請問一下 scanner cb 的用途是?

[lumpa7223]

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>> api.scanners?

Signature:
api.scanners(
    scanner_type: shioaji.constant.ScannerType, 
    ascending: bool = True,
    date: str = None,
    count: shioaji.shioaji.ConstrainedIntValue = 100, # 0 <= count <= 200
    timeout: int = 30000,
    cb: Callable[[List[shioaji.data.ChangePercentRank]], NoneType] = None,
) -> List[shioaji.data.ChangePercentRank]

callback 的用途可以幹嘛? 有試過這樣寫

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rk = api.scanners(
    scanner_type=sj.constant.ScannerType.AmountRank,
    count=3,
    cb=lambda x: print(x)
)

但是沒有任何print出來，請教高手的解答 謝謝.