Even if a class is inherited from ABC, it can still be instantiated unless it contains abstract methods.
Having the code below, what is the best way to prevent an Identifier object from being created: Identifier(['get', 'Name'])?
from abc import ABC
from typing import List
from dataclasses import dataclass
@dataclass
class Identifier(ABC):
sub_tokens: List[str]
@staticmethod
def from_sub_tokens(sub_tokens):
return SimpleIdentifier(sub_tokens) if len(sub_tokens) == 1 else CompoundIdentifier(sub_tokens)
@dataclass
class SimpleIdentifier(Identifier):
pass
@dataclass
class CompoundIdentifier(Identifier):
pass
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Answer
You can create a AbstractDataclass class which guarantees this behaviour, and you can use this every time you have a situation like the one you described.
@dataclass
class AbstractDataclass(ABC):
def __new__(cls, *args, **kwargs):
if cls == AbstractDataclass or cls.__bases__[0] == AbstractDataclass:
raise TypeError("Cannot instantiate abstract class.")
return super().__new__(cls)
So, if Identifier inherits from AbstractDataclass instead of from ABC directly, modifying the __post_init__ will not be needed.
@dataclass
class Identifier(AbstractDataclass):
sub_tokens: List[str]
@staticmethod
def from_sub_tokens(sub_tokens):
return SimpleIdentifier(sub_tokens) if len(sub_tokens) == 1 else CompoundIdentifier(sub_tokens)
@dataclass
class SimpleIdentifier(Identifier):
pass
@dataclass
class CompoundIdentifier(Identifier):
pass
Instantiating Identifier will raise TypeError but not instantiating SimpleIdentifier or CompountIdentifier.
And the AbstractDataclass can be re-used in other parts of the code.