README.md

Built-ins and libraries

@dataclasses.dataclass

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)

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