gmg7-data-class.md

Data Class

#datastructure #programming #data #storage #python

• Data class are the most complexity storage class on python compare to the [[01o9-collections-namedtuple]] and [[07fk-typing-namedtuple]]
• They are mutable, but can be immutable if you set the parameter frozen=true
  • If mutable, no type checking
• Data class does not modify the type, is not dependent of inheritance neither metaclass
• Class syntax
• Default methods implemented are: __init__, __repr__, __eq__
• If eq=true and frozen=true then the class is hashable
• Fields are initialized by __init__

Warning

BUG: Mutable variable as default value. Use default_factory

@dataclasss
class ClubMember:
	name: str
	guests: list = field(default_factory=list) # provide a function on the default_factory

• Data class has post_init when you need to make something after the initialization of the fields
• In addition, it is possible to pass init_var as well. This is variable used to initialize other variables and are not field neither class attribute

@dataclass
class HackerClubMember(ClubMember):
	all_handles: ClassVar[set[str]] = set()
    handle: str = ''
    def __post_init__(self):
        cls = self.__class__
        if self.handle == '':
            self.handle = self.name.split()[0]
        if self.handle in cls.all_handles:
            msg = f'handle {self.handle!r} already exists.'
            raise ValueError(msg)
        cls.all_handles.add(self.handle)

@dataclass
class C:
    i: int
    j: int = None
    database: InitVar[DatabaseType] = None
    def __post_init__(self, database):
        if self.j is None and database is not None:
            self.j = database.lookup('j')
c = C(10, database=my_database)

• [[sws0-data-class-as-code-smell]]#
• [[26qw-pattern-matching-as-data-class]]#

References

• Ramalho, 2022, p172-179
• Ramalho, 2022, p179-183