invalid-field-call / E3701

Message emitted:

Invalid usage of field(), %s

Description:

The dataclasses.field() specifier should only be used as the value of an assignment within a dataclass, or within the make_dataclass() function.

Problematic code:

from dataclasses import dataclass, field


@dataclass
class C:
    a: float
    b: float
    c: float

    field(init=False)  # [invalid-field-call]

    def __post_init__(self):
        self.c = self.a + self.b


print(field(init=False))  # [invalid-field-call]

Correct code:

from dataclasses import dataclass, field, make_dataclass

C = make_dataclass(
    "C",
    [("x", int), "y", ("z", int, field(default=5))],
    namespace={"add_one": lambda self: self.x + 1},
)


@dataclass
class C:
    a: float
    b: float
    c: float = field(init=False)

    def __post_init__(self):
        self.c = self.a + self.b

Created by the dataclass checker.