Type Hints and Dataclasses

Introduction

Python decides types as the program runs, which is flexible but can hide mistakes until it is too late. Type hints let you write down what types you expect. Python itself does not enforce them at runtime. Their value is that your editor and tools such as mypy can check them for you, and that they document your code for the next reader.

Type hints

Write a colon and the type after a variable or parameter, and -> before a function's return type.

Because the hints are ignored by Python, this still runs (and a checker like mypy would complain about it):

Optional values and unions

A value that may be missing is X | None. A value that can be one of several types uses |.

Checking is None first is exactly the habit that a type checker wants to see.

Dataclasses: less boilerplate

Many classes only hold data. Writing __init__, __repr__ and __eq__ by hand is repetitive. The @dataclass decorator writes them for you, based on type-hinted fields.

You get a readable repr and equality for free. Fields with defaults must come after fields without.

More dataclass options

frozen=True makes instances read-only, which also makes them usable as dictionary keys. For a mutable default such as a list, use field(default_factory=list). A plain = [] would be shared by every object.

Enums: a fixed set of choices

An enum is a named set of constants. It is clearer and safer than loose strings or numbers.

A misspelled Status.PUBLISHD is an immediate error, whereas the string "publishd" would silently be wrong.

Putting it together

Try it yourself

Create a dataclass Product with name: str, price: float and quantity: int = 0, and add a method total_value() that returns price * quantity. Create two products and print the total value of both.

Summary

• Type hints (x: int, -> str) document code and help tools, but Python does not enforce them.
• X | None marks optional values, so check is None before using them.
• @dataclass writes __init__, __repr__ and __eq__ for you. Use field(default_factory=...) for mutable defaults.
• An Enum names a fixed set of choices and prevents typos.