How to Add Types to Function Parameters/Signatures in Python

The aim of this explainer💡 is to show how you designate a return type in Python. This arrived with Python 3.5 (2014) via PEP 484 — Type Hints.

Note that these are purely hints not a type checker. If looking for a type checker, check out mypy — Optional Static Typing for Python

• Without hints you would write

def greeting(name):
  return 'Hello, {}'.format(name)

• Can now be written as this, utilizing also f-strings that arrived in 3.6 including the syntax for providing a default argument value for the name parameter

def greeting(name: str = "Mr Foo Bar") -> str:
  return "Hello {name}"

• This is powerful mainly for custom types, this is how I initialize CLI argument parser making it clear what type of object the function returns

def init_argparse() -> argparse.ArgumentParser:
    parser = argparse.ArgumentParser()
    parser.add_argument("--param1")
    parser.add_argument("--param2")
    return parser

SOURCES

• PEP 484 — Type Hints.
• mypy — Optional Static Typing for Python