How to Use Basic Type Hints (int, str) for Return Values in Python

Declare a simple function with int and str type hints and a typed return value in Python.

Easy Python 3.9+ Aug 9, 2026 Testing & modern typing 12 views 0 copies

type-hints annotations functions typing

Python code

6 lines
Python 3.9+
Run it 
def greet(name: str, age: int) -> str:
    return f"{name} is {age} years old."


if __name__ == "__main__":
    print(greet("Alice", 30))

Output

stdout 
Alice is 30 years old.

How it works

Type hints are annotations that tell developers (and tools like mypy) what types a function expects and returns. Here, name: str and age: int mark the parameters, and -> str marks the return type. Although Python does not enforce these hints at runtime, they make code self-documenting and enable static type checking and IDE autocompletion.

This simple pattern is the foundation for writing maintainable, typed Python code. The if __name__ == "__main__": guard ensures the test call runs only when the script is executed directly.

Common mistakes

  • Forgetting to add the return type annotation (-> str) after the closing parenthesis.
  • Using mutable defaults (like `list=[]`) with type hints, which can cause shared-state bugs.
  • Assuming type hints enforce runtime checks; they don't — use a linter like mypy for that.

Variations

  1. Use optional hints: `def greet(name: str, age: int | None = None) -> str:`
  2. Use `from __future__ import annotations` to defer evaluation of type hints.
  3. Add `from typing import Any` and use `Any` when type is unknown.

Real-world use cases

  • Defining API response formatters where the return type is guaranteed to be a string.
  • Building small utilities that log or format user info with typed parameters.
  • Writing test helpers that need clear input and output contracts for readability.

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