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CLI Parameter Types Intro

You can use several data types for the CLI options and CLI arguments, and you can add data validation requirements too.

Data conversion

When you declare a CLI parameter with some type Typer will convert the data received in the command line to that data type.

For example:

import typer

def main(name: str, age: int = 20, height_meters: float = 1.89, female: bool = True):
    print(f"NAME is {name}, of type: {type(name)}")
    print(f"--age is {age}, of type: {type(age)}")
    print(f"--height-meters is {height_meters}, of type: {type(height_meters)}")
    print(f"--female is {female}, of type: {type(female)}")

if __name__ == "__main__":

In this example, the value received for the CLI argument NAME will be treated as str.

The value for the CLI option --age will be converted to an int and --height-meters will be converted to a float.

And as female is a bool CLI option, Typer will convert it to a "flag" --female and the counterpart --no-female.

And here's how it looks like:

$ python --help

// Notice how --age is an INTEGER and --height-meters is a FLOAT

  NAME  [required]

  --age INTEGER           [default: 20]
  --height-meters FLOAT   [default: 1.89]
  --female / --no-female  [default: True]
  --help                  Show this message and exit.

// Call it with CLI parameters
$ python Camila --age 15 --height-meters 1.70 --female

// All the data has the correct Python type
NAME is Camila, of type: class 'str'
--age is 15, of type: class 'int'
--height-meters is 1.7, of type: class 'float'
--female is True, of type: class 'bool'

// And if you pass an incorrect type
$ python Camila --age 15.3

Try " --help" for help.

Error: Invalid value for '--age': '15.3' is not a valid integer

// Because 15.3 is not an INTEGER (it's a float)

Watch next

See more about specific types and validations in the next sections...

Technical Details

All the types you will see in the next sections are handled underneath by Click's Parameter Types.