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main.py

from typing import Optional

import numpy as np
import numpy.typing as npt


def f(a: Optional[npt.NDArray[np.int64]]):
    if a is None:
        a = np.repeat(1, 5).astype(np.int64)
    print(a[0])
    print(max([0], key=lambda x: a[x]))

f(np.array([0], dtype=np.int64))