The dataclasses module is a new addition in Python 3.7 that utilizes the type annotation syntax introduced in 3.6 to create fully-functional classes that are used to store data.
Some consider dataclass to be an upgrade from
namedtuple.
In short, the decorator will read the type annotations
and create methods like __init__, __repr__, __eq__, etc.
Example 1
(Script available)
from dataclasses import dataclass
@dataclass()
class Vector2(object):
x: float
y: float = 0
def __add__(self, other):
return Vector2(self.x + other.x, self.y + other.y)
if __name__ == '__main__':
help(Vector2)
v1 = Vector2(x=1)
print(v1)
v2 = Vector2(1, 0)
print(v2)
print(v1 == v2)
v1.y = 3
print(v1 + v2)
It is worth noting that type annotations will not enforce variable types, and dataclass has no special logic regarding this either. To enforce variable types please see next example.
Example 2
(Script available)
import sys
from dataclasses import dataclass
def enforce_types(data_cls):
__setattr = data_cls.__setattr__
def __setattr__(self, key, value):
if key in self.__class__.__dataclass_fields__:
value = self.__dataclass_fields__[key].type(value)
return __setattr(self, key, value)
data_cls.__setattr__ = __setattr__
return data_cls
def math_dataclass(data_cls):
def get_method(name):
def __method(self, other):
return self.__class__(**{
field_name: getattr(field.type, name)(
getattr(self, field_name),
getattr(other, field_name),
)
for field_name, field in self.__class__.__dataclass_fields__.items()
})
return __method
for name in ['__add__', '__sub__', '__mul__', '__truediv__']:
if hasattr(data_cls, name):
continue
setattr(data_cls, name, get_method(name))
return data_cls
class OperationNotAllowed(Exception):
pass
@math_dataclass
@enforce_types
@dataclass
class Vector2(object):
x: float
y: float
def __mul__(self, other):
return self.x * other.x + self.y * other.y
def __truediv__(self, other):
raise OperationNotAllowed
if __name__ == '__main__':
v1 = Vector2(1, 2)
print(v1)
v1.x = 0
print(v1)
v2 = Vector2(1.5, 2.5)
print(v1 + v2)
print(v1 * v2)
try:
print(v1 / v2)
except Exception as e:
print(repr(e), file=sys.stderr)