FastAPI Custom CLI Commands

What are custom CLI commands in FastAPI, and why are they useful?

Custom CLI commands in FastAPI allow you to add command-line functionality to your FastAPI application. These commands are useful for tasks like database migrations, data seeding, running background tasks, or any operation that needs to be executed from the terminal without starting the FastAPI server.

With custom CLI commands, you can automate tasks, run administrative operations, and integrate your FastAPI app with other tools or workflows through the command line.

How do you create a custom CLI command in FastAPI using Typer?

To create a custom CLI command in FastAPI, you can use the Typer library, which is built on top of Click and provides a simple way to define command-line interfaces. Typer is a great companion for FastAPI as it shares the same design philosophy and integrates smoothly with FastAPI applications.

Example of creating a custom CLI command with Typer:

import typer
from fastapi import FastAPI

app = FastAPI()
cli = typer.Typer()

@cli.command()
def greet(name: str):
    """Greets the user with their name."""
    typer.echo(f"Hello {name}!")

if __name__ == "__main__":
    cli()

In this example, a simple CLI command greet is created using Typer. When the command is run from the terminal, it greets the user by name.

To run the command:

python main.py greet John

How do you integrate FastAPI with Typer for custom CLI commands?

FastAPI and Typer can be easily integrated to create a single Python file that serves both as the application server and the CLI. By importing the FastAPI app into the Typer CLI file, you can run server-side operations through the command line.

Example of integrating FastAPI with Typer:

import typer
from fastapi import FastAPI

app = FastAPI()
cli = typer.Typer()

@cli.command()
def runserver():
    """Run the FastAPI server."""
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

if __name__ == "__main__":
    cli()

In this example, the runserver command starts the FastAPI application using Uvicorn. This provides a single CLI for managing both the FastAPI app and custom commands.

To start the FastAPI server:

python main.py runserver

How do you execute database migrations with a custom CLI in FastAPI?

You can use a custom CLI command to automate database migrations in FastAPI, making it easy to run migrations from the terminal. For this, tools like Alembic are commonly used, which can be integrated into Typer CLI commands to run migrations and upgrade the database schema.

Example of creating a CLI command for running database migrations:

import typer
import subprocess

cli = typer.Typer()

@cli.command()
def migrate():
    """Run database migrations using Alembic."""
    typer.echo("Running migrations...")
    subprocess.run(["alembic", "upgrade", "head"])

if __name__ == "__main__":
    cli()

In this example, the migrate command runs Alembic migrations. The command triggers the Alembic CLI tool to apply all pending database migrations.

To run the migration:

python cli.py migrate

How do you add multiple custom CLI commands in FastAPI?

Typer allows you to group multiple commands together by registering them with a Typer object. This way, you can define several commands related to different parts of your FastAPI application (e.g., server management, database operations, data seeding) in a single CLI tool.

Example of adding multiple custom CLI commands:

import typer
from fastapi import FastAPI

app = FastAPI()
cli = typer.Typer()

@cli.command()
def runserver():
    """Run the FastAPI server."""
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

@cli.command()
def migrate():
    """Run database migrations."""
    typer.echo("Running migrations...")
    # Migration logic here

@cli.command()
def seed_data():
    """Seed the database with initial data."""
    typer.echo("Seeding data...")
    # Data seeding logic here

if __name__ == "__main__":
    cli()

In this example, multiple commands—runserver, migrate, and seed_data—are defined in a single CLI tool. Each command performs a specific task related to managing the FastAPI application.

To run any of these commands:

python cli.py runserver
python cli.py migrate
python cli.py seed_data

How do you pass arguments and options to custom CLI commands in FastAPI?

Typer supports passing arguments and options to custom CLI commands, allowing you to make your commands more flexible. Arguments are positional and required, while options are optional and can have default values.

Example of passing arguments and options to a CLI command:

import typer

cli = typer.Typer()

@cli.command()
def create_user(name: str, age: int, admin: bool = typer.Option(False, help="Is the user an admin?")):
    """Create a new user."""
    typer.echo(f"Creating user: {name}, Age: {age}, Admin: {admin}")

if __name__ == "__main__":
    cli()

In this example, the create_user command accepts a required name and age argument, and an optional admin option (with a default value of False). The admin option also includes a help description.

To run the command:

python cli.py create-user John 30 --admin

How do you organize CLI commands into modules in FastAPI?

If your CLI tool becomes complex, you can organize the commands into multiple files or modules. Typer allows you to register commands from different modules and group them under the main CLI application.

Example of organizing CLI commands into modules:

Create separate modules for different commands:

# commands/server.py
import typer
from fastapi import FastAPI
import uvicorn

cli = typer.Typer()

@cli.command()
def runserver():
    """Run the FastAPI server."""
    app = FastAPI()
    uvicorn.run(app, host="0.0.0.0", port=8000)

Then, import and register these commands in the main CLI file:

# cli.py
import typer
from commands import server

cli = typer.Typer()

cli.add_typer(server.cli, name="server")

if __name__ == "__main__":
    cli()

In this example, the runserver command is organized in a separate module called server.py and then imported into the main CLI tool in cli.py.

To run the server command:

python cli.py server runserver

How do you test custom CLI commands in FastAPI?

You can test custom CLI commands using Typer's testing capabilities. Typer provides a CliRunner class, which simulates running commands from the command line and captures the output for testing purposes.

Example of testing a custom CLI command:

import typer
from typer.testing import CliRunner

cli = typer.Typer()
runner = CliRunner()

@cli.command()
def greet(name: str):
    typer.echo(f"Hello {name}")

def test_greet():
    result = runner.invoke(cli, ["greet", "John"])
    assert result.exit_code == 0
    assert "Hello John" in result.output

In this example, the test_greet function tests the greet command by simulating running the command with the argument "John". The output is then checked to ensure the command runs correctly.

What are some best practices for using custom CLI commands with FastAPI?

Some best practices for using custom CLI commands with FastAPI include:

• Use Typer for creating simple and intuitive CLI commands that integrate well with FastAPI.
• Organize your CLI commands into modules for better maintainability and clarity.
• Use argument validation and options to provide flexible and user-friendly commands.
• Write tests for your CLI commands to ensure they behave correctly.
• Leverage Typer's grouping and command structure to create a clean and scalable CLI interface for managing your FastAPI application.