CLI application tutorial

This tutorial shows how to build a CLI 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 CLI application that helps to search for the movies. Let’s call it Movie Lister.

How does Movie Lister work?

  • There is a movies database

  • Each movie has next fields:
    • Title

    • Year of the release

    • Director’s name

  • The database is distributed in two formats:
    • Csv

    • Sqlite

  • Application uses the movies database to search for the movies

  • Application can search for the movies by:
    • Director’s name

    • Year of the release

  • Other database formats can be added later

Movie Lister is a naive example from Martin Fowler’s article about the dependency injection and inversion of control:

Here is a class diagram of the Movie Lister application:

../_images/classes-011.png

The responsibilities are split next way:

  • MovieLister - is responsible for the search

  • MovieFinder - is responsible for the fetching from the database

  • Movie - the movie entity

Prepare the environment

Let’s create the environment for the project.

First we need to create a project folder:

mkdir movie-lister-tutorial
cd movie-lister-tutorial

Now let’s create and activate virtual environment:

python3 -m venv venv
. venv/bin/activate

Project layout

Create next structure in the project root directory. All files are empty. That’s ok for now.

Initial project layout:

./
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   └── containers.py
├── venv/
├── config.yml
└── requirements.txt

Move on to the project requirements.

Install the requirements

Now it’s time to install the project requirements. We will use next packages:

  • dependency-injector - the dependency injection framework

  • pyyaml - the YAML files parsing library, used for the reading of the configuration files

  • pytest - the test framework

  • pytest-cov - the helper library for measuring the test coverage

Put next lines into the requirements.txt file:

dependency-injector
pyyaml
pytest
pytest-cov

and run next in the terminal:

pip install -r requirements.txt

The requirements are setup. Now we will add the fixtures.

Fixtures

In this section we will add the fixtures.

We will create a script that creates database files.

First add the folder data/ in the root of the project and then add the file fixtures.py inside of it:

./
├── data/
│   └── fixtures.py
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   └── containers.py
├── venv/
├── config.yml
└── requirements.txt

Second put next in the fixtures.py:

"""Fixtures module."""

import csv
import sqlite3
import pathlib


SAMPLE_DATA = [
    ('The Hunger Games: Mockingjay - Part 2', 2015, 'Francis Lawrence'),
    ('Rogue One: A Star Wars Story', 2016, 'Gareth Edwards'),
    ('The Jungle Book', 2016, 'Jon Favreau'),
]

FILE = pathlib.Path(__file__)
DIR = FILE.parent
CSV_FILE = DIR / 'movies.csv'
SQLITE_FILE = DIR / 'movies.db'


def create_csv(movies_data, path):
    with open(path, 'w') as opened_file:
        writer = csv.writer(opened_file)
        for row in movies_data:
            writer.writerow(row)


def create_sqlite(movies_data, path):
    with sqlite3.connect(path) as db:
        db.execute(
            'CREATE TABLE IF NOT EXISTS movies '
            '(title text, year int, director text)'
        )
        db.execute('DELETE FROM movies')
        db.executemany('INSERT INTO movies VALUES (?,?,?)', movies_data)


def main():
    create_csv(SAMPLE_DATA, CSV_FILE)
    create_sqlite(SAMPLE_DATA, SQLITE_FILE)
    print('OK')


if __name__ == '__main__':
    main()

Now run in the terminal:

python data/fixtures.py

You should see:

OK

Check that files movies.csv and movies.db have appeared in the data/ folder:

./
├── data/
│   ├── fixtures.py
│   ├── movies.csv
│   └── movies.db
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   └── containers.py
├── venv/
├── config.yml
└── requirements.txt

Fixtures are created. Let’s move on.

Container

In this section we will add the main part of our application - the 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.

Let’s also create the main() function. Its responsibility is to run our application. For now it will just do nothing.

Edit __main__.py:

"""Main module."""

from .containers import Container


def main() -> None:
    ...


if __name__ == '__main__':
    container = Container()

    main()

Csv finder

In this section we will build everything we need for working with the csv file formats.

We will add:

  • The Movie entity

  • The MovieFinder base class

  • The CsvMovieFinder finder implementation

  • The MovieLister class

After each step we will add the provider to the container.

../_images/classes-021.png

Create the entities.py in the movies package:

./
├── data/
│   ├── fixtures.py
│   ├── movies.csv
│   └── movies.db
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   ├── containers.py
│   └── entities.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Movie entities module."""


class Movie:

    def __init__(self, title: str, year: int, director: str):
        self.title = str(title)
        self.year = int(year)
        self.director = str(director)

    def __repr__(self):
        return '{0}(title={1}, year={2}, director={3})'.format(
            self.__class__.__name__,
            repr(self.title),
            repr(self.year),
            repr(self.director),
        )

Now we need to add the Movie factory to the container. We need to add import of the providers module from the dependency_injector package, import entities module.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers

from . import entities


class Container(containers.DeclarativeContainer):

    movie = providers.Factory(entities.Movie)

Note

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

Let’s move on to the finders.

Create the finders.py in the movies package:

./
├── data/
│   ├── fixtures.py
│   ├── movies.csv
│   └── movies.db
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   ├── containers.py
│   ├── entities.py
│   └── finders.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Movie finders module."""

import csv
from typing import Callable, List

from .entities import Movie


class MovieFinder:

    def __init__(self, movie_factory: Callable[..., Movie]) -> None:
        self._movie_factory = movie_factory

    def find_all(self) -> List[Movie]:
        raise NotImplementedError()


class CsvMovieFinder(MovieFinder):

    def __init__(
            self,
            movie_factory: Callable[..., Movie],
            path: str,
            delimiter: str,
    ) -> None:
        self._csv_file_path = path
        self._delimiter = delimiter
        super().__init__(movie_factory)

    def find_all(self) -> List[Movie]:
        with open(self._csv_file_path) as csv_file:
            csv_reader = csv.reader(csv_file, delimiter=self._delimiter)
            return [self._movie_factory(*row) for row in csv_reader]

Now let’s add the csv finder into the container.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers

from . import finders, entities


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    movie = providers.Factory(entities.Movie)

    csv_finder = providers.Singleton(
        finders.CsvMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.csv.path,
        delimiter=config.finder.csv.delimiter,
    )

The csv finder needs the movie factory. It needs it to create the Movie entities when reads the csv rows. To provide the factory we use .provider factory attribute. This is also called the delegation of the provider. If we just pass the movie factory as the dependency, it will be called when csv finder is created and the Movie instance will be injected. With the .provider attribute the provider itself will be injected.

The csv finder also has a few dependencies on the configuration options. We added configuration provider to provide these dependencies.

Note

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

Use first, define later.

Not let’s define the configuration values.

Edit config.yml:

finder:

  csv:
    path: "data/movies.csv"
    delimiter: ","

The configuration file is ready. Now let’s update the main() function to specify its location.

Edit __main__.py:

"""Main module."""

from .containers import Container


def main() -> None:
    ...


if __name__ == '__main__':
    container = Container()
    container.config.from_yaml('config.yml')

    main()

Move on to the lister.

Create the listers.py in the movies package:

./
├── data/
│   ├── fixtures.py
│   ├── movies.csv
│   └── movies.db
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   ├── containers.py
│   ├── entities.py
│   ├── finders.py
│   └── listers.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Movie listers module."""

from .finders import MovieFinder


class MovieLister:

    def __init__(self, movie_finder: MovieFinder):
        self._movie_finder = movie_finder

    def movies_directed_by(self, director):
        return [
            movie for movie in self._movie_finder.find_all()
            if movie.director == director
        ]

    def movies_released_in(self, year):
        return [
            movie for movie in self._movie_finder.find_all()
            if movie.year == year
        ]

and edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers

from . import finders, listers, entities


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    movie = providers.Factory(entities.Movie)

    csv_finder = providers.Singleton(
        finders.CsvMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.csv.path,
        delimiter=config.finder.csv.delimiter,
    )

    lister = providers.Factory(
        listers.MovieLister,
        movie_finder=csv_finder,
    )

All the components are created and added to the container.

Let’s inject the lister into the main() function.

Edit __main__.py:

"""Main module."""

import sys

from dependency_injector.wiring import inject, Provide

from .listers import MovieLister
from .containers import Container


@inject
def main(lister: MovieLister = Provide[Container.lister]) -> None:
    ...


if __name__ == '__main__':
    container = Container()
    container.config.from_yaml('config.yml')
    container.wire(modules=[sys.modules[__name__]])

    main()

Now when we call main() the container will assemble and inject the movie lister.

Let’s add some payload to main() function. It will list movies directed by Francis Lawrence and movies released in 2016.

Edit __main__.py:

"""Main module."""

import sys

from dependency_injector.wiring import inject, Provide

from .listers import MovieLister
from .containers import Container


@inject
def main(lister: MovieLister = Provide[Container.lister]) -> None:
    print('Francis Lawrence movies:')
    for movie in lister.movies_directed_by('Francis Lawrence'):
        print('\t-', movie)

    print('2016 movies:')
    for movie in lister.movies_released_in(2016):
        print('\t-', movie)


if __name__ == '__main__':
    container = Container()
    container.config.from_yaml('config.yml')
    container.wire(modules=[sys.modules[__name__]])

    main()

All set. Now we run the application.

Run in the terminal:

python -m movies

You should see:

Francis Lawrence movies:
    - Movie(title='The Hunger Games: Mockingjay - Part 2', year=2015, director='Francis Lawrence')
2016 movies:
    - Movie(title='Rogue One: A Star Wars Story', year=2016, director='Gareth Edwards')
    - Movie(title='The Jungle Book', year=2016, director='Jon Favreau')

Our application can work with the movies database in the csv format. We also want to support the sqlite format. We will deal with it in the next section.

Sqlite finder

In this section we will add another type of the finder - the sqlite finder.

Let’s get to work.

Edit finders.py:

"""Movie finders module."""

import csv
import sqlite3
from typing import Callable, List

from .entities import Movie


class MovieFinder:

    def __init__(self, movie_factory: Callable[..., Movie]) -> None:
        self._movie_factory = movie_factory

    def find_all(self) -> List[Movie]:
        raise NotImplementedError()


class CsvMovieFinder(MovieFinder):

    def __init__(
            self,
            movie_factory: Callable[..., Movie],
            path: str,
            delimiter: str,
    ) -> None:
        self._csv_file_path = path
        self._delimiter = delimiter
        super().__init__(movie_factory)

    def find_all(self) -> List[Movie]:
        with open(self._csv_file_path) as csv_file:
            csv_reader = csv.reader(csv_file, delimiter=self._delimiter)
            return [self._movie_factory(*row) for row in csv_reader]


class SqliteMovieFinder(MovieFinder):

    def __init__(
            self,
            movie_factory: Callable[..., Movie],
            path: str,
    ) -> None:
        self._database = sqlite3.connect(path)
        super().__init__(movie_factory)

    def find_all(self) -> List[Movie]:
        with self._database as db:
            rows = db.execute('SELECT title, year, director FROM movies')
            return [self._movie_factory(*row) for row in rows]

Now we need to add the sqlite finder to the container and update lister’s dependency to use it.

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers

from . import finders, listers, entities


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    movie = providers.Factory(entities.Movie)

    csv_finder = providers.Singleton(
        finders.CsvMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.csv.path,
        delimiter=config.finder.csv.delimiter,
    )

    sqlite_finder = providers.Singleton(
        finders.SqliteMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.sqlite.path,
    )

    lister = providers.Factory(
        listers.MovieLister,
        movie_finder=sqlite_finder,
    )

The sqlite finder has a dependency on the configuration option. Let’s update the configuration file.

Edit config.yml:

finder:

  csv:
    path: "data/movies.csv"
    delimiter: ","

  sqlite:
    path: "data/movies.db"

All is ready. Let’s check.

Run in the terminal:

python -m movies

You should see:

Francis Lawrence movies:
    - Movie(title='The Hunger Games: Mockingjay - Part 2', year=2015, director='Francis Lawrence')
2016 movies:
    - Movie(title='Rogue One: A Star Wars Story', year=2016, director='Gareth Edwards')
    - Movie(title='The Jungle Book', year=2016, director='Jon Favreau')

Our application now supports both formats: csv files and sqlite databases. Every time when we need to work with the different format we need to make a code change in the container. We will improve this in the next section.

Selector

In this section we will make our application more flexible.

The code change will not be needed to switch between csv and sqlite formats. We implement the switch based on the environment variable MOVIE_FINDER_TYPE:

  • When MOVIE_FINDER_TYPE=csv application uses csv finder.

  • When MOVIE_FINDER_TYPE=sqlite application uses sqlite finder.

We will use the Selector provider. It selects the provider based on the configuration option (docs - Selector provider).

Edit containers.py:

"""Containers module."""

from dependency_injector import containers, providers

from . import finders, listers, entities


class Container(containers.DeclarativeContainer):

    config = providers.Configuration()

    movie = providers.Factory(entities.Movie)

    csv_finder = providers.Singleton(
        finders.CsvMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.csv.path,
        delimiter=config.finder.csv.delimiter,
    )

    sqlite_finder = providers.Singleton(
        finders.SqliteMovieFinder,
        movie_factory=movie.provider,
        path=config.finder.sqlite.path,
    )

    finder = providers.Selector(
        config.finder.type,
        csv=csv_finder,
        sqlite=sqlite_finder,
    )

    lister = providers.Factory(
        listers.MovieLister,
        movie_finder=finder,
    )

The switch is the config.finder.type option. When its value is csv, the provider with the csv key is used. The same is for sqlite.

Now we need to read the value of the config.finder.type option from the environment variable MOVIE_FINDER_TYPE.

Edit __main__.py:

"""Main module."""

import sys

from dependency_injector.wiring import inject, Provide

from .listers import MovieLister
from .containers import Container


@inject
def main(lister: MovieLister = Provide[Container.lister]) -> None:
    print('Francis Lawrence movies:')
    for movie in lister.movies_directed_by('Francis Lawrence'):
        print('\t-', movie)

    print('2016 movies:')
    for movie in lister.movies_released_in(2016):
        print('\t-', movie)


if __name__ == '__main__':
    container = Container()
    container.config.from_yaml('config.yml')
    container.config.finder.type.from_env('MOVIE_FINDER_TYPE')
    container.wire(modules=[sys.modules[__name__]])

    main()

Done.

Run in the terminal line by line:

MOVIE_FINDER_TYPE=csv python -m movies
MOVIE_FINDER_TYPE=sqlite python -m movies

The output should be similar for each command:

Francis Lawrence movies:
    - Movie(title='The Hunger Games: Mockingjay - Part 2', year=2015, director='Francis Lawrence')
2016 movies:
    - Movie(title='Rogue One: A Star Wars Story', year=2016, director='Gareth Edwards')
    - Movie(title='The Jungle Book', year=2016, director='Jon Favreau')

In the next section we will add some tests.

Tests

It would be nice to add some tests. Let’s do it.

We will use pytest and coverage.

Create tests.py in the movies package:

./
├── data/
│   ├── fixtures.py
│   ├── movies.csv
│   └── movies.db
├── movies/
│   ├── __init__.py
│   ├── __main__.py
│   ├── containers.py
│   ├── entities.py
│   ├── finders.py
│   ├── listers.py
│   └── tests.py
├── venv/
├── config.yml
└── requirements.txt

and put next into it:

"""Tests module."""

from unittest import mock

import pytest

from .containers import Container


@pytest.fixture
def container():
    container = Container()
    container.config.from_dict({
        'finder': {
            'type': 'csv',
            'csv': {
                'path': '/fake-movies.csv',
                'delimiter': ',',
            },
            'sqlite': {
                'path': '/fake-movies.db',
            },
        },
    })
    return container


def test_movies_directed_by(container):
    finder_mock = mock.Mock()
    finder_mock.find_all.return_value = [
        container.movie('The 33', 2015, 'Patricia Riggen'),
        container.movie('The Jungle Book', 2016, 'Jon Favreau'),
    ]

    with container.finder.override(finder_mock):
        lister = container.lister()
        movies = lister.movies_directed_by('Jon Favreau')

    assert len(movies) == 1
    assert movies[0].title == 'The Jungle Book'


def test_movies_released_in(container):
    finder_mock = mock.Mock()
    finder_mock.find_all.return_value = [
        container.movie('The 33', 2015, 'Patricia Riggen'),
        container.movie('The Jungle Book', 2016, 'Jon Favreau'),
    ]

    with container.finder.override(finder_mock):
        lister = container.lister()
        movies = lister.movies_released_in(2015)

    assert len(movies) == 1
    assert movies[0].title == 'The 33'

Run in the terminal:

pytest movies/tests.py --cov=movies

You should see:

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

movies/tests.py ..                                              [100%]

---------- coverage: platform darwin, python 3.8.5-final-0 -----------
Name                   Stmts   Miss  Cover
------------------------------------------
movies/__init__.py         0      0   100%
movies/__main__.py        18     18     0%
movies/containers.py       9      0   100%
movies/entities.py         7      1    86%
movies/finders.py         26     13    50%
movies/listers.py          8      0   100%
movies/tests.py           24      0   100%
------------------------------------------
TOTAL                     92     32    65%

Note

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

We use .override() method of the finder provider. Provider is overridden by the mock. Every time when any other provider will request finder provider to provide the dependency, the mock will be returned. So when we call the lister provider, the MovieLister instance is created with the mock, not an actual MovieFinder.

Conclusion

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

With a help of Containers and Providers we have defined how to assemble application components.

Selector provider served as a switch for selecting the database format based on a configuration. Configuration provider helped to deal with reading YAML file and environment variable.

We used Wiring feature to inject the dependencies into the main() function. 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?