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# Gotchas

PyWavelets utilizes `NumPy` under the hood. That’s why handling the data containing `None` values can be surprising. `None` values are converted to ‘not a number’ (`numpy.NaN`) values:

    >>> import numpy, pywt
    >>> x = [None, None]
    >>> mode = 'sym'
    >>> wavelet = 'db1'
    >>> cA, cD = pywt.dwt(x, wavelet, mode)
    >>> numpy.all(numpy.isnan(cA))
    True
    >>> numpy.all(numpy.isnan(cD))
    True
    >>> rec = pywt.idwt(cA, cD, wavelet, mode)
    >>> numpy.all(numpy.isnan(rec))
    True

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