How to initialize Python dataclass 2D list?

Question

Is is possible to create a default initializer to python dataclasses initializing a 2D array, i.e. resulting in the same as

from dataclasses import dataclass, field
from typing import List
MAX = 5
@dataclass
class AlgoData:
    list2D: List[List[int]]  # = ???
    list1D: List[int] = field(default_factory=list)

b = [[] for m in range(MAX)]
a = AlgoData(b)

but without the b argument. Result:

AlgoData(list2D=[[], [], [], [], []], list1D=[])

Answer 1

You can use a lambda function instead as a default_factory for field:

from dataclasses import dataclass, field
from typing import List
MAX = 5
@dataclass
class AlgoData:
    list2D: List[List[int]] = field(default_factory=lambda: [[] for m in range(MAX)])
    list1D: List[int] = field(default_factory=list)

a = AlgoData()

a becomes:

AlgoData(list2D=[[], [], [], [], []], list1D=[])