# documentação Django 2.0.8

# Código fonte para django.contrib.postgres.forms.ranges

    from psycopg2.extras import DateRange, DateTimeTZRange, NumericRange

    from django import forms
    from django.core import exceptions
    from django.forms.widgets import MultiWidget
    from django.utils.translation import gettext_lazy as _

    __all__ = [
        'BaseRangeField', 'IntegerRangeField', 'FloatRangeField',
        'DateTimeRangeField', 'DateRangeField', 'RangeWidget',
    ]

    class BaseRangeField(forms.MultiValueField):
        default_error_messages = {
            'invalid': _('Enter two valid values.'),
            'bound_ordering': _('The start of the range must not exceed the end of the range.'),
        }

        def __init__(self, **kwargs):
            if 'widget' not in kwargs:
                kwargs['widget'] = RangeWidget(self.base_field.widget)
            if 'fields' not in kwargs:
                kwargs['fields'] = [self.base_field(required=False), self.base_field(required=False)]
            kwargs.setdefault('required', False)
            kwargs.setdefault('require_all_fields', False)
            super().__init__(**kwargs)

        def prepare_value(self, value):
            lower_base, upper_base = self.fields
            if isinstance(value, self.range_type):
                return [
                    lower_base.prepare_value(value.lower),
                    upper_base.prepare_value(value.upper),
                ]
            if value is None:
                return [
                    lower_base.prepare_value(None),
                    upper_base.prepare_value(None),
                ]
            return value

        def compress(self, values):
            if not values:
                return None
            lower, upper = values
            if lower is not None and upper is not None and lower > upper:
                raise exceptions.ValidationError(
                    self.error_messages['bound_ordering'],
                    code='bound_ordering',
                )
            try:
                range_value = self.range_type(lower, upper)
            except TypeError:
                raise exceptions.ValidationError(
                    self.error_messages['invalid'],
                    code='invalid',
                )
            else:
                return range_value

    class IntegerRangeField(BaseRangeField):
        default_error_messages = {'invalid': _('Enter two whole numbers.')}
        base_field = forms.IntegerField
        range_type = NumericRange

    class FloatRangeField(BaseRangeField):
        default_error_messages = {'invalid': _('Enter two numbers.')}
        base_field = forms.FloatField
        range_type = NumericRange

    class DateTimeRangeField(BaseRangeField):
        default_error_messages = {'invalid': _('Enter two valid date/times.')}
        base_field = forms.DateTimeField
        range_type = DateTimeTZRange

    class DateRangeField(BaseRangeField):
        default_error_messages = {'invalid': _('Enter two valid dates.')}
        base_field = forms.DateField
        range_type = DateRange

    class RangeWidget(MultiWidget):
        def __init__(self, base_widget, attrs=None):
            widgets = (base_widget, base_widget)
            super().__init__(widgets, attrs)

        def decompress(self, value):
            if value:
                return (value.lower, value.upper)
            return (None, None)