Is the Dataclasses lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Dataclasses runs real Python in your browser via Pyodide.

How long does the Dataclasses lesson take?

About 6 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Dataclasses lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Dataclasses lesson?

Advanced level. Assumes comfort with classes, functions, and standard library. Not for first-time Python learners.

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Advanced·6 min·advanced · oop

Dataclasses

You’ll be able to

Replace boilerplate __init__ with @dataclass
Add default values and default factories to dataclass fields
Get free __repr__, __eq__, and comparison methods
Choose between frozen=True (immutable) and mutable dataclasses

Why this matters

FastAPI request models, Pydantic v1 internals, and half of modern config layers use @dataclass or its stricter cousin attrs to kill boilerplate. Staff engineers reach for frozen=True + slots=True when they need hashable, memory-lean value objects for caching keys or event payloads.

Common pitfalls

Using a mutable default like field=[] — raises ValueError; use field(default_factory=list) instead.
Assuming frozen=True means deeply immutable — nested lists still mutate; wrap in tuples or use MappingProxyType.
Skipping slots=True on hot-path classes — costs ~40% more memory per instance versus a slotted dataclass.

@dataclass auto-generates __init__, __repr__, and __eq__ from your annotated fields. Modern Python's answer to "a class that just holds data."

Two things to remember

Use field(default_factory=list) for mutable defaults (never = []!).
Add frozen=True to make instances immutable (hashable, dict-key safe).

Versus namedtuple / dict

More readable than dicts (you get attributes, not string lookups)
More flexible than namedtuple (mutable by default, supports inheritance)
Less ceremony than a hand-written class

Try it

Make Task frozen and put one in a set.
Add a due: datetime | None = None field.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Use @dataclass to define Point with two float fields x and y. Create Point(3, 4), print it — the auto-generated repr should include Point(x=3, y=4).
Exercise 2
Define a Task dataclass with title: str, priority: int = 1, and done: bool = False. Add a method mark_done(self) that sets done to True and returns self. Then create Task("Ship"), call .mark_done(), print it — expected repr includes done=True.
Exercise 3
Define a Team dataclass with name: str and members: list[str]. Use field(default_factory=list) for members so every instance gets its own empty list (never a shared default). Create two Teams, append to one — the other must stay empty. Print both.

← PreviousType Hints Next →Iterators

Dataclasses

You’ll be able to

Replace boilerplate __init__ with @dataclass
Add default values and default factories to dataclass fields
Get free __repr__, __eq__, and comparison methods
Choose between frozen=True (immutable) and mutable dataclasses

Why this matters

Common pitfalls

Using a mutable default like field=[] — raises ValueError; use field(default_factory=list) instead.
Assuming frozen=True means deeply immutable — nested lists still mutate; wrap in tuples or use MappingProxyType.
Skipping slots=True on hot-path classes — costs ~40% more memory per instance versus a slotted dataclass.

@dataclass auto-generates __init__, __repr__, and __eq__ from your annotated fields. Modern Python's answer to "a class that just holds data."

Two things to remember

Use field(default_factory=list) for mutable defaults (never = []!).
Add frozen=True to make instances immutable (hashable, dict-key safe).

Versus namedtuple / dict

More readable than dicts (you get attributes, not string lookups)
More flexible than namedtuple (mutable by default, supports inheritance)
Less ceremony than a hand-written class

Try it

Make Task frozen and put one in a set.
Add a due: datetime | None = None field.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Use @dataclass to define Point with two float fields x and y. Create Point(3, 4), print it — the auto-generated repr should include Point(x=3, y=4).
Exercise 2
Define a Task dataclass with title: str, priority: int = 1, and done: bool = False. Add a method mark_done(self) that sets done to True and returns self. Then create Task("Ship"), call .mark_done(), print it — expected repr includes done=True.
Exercise 3
Define a Team dataclass with name: str and members: list[str]. Use field(default_factory=list) for members so every instance gets its own empty list (never a shared default). Create two Teams, append to one — the other must stay empty. Print both.