What is Data class?

A data class is a regular Python class. The only thing that sets it apart is that it has basic data model methods like .init(), .repr(), and .eq() implemented for you.

from dataclasses import dataclass
@dataclass
class Position:
    name: str
    lon: float = 0.0
    lat: float = 0.0

In data class is necessary to specify data type (which name is data annotation).

All parameter that dataclass support

• init: Add .init() method? (Default is True.)
• repr: Add .repr() method? (Default is True.)
• eq: Add .eq() method? (Default is True.)
• order: Add ordering methods? (Default is False.)
• unsafe_hash: Force the addition of a .hash() method? (Default is False.)
• frozen: If True, assigning to fields raise an exception. (Default is False.)

see below examples

from dataclasses import dataclass
@dataclass(eq=True)
class Position:
    name: str
    lon: float = 0.0
    lat: float = 0.0
city1 = Position('ISF', 1.2, 3.2)
city2 = Position('Isfahan', 1.2, 3.2)
print(city1 == city2)

True

We should make data class immutable if we want declear a set of instances of it

@dataclass(eq=True, frozen=True)
class Position:
    name: str
    lon: float = 0.0
    lat: float = 0.0
city1 = Position('ISF', 1.2, 3.2)
city2 = Position('Isfahan', 1.2, 3.2)
li = {city1, city2}

If we want validate the value of class attributes when an object has initialized we can use post_init method.

@dataclass(eq=True, frozen=True)
class Position:
    name: str
    lon: float = 0.0
    lat: float = 0.0
    def __post_init__(self):
        if len(self.name) == 0:
            raise ValueError("name could not be empty")
        
city1 = Position("", 1,4)

ValueError: name could not be empty