dataclasses_frozen.py

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"""
Tests validation of frozen dataclass instances.
"""

# Specification: https://peps.python.org/pep-0557/#frozen-instances

from dataclasses import dataclass

@dataclass(frozen=True)
class DC1:
    a: float
    b: str

dc1 = DC1(1, "")

dc1.a = 1  # E: dataclass is frozen
[invalid-assignment] Property `a` defined in `DC1` is read-only
dc1.b = "" # E: dataclass is frozen
[invalid-assignment] Property `b` defined in `DC1` is read-only


# This should generate an error because a non-frozen dataclass
# cannot inherit from a frozen dataclass.
@dataclass  # E[DC2]
class DC2(DC1):  # E[DC2]
Tag 'DC2'
[invalid-frozen-dataclass-subclass] Non-frozen dataclass `DC2` cannot inherit from frozen dataclass `DC1`
    pass

@dataclass
class DC3:
    a: int

# This should generate an error because a frozen dataclass
# cannot inherit from a non-frozen dataclass.
@dataclass(frozen=True)  # E[DC4]
class DC4(DC3):  # E[DC4]
Tag 'DC4'
[invalid-frozen-dataclass-subclass] Frozen dataclass `DC4` cannot inherit from non-frozen dataclass `DC3`
    pass


@dataclass(frozen=True)
class DC1Child(DC1):
    # This should be allowed because attributes within a frozen
    # dataclass are covariant rather than invariant.
    a: int