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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _fillna(expr, value):
 """ Fill null with value. :param expr: sequence or scalar :param value: value to fill into :return: sequence or scalar """


    if isinstance(expr, SequenceExpr):
        return FillNa(_input=expr, _fill_value=value, _data_type=expr.dtype)
    elif isinstance(expr, Scalar):
        return FillNa(_input=expr, _fill_value=value, _value_type=expr.dtype)