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. Letâs call it Movie Lister.
How does Movie Lister work?
There is a movies database
Title
Year of the release
Directorâs name
Csv
Sqlite
Application uses the movies database to search for the movies
Directorâs name
Year of the release
Other database formats can be added later
Movie Lister is a naive example from Martin Fowlerâs article about the dependency injection and inversion of control:
Here is a class diagram of the Movie Lister application:
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
Letâs create the environment for the project.
First we need to create a project folder:
mkdir movie-lister-tutorial cd movie-lister-tutorial
Now letâs create and activate virtual environment:
python3 -m venv venv . venv/bin/activateProject layout¶
Create next structure in the project root directory. All files are empty. Thatâs ok for now.
Initial project layout:
./ âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py âââ venv/ âââ config.yml âââ requirements.txt
Move on to the project requirements.
Install the requirements¶Now itâs 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:
./ âââ data/ â âââ fixtures.py âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py âââ venv/ âââ config.yml âââ 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:
You should see:
Check that files movies.csv and movies.db have appeared in the data/ folder:
./ âââ data/ â âââ fixtures.py â âââ movies.csv â âââ movies.db âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py âââ venv/ âââ config.yml âââ requirements.txt
Fixtures are created. Letâs 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.
Letâs 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.
Create the entities.py in the movies package:
./ âââ data/ â âââ fixtures.py â âââ movies.csv â âââ movies.db âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py â âââ entities.py âââ venv/ âââ config.yml âââ 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
Donât forget to remove the Ellipsis ... from the container. We donât need it anymore since we container is not empty.
Letâs move on to the finders.
Create the finders.py in the movies package:
./ âââ data/ â âââ fixtures.py â âââ movies.csv â âââ movies.db âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py â âââ entities.py â âââ finders.py âââ venv/ âââ config.yml âââ 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 letâs 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(yaml_files=["config.yml"])
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 a configuration provider to provide these dependencies and specified the location of the configuration file. The configuration provider will parse the configuration file when we create a container instance.
Not letâs define the configuration values.
Edit config.yml:
finder:
csv:
path: "data/movies.csv"
delimiter: ","
The configuration file is ready. Move on to the lister.
Create the listers.py in the movies package:
./ âââ data/ â âââ fixtures.py â âââ movies.csv â âââ movies.db âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py â âââ entities.py â âââ finders.py â âââ listers.py âââ venv/ âââ config.yml âââ 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(yaml_files=["config.yml"])
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.
Letâs inject the lister into the main() function.
Edit __main__.py:
"""Main module."""
from dependency_injector.wiring import Provide, inject
from .listers import MovieLister
from .containers import Container
@inject
def main(lister: MovieLister = Provide[Container.lister]) -> None:
...
if __name__ == "__main__":
container = Container()
container.wire(modules=[__name__])
main()
Now when we call main() the container will assemble and inject the movie lister.
Letâs add some payload to main() function. It will list movies directed by Francis Lawrence and movies released in 2016.
Edit __main__.py:
"""Main module."""
from dependency_injector.wiring import Provide, inject
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.wire(modules=[__name__])
main()
All set. Now we run the application.
Run in the terminal:
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.
Letâs 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 listerâs 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(yaml_files=["config.yml"])
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. Letâs update the configuration file.
Edit config.yml:
finder:
csv:
path: "data/movies.csv"
delimiter: ","
sqlite:
path: "data/movies.db"
All is ready. Letâs check.
Run in the terminal:
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(yaml_files=["config.yml"])
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."""
from dependency_injector.wiring import Provide, inject
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.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. Letâs do it.
We will use pytest and coverage.
Create tests.py in the movies package:
./ âââ data/ â âââ fixtures.py â âââ movies.csv â âââ movies.db âââ movies/ â âââ __init__.py â âââ __main__.py â âââ containers.py â âââ entities.py â âââ finders.py â âââ listers.py â âââ tests.py âââ venv/ âââ config.yml âââ 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(
config={
"finder": {
"type": "csv",
"csv": {
"path": "/fake-movies.csv",
"delimiter": ",",
},
"sqlite": {
"path": "/fake-movies.db",
},
},
},
)
return container
@pytest.fixture
def finder_mock(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"),
]
return finder_mock
def test_movies_directed_by(container, finder_mock):
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):
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 3.10.0, pytest-6.2.5, py-1.10.0, pluggy-1.0.0 plugins: cov-3.0.0 collected 2 items movies/tests.py .. [100%] ---------- coverage: platform darwin, python 3.10 ----------- Name Stmts Miss Cover ------------------------------------------ movies/__init__.py 0 0 100% movies/__main__.py 16 16 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 90 30 67%
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.
In this tutorial weâve built a CLI application following the dependency injection principle. Weâve 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 a YAML file and environment variables.
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.
Whatâs next?
Sponsor the project on GitHub:
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