Passing default list argument to dataclasses
From the dataclasses.field docs:
The parameters to
field()are:
- default_factory: If provided, it must be a zero-argument callable that will be called when a default value is needed for this field. Among other purposes, this can be used to specify fields with mutable default values, as discussed below. It is an error to specify both default and default_factory.
Your default_factory is not a 0-argument callable but a list, which is the reason for the error:
@dataclassclass Pizza(): ingredients: List = field(default_factory=['dow', 'tomatoes']) # <- wrong!Use a lambda function instead:
@dataclassclass Pizza(): ingredients: List = field(default_factory=lambda: ['dow', 'tomatoes'])
For complex datatypes i tend to abbreviate like so:
import copyfrom dataclasses import dataclass, fieldfrom typing import Dict, Tupledef default_field(obj): return field(default_factory=lambda: copy.copy(obj))@dataclassclass C: complex_attribute: Dict[str, Tuple[int, str]] = default_field({"a": (1, "x"), "b": (1, "y")})