4.8. Dataclass Metadata¶
metadata- For storing extra information about fielddict | NoneNoneis treated as an emptydictMetadata is not used at all by Data Classes
Metadata is provided as a third-party extension mechanism
Use Case: SQLAlchemy https://python3.info/database/sqlalchemy/model-dataclass.html
def field(*,
default: Any,
default_factory: Callable,
init: bool = True,
repr: bool = True,
hash: bool|None = None,
compare: bool = True,
metadata: dict = None) -> None
4.8.1. SetUp¶
>>> from dataclasses import dataclass, field
4.8.2. Syntax¶
>>> @dataclass
... class User:
... firstname: str
... lastname: str
... age: int = field(metadata={'min': 27, 'max': 50})
... role: str = field(metadata={'choices': ['user', 'staff', 'admin']})
... height: float = field(metadata={'unit': 'cm'})
... weight: float = field(metadata={'unit': 'kg'})
4.8.3. Validation¶
>>> @dataclass
... class User:
... firstname: str
... lastname: str
... age: int = field(default=None, metadata={'min': 27, 'max': 50})
...
... def __post_init__(self):
... AGE_MIN = self.__dataclass_fields__['age'].metadata['min']
... AGE_MAX = self.__dataclass_fields__['age'].metadata['max']
...
... if self.age not in range(AGE_MIN, AGE_MAX):
... raise ValueError('Invalid age')
>>>
>>>
>>> mark = User('Mark', 'Watney', age=99)
Traceback (most recent call last):
ValueError: Invalid age
4.8.4. Use Case - 0x01¶
Validation
>>> from dataclasses import dataclass, field, KW_ONLY
>>> from datetime import date, time, datetime, timezone, timedelta
>>>
>>>
>>> @dataclass
... class Group:
... gid: int
... name: str
>>>
>>>
>>> @dataclass(frozen=True)
... class User:
... firstname: str
... lastname: str
... _: KW_ONLY
... birthdate: date
... job: str = 'admin'
... role: str = field(default='user', metadata={'choices': ['user', 'staff']})
... age: int | None = None
... height: int | float | None = field(default=None, metadata={'unit': 'cm', 'min': 156, 'max': 210})
... weight: int | float | None = field(default=None, metadata={'unit': 'kg', 'min': 50, 'max': 90})
... role: list[str] = field(default_factory=lambda: ['user', 'staff', 'admin'])
... friends: dict[str,str] = field(default_factory=dict)
... assignments: list[str] | None = field(default=None, metadata={'choices': ['Apollo18', 'Ares3', 'STS-136']})
... missions: list[Group] = field(default_factory=list)
... experience: timedelta = timedelta(hours=0)
... account_last_login: datetime | None = None
... account_created: datetime = datetime.now(tz=timezone.utc)
... AGE_MIN: int = field(default=30, init=False, repr=False)
... AGE_MAX: int = field(default=50, init=False, repr=False)
...
... def __post_init__(self):
... HEIGHT_MIN = self.__dataclass_fields__['height'].metadata['min']
... HEIGHT_MAX = self.__dataclass_fields__['height'].metadata['max']
... WEIGHT_MIN = self.__dataclass_fields__['weight'].metadata['min']
... WEIGHT_MAX = self.__dataclass_fields__['weight'].metadata['max']
... if not HEIGHT_MIN <= self.height < HEIGHT_MAX:
... raise ValueError(f'Height {self.height} is not in between {HEIGHT_MIN} and {HEIGHT_MAX}')
... if not WEIGHT_MIN <= self.weight < WEIGHT_MAX:
... raise ValueError(f'Height {self.weight} is not in between {WEIGHT_MIN} and {WEIGHT_MAX}')
... if self.age not in range(self.AGE_MIN, self.AGE_MAX):
... raise ValueError('Age is not valid for an user')
>>>
>>>
>>> mark = User(
... firstname='Mark',
... lastname='Watney',
... birthdate=date(1961, 4, 12),
... age=42,
... height=178.0,
... weight=75.5,
... assignments=['STS-136'],
... missions=[Group(gid=1, name='admin'),
... Group(gid=2, name='staff')],
... )
>>>
>>> print(mark)
User(firstname='Mark', lastname='Watney', birthdate=datetime.date(1961, 4, 12),
job='admin', role='user', age=42, height=178.0, weight=75.5,
role=['user', 'staff', 'admin'], friends={}, assignments=['STS-136'],
missions=[Group(gid=1, name='admin'), Group(gid=2, name='staff')],
experience=datetime.timedelta(0), account_last_login=None,
account_created=datetime.datetime(1969, 7, 21, 2, 56, 15, 123456, tzinfo=datetime.timezone.utc))
4.8.5. Use Case - 0x02¶
Setattr
>>> from dataclasses import dataclass, field
>>>
>>>
>>> @dataclass
... class User:
... firstname: str
... lastname: str
... age: float = field(default=None, metadata={'unit': 'year', 'min': 30, 'max': 50})
... height: float = field(default=None, metadata={'unit': 'cm', 'min': 156, 'max': 210})
... weight: float = field(default=None, metadata={'unit': 'kg', 'min': 50, 'max': 90})
...
... def __setattr__(self, attrname, attrvalue):
... if attrvalue is None:
... return super().__setattr__(attrname, attrvalue)
... try:
... min = User.__dataclass_fields__[attrname].metadata['min']
... max = User.__dataclass_fields__[attrname].metadata['max']
... except KeyError:
... # field does not have min and max metadata
... pass
... else:
... assert min <= attrvalue < max, f'{attrname} value {attrvalue} is not between {min} and {max}'
... finally:
... super().__setattr__(attrname, attrvalue)
>>>
>>>
>>>
>>> User('Mark', 'Watney')
User(firstname='Mark', lastname='Watney', age=None, height=None, weight=None)
>>>
>>> User('Mark', 'Watney', age=42)
User(firstname='Mark', lastname='Watney', age=42, height=None, weight=None)
>>>
>>> User('Mark', 'Watney', age=42, height=178.0, weight=75.5)
User(firstname='Mark', lastname='Watney', age=42, height=178.0, weight=75.5)
>>>
>>> User('Mark', 'Watney', age=99)
Traceback (most recent call last):
AssertionError: age value 99 is not between 30 and 50
>>>
>>> User('Mark', 'Watney', age=42, weight=200)
Traceback (most recent call last):
AssertionError: weight value 200 is not between 50 and 90
>>>
>>> User('Mark', 'Watney', age=42, height=120)
Traceback (most recent call last):
AssertionError: height value 120 is not between 156 and 210
4.8.6. Use Case - 0x03¶
>>> from dataclasses import field, dataclass
>>>
>>>
>>>
>>> @dataclass
... class User:
... firstname: str
... lastname: str
... age: int = field(metadata={'unit': 'year', 'type': 'range', 'min': 30, 'max': 50})
... height: float = field(metadata={'unit': 'cm', 'type': 'range', 'min': 150, 'max': 200})
... weight: float = field(metadata={'unit': 'kg', 'type': 'range', 'min': 50, 'max': 90})
... group: str = field(metadata={'type': 'choice', 'options': ['user', 'staff', 'admin']})
...
... def __post_init__(self):
... for fieldname, field in self.__dataclass_fields__.items():
... if not hasattr(field, 'metadata'):
... continue
... if 'type' not in field.metadata:
... continue
... value = getattr(self, field.name)
... match field.metadata['type']:
... case 'range': self._validate_range(field, value)
... case 'choice': self._validate_choice(field, value)
...
... def _validate_range(self, field, value):
... min = field.metadata['min']
... max = field.metadata['max']
... if not min <= value < max:
... raise ValueError(f'{field.name} value ({value}) is not between {min} and {max}')
...
... def _validate_choice(self, field, value):
... options = field.metadata['options']
... if value not in options:
... raise ValueError(f'{field.name} value ({value}) not in options: {options}')
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=178.0, group='user')
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=178.0, group='staff')
>>>
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=178.0, group='root')
Traceback (most recent call last):
ValueError: group value (root) not in options: ['user', 'staff', 'admin']
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=178.0, group='user')
>>>
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=100, group='user')
Traceback (most recent call last):
ValueError: height value (100) is not between 150 and 200
>>>
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=210, group='user')
Traceback (most recent call last):
ValueError: height value (210) is not between 150 and 200
>>> mark = User('Mark', 'Watney', age=42, weight=75.5, height=178.0, group='user')
>>>
>>> mark = User('Mark', 'Watney', age=20, weight=75.5, height=178.0, group='user')
Traceback (most recent call last):
ValueError: age value (20) is not between 30 and 50
>>>
>>> mark = User('Mark', 'Watney', age=99, weight=75, height=178.0, group='user')
Traceback (most recent call last):
ValueError: age value (99) is not between 30 and 50
4.8.7. Use Case - 0x03¶
>>> from dataclasses import dataclass, field
>>>
>>>
>>> @dataclass
... class User:
... firstname: str
... lastname: str
... age: int = field(default=None, metadata={'type': 'range', 'unit': 'year', 'min': 30, 'max': 50})
... height: float | None = field(default=None, metadata={'type': 'range', 'unit': 'cm', 'min': 156, 'max': 210})
... group: str | None = field(default='user', metadata={'type': 'choice', 'options': ['user', 'staff', 'admin']})
...
... def _validate_range(self, attrname, value):
... min = self.__dataclass_fields__[attrname].metadata['min']
... max = self.__dataclass_fields__[attrname].metadata['max']
... if value and not min <= value <= max:
... raise ValueError(f'Attribute {attrname} is not between {min} and {max}')
...
... def _validate_choice(self, attrname, value):
... options = self.__dataclass_fields__[attrname].metadata['options']
... if options and value not in options:
... raise ValueError(f'Attribute {attrname} is not a good choice, options are: {options}')
...
... def __setattr__(self, attrname, value):
... try:
... attrtype = self.__dataclass_fields__[attrname].metadata['type']
... except KeyError:
... return super().__setattr__(attrname, value)
... match attrtype:
... case 'range': self._validate_range(attrname, value)
... case 'choice': self._validate_choice(attrname, value)
... case _: raise NotImplementedError
>>>
>>>
>>> mark = User('Mark', 'Watney')
>>>
>>> mark
User(firstname='Mark', lastname='Watney', age=None, height=None, group='user')
>>>
>>> mark.group = 'staff'
>>> mark.group = 'root'
Traceback (most recent call last):
ValueError: Attribute group is not a good choice, options are: ['user', 'staff', 'admin']
>>>
>>> mark.age = 42
>>> mark.age = 69
Traceback (most recent call last):
ValueError: Attribute age is not between 30 and 50
4.8.8. Use Case - 0x04¶
>>>
... from __future__ import annotations
... from dataclasses import dataclass, field
... from sqlalchemy import Column, ForeignKey, Integer, String
... from sqlalchemy.orm import registry, relationship
...
... mapper_registry = registry()
...
...
... @mapper_registry.mapped
... @dataclass
... class User:
... __tablename__ = "user"
... __sa_dataclass_metadata_key__ = "db"
...
... id: int = field(init=False, metadata={"db": Column(Integer, primary_key=True)})
... name: str = field(default=None, metadata={"db": Column(String(50))})
... fullname: str = field(default=None, metadata={"db": Column(String(50))})
... nickname: str = field(default=None, metadata={"db": Column(String(12))})
... addresses: list[Address] = field(default_factory=list, metadata={"db": relationship("Address")})
...
...
... @mapper_registry.mapped
... @dataclass
... class Address:
... __tablename__ = "address"
... __sa_dataclass_metadata_key__ = "db"
...
... id: int = field(init=False, metadata={"db": Column(Integer, primary_key=True)})
... user_id: int = field(init=False, metadata={"db": Column(ForeignKey("user.id"))})
... email_address: str = field(default=None, metadata={"db": Column(String(50))})
4.8.9. Use Case - 0x05¶
>>> from dataclasses import dataclass, field
>>> @dataclass
... class BaseModel:
... def _validate_range(self, fieldname, field):
... value = getattr(self, fieldname)
... MIN = field.metadata['min']
... MAX = field.metadata['max']
... if not MIN <= value < MAX:
... raise ValueError(f'{fieldname} value ({value}) not between {MIN} and {MAX}')
...
... def _validate_choice(self, fieldname, field):
... value = getattr(self, fieldname)
... OPTIONS = field.metadata['options']
... if value not in OPTIONS:
... raise ValueError(f'{fieldname} value ({value}) not in {OPTIONS}')
...
... def __post_init__(self):
... for fieldname, field in self.__dataclass_fields__.items():
... if 'type' in field.metadata:
... match field.metadata['type']:
... case 'range': self._validate_range(fieldname, field)
... case 'choice': self._validate_choice(fieldname, field)
... case _: raise TypeError
>>> @dataclass
... class User(BaseModel):
... firstname: str
... lastname: str
... age: int = field(default=None, metadata={'unit': 'year', 'type': 'range', 'min': 30, 'max': 50, 'database': 'SmallPositiveInteger'})
... height: float = field(default=None, metadata={'unit': 'cm', 'type': 'range', 'min': 150, 'max': 210, 'database':'Decimal(3,2)'})
... weight: float = field(default=None, metadata={'unit': 'kg', 'type': 'range', 'min': 55, 'max': 85, 'database':'Decimal(3,2)'})
... group: str = field(default=None, metadata={'type': 'choice', 'options': ['user', 'staff', 'admin'], 'database':'VarChar(30)'})
>>> mark = User('Mark', 'Watney', age=42, height=178.0, weight=75.5, group='user')
>>> mark = User('Mark', 'Watney', age=42, height=178.0, weight=75.5, group='root')
Traceback (most recent call last):
ValueError: group value (root) not in ['user', 'staff', 'admin']
>>> mark = User('Mark', 'Watney', age=42, height=178.0, weight=90, group='user')
Traceback (most recent call last):
ValueError: weight value (90) not between 55 and 85
>>> mark = User('Mark', 'Watney', age=69, height=178.0, weight=75.5, group='user')
Traceback (most recent call last):
ValueError: age value (69) not between 30 and 50
4.8.10. Use Case - 0x06¶
>>> from dataclasses import dataclass, field
>>>
>>>
>>> @dataclass
... class Date:
... gid: int = field(metadata={'type': 'range', 'min': 1902, 'max': 2038})
... month: str = field(metadata={'type': 'choice', 'options': ['Jan', 'Feb', 'Mar', 'Apr']})
... day: int = field(metadata={'type': 'range', 'min': 1, 'max': 31})
...
... def _validate_range(self, field):
... min = self.__dataclass_fields__[field].metadata['min']
... max = self.__dataclass_fields__[field].metadata['max']
... value = getattr(self, field)
... if not min <= value <= max:
... raise ValueError(f'Field {field} value {value} is not between {min} and {max}')
...
... def _validate_choice(self, field):
... options = self.__dataclass_fields__[field].metadata['options']
... value = getattr(self, field)
... if value not in options:
... raise ValueError(f'Field {field} value {value} not in {options}')
...
... def __post_init__(self):
... for attrname, field in self.__dataclass_fields__.items():
... match field.metadata['type']:
... case 'range': self._validate_range(attrname)
... case 'choice': self._validate_choice(attrname)
... case _: raise NotImplementedError
>>> d = Date(1969, 'Feb', 30)
>>> d = Date(1969, 'Feb', 33)
Traceback (most recent call last):
ValueError: Field day value 33 is not between 1 and 31
>>> d = Date(1969, 'XYZ', 33)
Traceback (most recent call last):
ValueError: Field month value XYZ not in ['Jan', 'Feb', 'Mar', 'Apr']
4.8.11. Use Case - 0x07¶
>>> import logging
>>> from abc import ABC
>>> from dataclasses import field, dataclass
>>> from datetime import datetime, timezone
>>> from uuid import uuid4
>>>
>>> DEBUG = True
>>>
>>> @dataclass
... class BaseModel(ABC):
... def _validate_range(self, fieldname):
... MIN = self.__dataclass_fields__[fieldname].metadata['min']
... MAX = self.__dataclass_fields__[fieldname].metadata['max']
... UNIT = self.__dataclass_fields__[fieldname].metadata['unit']
... value = getattr(self, fieldname)
... if not MIN <= value <= MAX:
... raise ValueError(f'Field `{fieldname}` not between {MIN} and {MAX}.'
... f'Please remember that values need to be in {UNIT}')
...
... def _validate_choice(self, fieldname):
... OPTIONS = self.__dataclass_fields__[fieldname].metadata['options']
... value = getattr(self, fieldname)
... if value not in OPTIONS:
... raise ValueError(f'Field `{fieldname}` not in {OPTIONS}')
...
... def __post_init__(self):
... if DEBUG:
... self._since = datetime.now(tz=timezone.utc)
... self._uuid = str(uuid4())
... self._log = logging.getLogger(self.__class__.__name__)
...
... for fieldname in self.__dataclass_fields__.keys():
... match self.__dataclass_fields__[fieldname].metadata.get('type'):
... case 'range': self._validate_range(fieldname)
... case 'choice': self._validate_choice(fieldname)
... case _: continue
Define a model:
>>> @dataclass
... class User(BaseModel):
... firstname: str
... lastname: str
... age: int = field(metadata={'type':'range', 'min':30, 'max':50, 'unit':'years'})
... weight: float = field(metadata={'type':'range', 'min':50, 'max':100, 'unit':'kg'})
... height: float = field(metadata={'type':'range', 'min':150, 'max':200, 'unit':'cm'})
... group: str = field(metadata={'type':'choice', 'options':['users','staff','admins']})
Use:
>>> mark = User(
... firstname='Mark',
... lastname='Watney',
... age=40,
... weight=75.5,
... height=185.0,
... group='users',
... )