: bash (venv) $ python -c "import dependency_injector; print(dependency_injector.__version__)" 4.37.0 (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``: .. code-block:: python """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``: .. code-block:: python """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``: .. code-block:: python """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: .. code-block:: bash export FLASK_APP=githubnavigator.application export FLASK_ENV=development flask run The output should be something like: .. code-block:: bash * 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``: .. code-block:: bash :emphasize-lines: 3 dependency-injector flask bootstrap-flask and run in the terminal: .. code-block:: bash pip install -r requirements.txt Let's initialize ``bootstrap-flask`` extension. We will need to modify ``create_app()``. Edit ``application.py``: .. code-block:: python :emphasize-lines: 4,17-18 """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``: .. code-block:: bash :emphasize-lines: 3-5 ./ ├── 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``: .. code-block:: html {% block head %} {% block styles %} {{ bootstrap.load_css() }} {% endblock %} {% block title %}{% endblock %} {% endblock %} {% block content %}{% endblock %} {% block scripts %} {{ bootstrap.load_js() }} {% endblock %} And put something to the index page. Put next into the ``index.html``: .. code-block:: html {% extends "base.html" %} {% block title %}Github Navigator{% endblock %} {% block content %}

Github Navigator

Results found: {{ repositories|length }}

{% for repository in repositories %} {{n}} {% endfor %}
# Repository Repository owner Last commit
{{ loop.index }} {{ repository.name }} avatar {{ repository.owner.login }} {{ repository.latest_commit.sha }} {{ repository.latest_commit.message }} {{ repository.latest_commit.author_name }}
{% endblock %} Ok, almost there. The last step is to make ``index`` view to render the ``index.html`` template. Edit ``views.py``: .. code-block:: python """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: .. image:: flask-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``: .. code-block:: bash :emphasize-lines: 4 dependency-injector flask bootstrap-flask pygithub and run in the terminal: .. code-block:: bash 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``: .. code-block:: python :emphasize-lines: 3-4,9,11-15 """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: .. code-block:: bash :emphasize-lines: 11 ./ ├── githubnavigator/ │ ├── templates/ │ │ ├── base.html │ │ └── index.html │ ├── __init__.py │ ├── application.py │ ├── containers.py │ └── views.py ├── venv/ ├── config.yml └── requirements.txt and put next into it: .. code-block:: yaml 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``: .. code-block:: bash :emphasize-lines: 5 dependency-injector flask bootstrap-flask pygithub pyyaml and install it: .. code-block:: bash 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``: .. code-block:: python :emphasize-lines: 12-13 """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: .. code-block:: bash export GITHUB_TOKEN=cbde697a6e01424856fde2b7f94a88d1b501320e .. note:: To create your own token: - Follow the `Github guide `_. - Set the token to the environment variable: .. code-block:: bash export GITHUB_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: .. code-block:: bash :emphasize-lines: 9 ./ ├── 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: .. code-block:: python """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``: .. code-block:: python :emphasize-lines: 6,19-22 """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 :ref:`Wiring` feature. Edit ``views.py``: .. code-block:: python :emphasize-lines: 4,6-7,10-11,15 """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``: .. code-block:: python :emphasize-lines: 14 """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: .. image:: flask-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``: .. code-block:: python :emphasize-lines: 11-17 """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``: .. code-block:: yaml :emphasize-lines: 3-5 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``: .. code-block:: bash :emphasize-lines: 6-7 dependency-injector flask bootstrap-flask pygithub pyyaml pytest-flask pytest-cov And install added packages: .. code-block:: bash pip install -r requirements.txt Create empty file ``tests.py`` in the ``githubnavigator`` package: .. code-block:: bash :emphasize-lines: 10 ./ ├── 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: .. code-block:: python :emphasize-lines: 44,67 """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: .. code-block:: bash py.test githubnavigator/tests.py --cov=githubnavigator You should see: .. code-block:: bash platform darwin -- Python 3.9, 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 3.9 ----------- 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. :ref:`containers` and :ref:`providers` helped to specify how to assemble search service and integrate it with a 3rd-party library. :ref:`configuration-provider` helped to deal with reading YAML file and environment variable. We used :ref:`wiring` feature to inject the dependencies into the ``index()`` view. :ref:`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? - Look at the other :ref:`tutorials` - Know more about the :ref:`providers` - Go to the :ref:`contents` .. disqus::