Summary - Dataclasses
- Python course
- Intermediate
- Lesson 58/116
- 5 min
Combine @dataclass, field and frozen to write a Product class.
Summary - Dataclasses
In this group we covered @dataclass basics, mutable defaults with field(), immutability with frozen=True, validation with __post_init__, and the asdict/astuple conversions.
A dataclass auto-generates constructor, repr and eq; field(default_factory=...) gives each instance its own container; frozen=True provides immutability; __post_init__ is for validation; asdict and astuple convert data to other forms. Now we'll write an example that combines these.
Practice Combined summary example
Start with from dataclasses import dataclass, field, asdict. Define a Product class with @dataclass; include fields name: str = "", price: int = 0 and tags: list = field(default_factory=list). Add a method def __post_init__(self):; in the body write if self.price < 0: and raise ValueError("negative not allowed"). Then create p = Product(name="Pen", price=10) and call print(p.name, p.price) and print(asdict(p)).
Starter code
# Write the Product class with field, __post_init__ and asdictSample solution
# Write the Product class with field, __post_init__ and asdict
from dataclasses import dataclass, field, asdict
@dataclass
class Product:
name: str = ""
price: int = 0
tags: list = field(default_factory=list)
def __post_init__(self):
if self.price < 0:
raise ValueError("negative not allowed")
p = Product(name="Pen", price=10)
print(p.name, p.price)
print(asdict(p))Expected output
Pen 10
{'name': 'Pen', 'price': 10, 'tags': []}
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which combination is wrong?
- @dataclass + field(default_factory=list)
- @dataclass(frozen=True) + members: list = [] Correct answer
- @dataclass + asdict + astuple
- @dataclass(frozen=True) + __post_init__
Explanation: A frozen class cannot hold a mutable list.
2 Match the dataclass concepts
| Term | Meaning |
|---|---|
@dataclass |
Auto constructor |
field(default_factory=list) |
Empty-list factory |
frozen=True |
Immutability |
__post_init__ |
Validation |
3 Import asdict
from dataclasses import dataclass, field, ____
@dataclass
class Product:
name: str = ""
price: int = 0from dataclasses import dataclass, field, asdict
@dataclass
class Product:
name: str = ""
price: int = 0Explanation: asdict is in the dataclasses module.
4 Match the class parts
| Term | Meaning |
|---|---|
@dataclass |
Decorator |
field(default_factory=list) |
Mutable default |
__post_init__ |
Validation method |
asdict(p) |
Dict conversion |