Fluent Python, 2nd Edition

Chapter 8. Type Hints in Functions

It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention.

Guido van Rossum, Jukka Lehtosalo, and Łukasz Langa, PEP 484—Type Hints¹

Type hints are the biggest change in the history of Python since the unification of types and classes in Python 2.2, released in 2001. However, type hints do not benefit all Python users equally. That’s why they should always be optional.

PEP 484—Type Hints introduced syntax and semantics for explicit type declarations in function arguments, return values, and variables. The goal is to help developer tools find bugs in Python codebases via static analysis, i.e., without actually running the code through tests.

The main beneficiaries are professional software engineers using IDEs (Integrated Development Environments) and CI (Continuous Integration). The cost-benefit analysis that makes type hints attractive to that group does not apply to all users of Python.

Python’s user base is much wider than that. It includes scientists, traders, journalists, artists, makers, analysts, and students in many fields—among others. For most of them, the cost of learning type hints is likely higher—unless they already know a language with static types, subtyping, and generics. The benefits will be lower for many of those users, given how they interact with Python, and the smaller size of their codebases and teams—often “teams” ...