Python Typer Boolean Flags and Multi-Value Options

Python Typer makes CLI development easy. This guide covers boolean flags and multi-value options.

Boolean Flags in Typer

Boolean flags are options that don't need values. They're either on or off.

Use typer.Option() with default values to create them. Here's an example:

import typer

app = typer.Typer()

@app.command()
def greet(verbose: bool = typer.Option(False, "--verbose", "-v")):
    if verbose:
        typer.echo("Verbose mode activated!")
    typer.echo("Hello World")

if __name__ == "__main__":
    app()

Run it with or without the flag:

$ python app.py
Hello World

$ python app.py --verbose
Verbose mode activated!
Hello World

Important: The flag name determines if it's True or False. No value needed.

Multi-Value Options

Sometimes you need multiple values for an option. Typer supports this with multiple=True.

This creates a list of values. Here's how to use it:

@app.command()
def process_files(files: list[str] = typer.Option(..., "--file", "-f", multiple=True)):
    for file in files:
        typer.echo(f"Processing {file}")

if __name__ == "__main__":
    app()

Run it with multiple values:

$ python app.py --file a.txt --file b.txt
Processing a.txt
Processing b.txt

Combining Boolean and Multi-Value

You can combine these features for powerful CLIs. Here's an example:

@app.command()
def search(
    query: list[str] = typer.Option(..., "--query", "-q", multiple=True),
    case_sensitive: bool = typer.Option(False, "--case", "-c")
):
    for q in query:
        if case_sensitive:
            typer.echo(f"Case-sensitive search: {q}")
        else:
            typer.echo(f"Search: {q.lower()}")

Try it with different combinations:

$ python app.py -q python -q typer
Search: python
Search: typer

$ python app.py -q Python -c
Case-sensitive search: Python

Best Practices

Follow these tips for better CLI design:

1. Use meaningful flag names. --verbose is better than -v alone.

2. Document your options. Add help text using the help parameter.

3. Consider using help text to guide users.

4. For complex CLIs, explore subcommands.

Type Hints Matter

Typer uses type hints extensively. Always specify types for your options.

For boolean flags, use bool. For multi-value, use list[str].

Learn more in our type hints guide.

Conclusion

Boolean flags and multi-value options make your CLI powerful. Typer handles them elegantly.

Remember to use proper type hints and document your options. This improves user experience.

For more advanced features, explore Typer's envvar support or shell completion.