Python Typer Error Handling Guide

Error handling is key in CLI apps. Python Typer offers tools like BadParameter, Abort, and Exit. These help manage user input errors.

Understanding Typer Error Types

Typer provides three main error types. Each serves a specific purpose in CLI apps.

BadParameter handles invalid input. Abort stops execution gracefully. Exit ends the program with a status code.

Using BadParameter for Input Validation

BadParameter raises when input fails validation. It shows helpful error messages to users.

import typer

app = typer.Typer()

@app.command()
def greet(name: str):
    if len(name) < 3:
        raise typer.BadParameter("Name must be 3+ characters")
    typer.echo(f"Hello, {name}!")

if __name__ == "__main__":
    app()

$ python app.py greet ab
Error: Name must be 3+ characters

This example validates name length. The error message guides users to correct input.

Graceful Abort with typer.Abort

Abort stops command execution. It's useful when conditions aren't met.

@app.command()
def process(file: str):
    if not os.path.exists(file):
        typer.echo("File not found!", err=True)
        raise typer.Abort()
    # Process file

$ python app.py process missing.txt
File not found!
Aborted!

The command stops if the file doesn't exist. Users see a clear message.

Controlled Exit with typer.Exit

Exit ends the program with a status code. It's good for script completion.

@app.command()
def check(verbose: bool):
    if not verbose:
        typer.echo("Running in quiet mode")
        raise typer.Exit(code=0)
    typer.echo("Verbose output enabled")

$ python app.py check
Running in quiet mode

The command exits early in quiet mode. Status code 0 means success.

Error Handling Best Practices

Combine these methods for robust CLI apps. Use BadParameter for input issues.

For runtime problems, use Abort. For controlled exits, choose Exit.

Learn more about input handling in our Python Typer: Arguments vs Options guide.

Advanced Error Scenarios

You can customize error messages. Add colors or formatting for better UX.

@app.command()
def calculate(value: int):
    if value < 0:
        raise typer.BadParameter(
            typer.style("Value must be positive!", fg=typer.colors.RED)
        )

For enum validation, see our Python Typer Enum Choices guide.

Conclusion

Typer's error handling makes CLI apps user-friendly. BadParameter validates input.

Abort stops execution when needed. Exit provides controlled termination.

For more on Typer features, check our Python Typer Help Text Guide.