Python 3.7+dataclassesterm

__post_init__

initialization

The __post_init__ method is called automatically after __init__, for extra validation or processing.

Syntax

def __post_init__(self): ...

It is called only by the __init__ that the dataclass itself generated: with init=False, or with a hand-written __init__, nothing invokes it. Under frozen=True a plain self.x = ... raises FrozenInstanceError there, so you have to write object.__setattr__(self, "x", ...). InitVar fields land here too — they never become attributes, they are passed to __post_init__ as arguments, which is the sanctioned way to feed in data needed only during construction.

Examples

from dataclasses import dataclass
@dataclass
class Circle:
    radius: float
    def __post_init__(self):
        if self.radius < 0:
            raise ValueError('radius < 0')
        self.area = 3.14 * self.radius ** 2
c = Circle(3.0)
c.area # → 28.26

# Circle(-1) # → ValueError

__post_init__

Syntax

Examples

See also