# documentação Django 2.0.8

# Código fonte para django.db.models.functions.window

    from django.db.models import FloatField, Func, IntegerField

    __all__ = [
        'CumeDist', 'DenseRank', 'FirstValue', 'Lag', 'LastValue', 'Lead',
        'NthValue', 'Ntile', 'PercentRank', 'Rank', 'RowNumber',
    ]

    class CumeDist(Func):
        function = 'CUME_DIST'
        name = 'CumeDist'
        output_field = FloatField()
        window_compatible = True

    class DenseRank(Func):
        function = 'DENSE_RANK'
        name = 'DenseRank'
        output_field = IntegerField()
        window_compatible = True

    class FirstValue(Func):
        arity = 1
        function = 'FIRST_VALUE'
        name = 'FirstValue'
        window_compatible = True

    class LagLeadFunction(Func):
        window_compatible = True

        def __init__(self, expression, offset=1, default=None, **extra):
            if expression is None:
                raise ValueError(
                    '%s requires a non-null source expression.' %
                    self.__class__.__name__
                )
            if offset is None or offset <= 0:
                raise ValueError(
                    '%s requires a positive integer for the offset.' %
                    self.__class__.__name__
                )
            args = (expression, offset)
            if default is not None:
                args += (default,)
            super().__init__(*args, **extra)

        def _resolve_output_field(self):
            sources = self.get_source_expressions()
            return sources[0].output_field

    class Lag(LagLeadFunction):
        function = 'LAG'
        name = 'Lag'

    class LastValue(Func):
        arity = 1
        function = 'LAST_VALUE'
        name = 'LastValue'
        window_compatible = True

    class Lead(LagLeadFunction):
        function = 'LEAD'
        name = 'Lead'

    class NthValue(Func):
        function = 'NTH_VALUE'
        name = 'NthValue'
        window_compatible = True

        def __init__(self, expression, nth=1, **extra):
            if expression is None:
                raise ValueError('%s requires a non-null source expression.' % self.__class__.__name__)
            if nth is None or nth <= 0:
                raise ValueError('%s requires a positive integer as for nth.' % self.__class__.__name__)
            super().__init__(expression, nth, **extra)

        def _resolve_output_field(self):
            sources = self.get_source_expressions()
            return sources[0].output_field

    class Ntile(Func):
        function = 'NTILE'
        name = 'Ntile'
        output_field = IntegerField()
        window_compatible = True

        def __init__(self, num_buckets=1, **extra):
            if num_buckets <= 0:
                raise ValueError('num_buckets must be greater than 0.')
            super().__init__(num_buckets, **extra)

    class PercentRank(Func):
        function = 'PERCENT_RANK'
        name = 'PercentRank'
        output_field = FloatField()
        window_compatible = True

    class Rank(Func):
        function = 'RANK'
        name = 'Rank'
        output_field = IntegerField()
        window_compatible = True

    class RowNumber(Func):
        function = 'ROW_NUMBER'
        name = 'RowNumber'
        output_field = IntegerField()
        window_compatible = True