# Django 2.2.9.dev20191202104323 文档

# django.forms.fields 源代码

    """
    Field classes.
    """

    import copy
    import datetime
    import math
    import operator
    import os
    import re
    import uuid
    from decimal import Decimal, DecimalException
    from io import BytesIO
    from urllib.parse import urlsplit, urlunsplit

    from django.core import validators
    from django.core.exceptions import ValidationError
    # Provide this import for backwards compatibility.
    from django.core.validators import EMPTY_VALUES  # NOQA
    from django.forms.boundfield import BoundField
    from django.forms.utils import from_current_timezone, to_current_timezone
    from django.forms.widgets import (
        FILE_INPUT_CONTRADICTION, CheckboxInput, ClearableFileInput, DateInput,
        DateTimeInput, EmailInput, FileInput, HiddenInput, MultipleHiddenInput,
        NullBooleanSelect, NumberInput, Select, SelectMultiple,
        SplitDateTimeWidget, SplitHiddenDateTimeWidget, TextInput, TimeInput,
        URLInput,
    )
    from django.utils import formats
    from django.utils.dateparse import parse_duration
    from django.utils.duration import duration_string
    from django.utils.ipv6 import clean_ipv6_address
    from django.utils.translation import gettext_lazy as _, ngettext_lazy

    __all__ = (
        'Field', 'CharField', 'IntegerField',
        'DateField', 'TimeField', 'DateTimeField', 'DurationField',
        'RegexField', 'EmailField', 'FileField', 'ImageField', 'URLField',
        'BooleanField', 'NullBooleanField', 'ChoiceField', 'MultipleChoiceField',
        'ComboField', 'MultiValueField', 'FloatField', 'DecimalField',
        'SplitDateTimeField', 'GenericIPAddressField', 'FilePathField',
        'SlugField', 'TypedChoiceField', 'TypedMultipleChoiceField', 'UUIDField',
    )

    class Field:
        widget = TextInput  # Default widget to use when rendering this type of Field.
        hidden_widget = HiddenInput  # Default widget to use when rendering this as "hidden".
        default_validators = []  # Default set of validators
        # Add an 'invalid' entry to default_error_message if you want a specific
        # field error message not raised by the field validators.
        default_error_messages = {
            'required': _('This field is required.'),
        }
        empty_values = list(validators.EMPTY_VALUES)

        def __init__(self, *, required=True, widget=None, label=None, initial=None,
                     help_text='', error_messages=None, show_hidden_initial=False,
                     validators=(), localize=False, disabled=False, label_suffix=None):
            # required -- Boolean that specifies whether the field is required.
            #             True by default.
            # widget -- A Widget class, or instance of a Widget class, that should
            #           be used for this Field when displaying it. Each Field has a
            #           default Widget that it'll use if you don't specify this. In
            #           most cases, the default widget is TextInput.
            # label -- A verbose name for this field, for use in displaying this
            #          field in a form. By default, Django will use a "pretty"
            #          version of the form field name, if the Field is part of a
            #          Form.
            # initial -- A value to use in this Field's initial display. This value
            #            is *not* used as a fallback if data isn't given.
            # help_text -- An optional string to use as "help text" for this Field.
            # error_messages -- An optional dictionary to override the default
            #                   messages that the field will raise.
            # show_hidden_initial -- Boolean that specifies if it is needed to render a
            #                        hidden widget with initial value after widget.
            # validators -- List of additional validators to use
            # localize -- Boolean that specifies if the field should be localized.
            # disabled -- Boolean that specifies whether the field is disabled, that
            #             is its widget is shown in the form but not editable.
            # label_suffix -- Suffix to be added to the label. Overrides
            #                 form's label_suffix.
            self.required, self.label, self.initial = required, label, initial
            self.show_hidden_initial = show_hidden_initial
            self.help_text = help_text
            self.disabled = disabled
            self.label_suffix = label_suffix
            widget = widget or self.widget
            if isinstance(widget, type):
                widget = widget()
            else:
                widget = copy.deepcopy(widget)

            # Trigger the localization machinery if needed.
            self.localize = localize
            if self.localize:
                widget.is_localized = True

            # Let the widget know whether it should display as required.
            widget.is_required = self.required

            # Hook into self.widget_attrs() for any Field-specific HTML attributes.
            extra_attrs = self.widget_attrs(widget)
            if extra_attrs:
                widget.attrs.update(extra_attrs)

            self.widget = widget

            messages = {}
            for c in reversed(self.__class__.__mro__):
                messages.update(getattr(c, 'default_error_messages', {}))
            messages.update(error_messages or {})
            self.error_messages = messages

            self.validators = [*self.default_validators, *validators]

            super().__init__()

        def prepare_value(self, value):
            return value

        def to_python(self, value):
            return value

        def validate(self, value):
            if value in self.empty_values and self.required:
                raise ValidationError(self.error_messages['required'], code='required')

        def run_validators(self, value):
            if value in self.empty_values:
                return
            errors = []
            for v in self.validators:
                try:
                    v(value)
                except ValidationError as e:
                    if hasattr(e, 'code') and e.code in self.error_messages:
                        e.message = self.error_messages[e.code]
                    errors.extend(e.error_list)
            if errors:
                raise ValidationError(errors)

        def clean(self, value):
            """
            Validate the given value and return its "cleaned" value as an
            appropriate Python object. Raise ValidationError for any errors.
            """
            value = self.to_python(value)
            self.validate(value)
            self.run_validators(value)
            return value

        def bound_data(self, data, initial):
            """
            Return the value that should be shown for this field on render of a
            bound form, given the submitted POST data for the field and the initial
            data, if any.

            For most fields, this will simply be data; FileFields need to handle it
            a bit differently.
            """
            if self.disabled:
                return initial
            return data

        def widget_attrs(self, widget):
            """
            Given a Widget instance (*not* a Widget class), return a dictionary of
            any HTML attributes that should be added to the Widget, based on this
            Field.
            """
            return {}

        def has_changed(self, initial, data):
            """Return True if data differs from initial."""
            # Always return False if the field is disabled since self.bound_data
            # always uses the initial value in this case.
            if self.disabled:
                return False
            try:
                data = self.to_python(data)
                if hasattr(self, '_coerce'):
                    return self._coerce(data) != self._coerce(initial)
            except ValidationError:
                return True
            # For purposes of seeing whether something has changed, None is
            # the same as an empty string, if the data or initial value we get
            # is None, replace it with ''.
            initial_value = initial if initial is not None else ''
            data_value = data if data is not None else ''
            return initial_value != data_value

        def get_bound_field(self, form, field_name):
            """
            Return a BoundField instance that will be used when accessing the form
            field in a template.
            """
            return BoundField(form, self, field_name)

        def __deepcopy__(self, memo):
            result = copy.copy(self)
            memo[id(self)] = result
            result.widget = copy.deepcopy(self.widget, memo)
            result.validators = self.validators[:]
            return result

    class CharField(Field):
        def __init__(self, *, max_length=None, min_length=None, strip=True, empty_value='', **kwargs):
            self.max_length = max_length
            self.min_length = min_length
            self.strip = strip
            self.empty_value = empty_value
            super().__init__(**kwargs)
            if min_length is not None:
                self.validators.append(validators.MinLengthValidator(int(min_length)))
            if max_length is not None:
                self.validators.append(validators.MaxLengthValidator(int(max_length)))
            self.validators.append(validators.ProhibitNullCharactersValidator())

        def to_python(self, value):
            """Return a string."""
            if value not in self.empty_values:
                value = str(value)
                if self.strip:
                    value = value.strip()
            if value in self.empty_values:
                return self.empty_value
            return value

        def widget_attrs(self, widget):
            attrs = super().widget_attrs(widget)
            if self.max_length is not None and not widget.is_hidden:
                # The HTML attribute is maxlength, not max_length.
                attrs['maxlength'] = str(self.max_length)
            if self.min_length is not None and not widget.is_hidden:
                # The HTML attribute is minlength, not min_length.
                attrs['minlength'] = str(self.min_length)
            return attrs

    class IntegerField(Field):
        widget = NumberInput
        default_error_messages = {
            'invalid': _('Enter a whole number.'),
        }
        re_decimal = re.compile(r'\.0*\s*$')

        def __init__(self, *, max_value=None, min_value=None, **kwargs):
            self.max_value, self.min_value = max_value, min_value
            if kwargs.get('localize') and self.widget == NumberInput:
                # Localized number input is not well supported on most browsers
                kwargs.setdefault('widget', super().widget)
            super().__init__(**kwargs)

            if max_value is not None:
                self.validators.append(validators.MaxValueValidator(max_value))
            if min_value is not None:
                self.validators.append(validators.MinValueValidator(min_value))

        def to_python(self, value):
            """
            Validate that int() can be called on the input. Return the result
            of int() or None for empty values.
            """
            value = super().to_python(value)
            if value in self.empty_values:
                return None
            if self.localize:
                value = formats.sanitize_separators(value)
            # Strip trailing decimal and zeros.
            try:
                value = int(self.re_decimal.sub('', str(value)))
            except (ValueError, TypeError):
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            return value

        def widget_attrs(self, widget):
            attrs = super().widget_attrs(widget)
            if isinstance(widget, NumberInput):
                if self.min_value is not None:
                    attrs['min'] = self.min_value
                if self.max_value is not None:
                    attrs['max'] = self.max_value
            return attrs

    class FloatField(IntegerField):
        default_error_messages = {
            'invalid': _('Enter a number.'),
        }

        def to_python(self, value):
            """
            Validate that float() can be called on the input. Return the result
            of float() or None for empty values.
            """
            value = super(IntegerField, self).to_python(value)
            if value in self.empty_values:
                return None
            if self.localize:
                value = formats.sanitize_separators(value)
            try:
                value = float(value)
            except (ValueError, TypeError):
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            return value

        def validate(self, value):
            super().validate(value)
            if value in self.empty_values:
                return
            if not math.isfinite(value):
                raise ValidationError(self.error_messages['invalid'], code='invalid')

        def widget_attrs(self, widget):
            attrs = super().widget_attrs(widget)
            if isinstance(widget, NumberInput) and 'step' not in widget.attrs:
                attrs.setdefault('step', 'any')
            return attrs

    class DecimalField(IntegerField):
        default_error_messages = {
            'invalid': _('Enter a number.'),
        }

        def __init__(self, *, max_value=None, min_value=None, max_digits=None, decimal_places=None, **kwargs):
            self.max_digits, self.decimal_places = max_digits, decimal_places
            super().__init__(max_value=max_value, min_value=min_value, **kwargs)
            self.validators.append(validators.DecimalValidator(max_digits, decimal_places))

        def to_python(self, value):
            """
            Validate that the input is a decimal number. Return a Decimal
            instance or None for empty values. Ensure that there are no more
            than max_digits in the number and no more than decimal_places digits
            after the decimal point.
            """
            if value in self.empty_values:
                return None
            if self.localize:
                value = formats.sanitize_separators(value)
            value = str(value).strip()
            try:
                value = Decimal(value)
            except DecimalException:
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            return value

        def validate(self, value):
            super().validate(value)
            if value in self.empty_values:
                return
            if not value.is_finite():
                raise ValidationError(self.error_messages['invalid'], code='invalid')

        def widget_attrs(self, widget):
            attrs = super().widget_attrs(widget)
            if isinstance(widget, NumberInput) and 'step' not in widget.attrs:
                if self.decimal_places is not None:
                    # Use exponential notation for small values since they might
                    # be parsed as 0 otherwise. ref #20765
                    step = str(Decimal(1).scaleb(-self.decimal_places)).lower()
                else:
                    step = 'any'
                attrs.setdefault('step', step)
            return attrs

    class BaseTemporalField(Field):

        def __init__(self, *, input_formats=None, **kwargs):
            super().__init__(**kwargs)
            if input_formats is not None:
                self.input_formats = input_formats

        def to_python(self, value):
            value = value.strip()
            # Try to strptime against each input format.
            for format in self.input_formats:
                try:
                    return self.strptime(value, format)
                except (ValueError, TypeError):
                    continue
            raise ValidationError(self.error_messages['invalid'], code='invalid')

        def strptime(self, value, format):
            raise NotImplementedError('Subclasses must define this method.')

    class DateField(BaseTemporalField):
        widget = DateInput
        input_formats = formats.get_format_lazy('DATE_INPUT_FORMATS')
        default_error_messages = {
            'invalid': _('Enter a valid date.'),
        }

        def to_python(self, value):
            """
            Validate that the input can be converted to a date. Return a Python
            datetime.date object.
            """
            if value in self.empty_values:
                return None
            if isinstance(value, datetime.datetime):
                return value.date()
            if isinstance(value, datetime.date):
                return value
            return super().to_python(value)

        def strptime(self, value, format):
            return datetime.datetime.strptime(value, format).date()

    class TimeField(BaseTemporalField):
        widget = TimeInput
        input_formats = formats.get_format_lazy('TIME_INPUT_FORMATS')
        default_error_messages = {
            'invalid': _('Enter a valid time.')
        }

        def to_python(self, value):
            """
            Validate that the input can be converted to a time. Return a Python
            datetime.time object.
            """
            if value in self.empty_values:
                return None
            if isinstance(value, datetime.time):
                return value
            return super().to_python(value)

        def strptime(self, value, format):
            return datetime.datetime.strptime(value, format).time()

    class DateTimeField(BaseTemporalField):
        widget = DateTimeInput
        input_formats = formats.get_format_lazy('DATETIME_INPUT_FORMATS')
        default_error_messages = {
            'invalid': _('Enter a valid date/time.'),
        }

        def prepare_value(self, value):
            if isinstance(value, datetime.datetime):
                value = to_current_timezone(value)
            return value

        def to_python(self, value):
            """
            Validate that the input can be converted to a datetime. Return a
            Python datetime.datetime object.
            """
            if value in self.empty_values:
                return None
            if isinstance(value, datetime.datetime):
                return from_current_timezone(value)
            if isinstance(value, datetime.date):
                result = datetime.datetime(value.year, value.month, value.day)
                return from_current_timezone(result)
            result = super().to_python(value)
            return from_current_timezone(result)

        def strptime(self, value, format):
            return datetime.datetime.strptime(value, format)

    class DurationField(Field):
        default_error_messages = {
            'invalid': _('Enter a valid duration.'),
            'overflow': _('The number of days must be between {min_days} and {max_days}.')
        }

        def prepare_value(self, value):
            if isinstance(value, datetime.timedelta):
                return duration_string(value)
            return value

        def to_python(self, value):
            if value in self.empty_values:
                return None
            if isinstance(value, datetime.timedelta):
                return value
            try:
                value = parse_duration(str(value))
            except OverflowError:
                raise ValidationError(self.error_messages['overflow'].format(
                    min_days=datetime.timedelta.min.days,
                    max_days=datetime.timedelta.max.days,
                ), code='overflow')
            if value is None:
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            return value

    class RegexField(CharField):
        def __init__(self, regex, **kwargs):
            """
            regex can be either a string or a compiled regular expression object.
            """
            kwargs.setdefault('strip', False)
            super().__init__(**kwargs)
            self._set_regex(regex)

        def _get_regex(self):
            return self._regex

        def _set_regex(self, regex):
            if isinstance(regex, str):
                regex = re.compile(regex)
            self._regex = regex
            if hasattr(self, '_regex_validator') and self._regex_validator in self.validators:
                self.validators.remove(self._regex_validator)
            self._regex_validator = validators.RegexValidator(regex=regex)
            self.validators.append(self._regex_validator)

        regex = property(_get_regex, _set_regex)

    class EmailField(CharField):
        widget = EmailInput
        default_validators = [validators.validate_email]

        def __init__(self, **kwargs):
            super().__init__(strip=True, **kwargs)

    class FileField(Field):
        widget = ClearableFileInput
        default_error_messages = {
            'invalid': _("No file was submitted. Check the encoding type on the form."),
            'missing': _("No file was submitted."),
            'empty': _("The submitted file is empty."),
            'max_length': ngettext_lazy(
                'Ensure this filename has at most %(max)d character (it has %(length)d).',
                'Ensure this filename has at most %(max)d characters (it has %(length)d).',
                'max'),
            'contradiction': _('Please either submit a file or check the clear checkbox, not both.')
        }

        def __init__(self, *, max_length=None, allow_empty_file=False, **kwargs):
            self.max_length = max_length
            self.allow_empty_file = allow_empty_file
            super().__init__(**kwargs)

        def to_python(self, data):
            if data in self.empty_values:
                return None

            # UploadedFile objects should have name and size attributes.
            try:
                file_name = data.name
                file_size = data.size
            except AttributeError:
                raise ValidationError(self.error_messages['invalid'], code='invalid')

            if self.max_length is not None and len(file_name) > self.max_length:
                params = {'max': self.max_length, 'length': len(file_name)}
                raise ValidationError(self.error_messages['max_length'], code='max_length', params=params)
            if not file_name:
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            if not self.allow_empty_file and not file_size:
                raise ValidationError(self.error_messages['empty'], code='empty')

            return data

        def clean(self, data, initial=None):
            # If the widget got contradictory inputs, we raise a validation error
            if data is FILE_INPUT_CONTRADICTION:
                raise ValidationError(self.error_messages['contradiction'], code='contradiction')
            # False means the field value should be cleared; further validation is
            # not needed.
            if data is False:
                if not self.required:
                    return False
                # If the field is required, clearing is not possible (the widget
                # shouldn't return False data in that case anyway). False is not
                # in self.empty_value; if a False value makes it this far
                # it should be validated from here on out as None (so it will be
                # caught by the required check).
                data = None
            if not data and initial:
                return initial
            return super().clean(data)

        def bound_data(self, data, initial):
            if data in (None, FILE_INPUT_CONTRADICTION):
                return initial
            return data

        def has_changed(self, initial, data):
            return not self.disabled and data is not None

    class ImageField(FileField):
        default_validators = [validators.validate_image_file_extension]
        default_error_messages = {
            'invalid_image': _(
                "Upload a valid image. The file you uploaded was either not an "
                "image or a corrupted image."
            ),
        }

        def to_python(self, data):
            """
            Check that the file-upload field data contains a valid image (GIF, JPG,
            PNG, etc. -- whatever Pillow supports).
            """
            f = super().to_python(data)
            if f is None:
                return None

            from PIL import Image

            # We need to get a file object for Pillow. We might have a path or we might
            # have to read the data into memory.
            if hasattr(data, 'temporary_file_path'):
                file = data.temporary_file_path()
            else:
                if hasattr(data, 'read'):
                    file = BytesIO(data.read())
                else:
                    file = BytesIO(data['content'])

            try:
                # load() could spot a truncated JPEG, but it loads the entire
                # image in memory, which is a DoS vector. See #3848 and #18520.
                image = Image.open(file)
                # verify() must be called immediately after the constructor.
                image.verify()

                # Annotating so subclasses can reuse it for their own validation
                f.image = image
                # Pillow doesn't detect the MIME type of all formats. In those
                # cases, content_type will be None.
                f.content_type = Image.MIME.get(image.format)
            except Exception as exc:
                # Pillow doesn't recognize it as an image.
                raise ValidationError(
                    self.error_messages['invalid_image'],
                    code='invalid_image',
                ) from exc
            if hasattr(f, 'seek') and callable(f.seek):
                f.seek(0)
            return f

        def widget_attrs(self, widget):
            attrs = super().widget_attrs(widget)
            if isinstance(widget, FileInput) and 'accept' not in widget.attrs:
                attrs.setdefault('accept', 'image/*')
            return attrs

    class URLField(CharField):
        widget = URLInput
        default_error_messages = {
            'invalid': _('Enter a valid URL.'),
        }
        default_validators = [validators.URLValidator()]

        def __init__(self, **kwargs):
            super().__init__(strip=True, **kwargs)

        def to_python(self, value):

            def split_url(url):
                """
                Return a list of url parts via urlparse.urlsplit(), or raise
                ValidationError for some malformed URLs.
                """
                try:
                    return list(urlsplit(url))
                except ValueError:
                    # urlparse.urlsplit can raise a ValueError with some
                    # misformatted URLs.
                    raise ValidationError(self.error_messages['invalid'], code='invalid')

            value = super().to_python(value)
            if value:
                url_fields = split_url(value)
                if not url_fields[0]:
                    # If no URL scheme given, assume http://
                    url_fields[0] = 'http'
                if not url_fields[1]:
                    # Assume that if no domain is provided, that the path segment
                    # contains the domain.
                    url_fields[1] = url_fields[2]
                    url_fields[2] = ''
                    # Rebuild the url_fields list, since the domain segment may now
                    # contain the path too.
                    url_fields = split_url(urlunsplit(url_fields))
                value = urlunsplit(url_fields)
            return value

    class BooleanField(Field):
        widget = CheckboxInput

        def to_python(self, value):
            """Return a Python boolean object."""
            # Explicitly check for the string 'False', which is what a hidden field
            # will submit for False. Also check for '0', since this is what
            # RadioSelect will provide. Because bool("True") == bool('1') == True,
            # we don't need to handle that explicitly.
            if isinstance(value, str) and value.lower() in ('false', '0'):
                value = False
            else:
                value = bool(value)
            return super().to_python(value)

        def validate(self, value):
            if not value and self.required:
                raise ValidationError(self.error_messages['required'], code='required')

        def has_changed(self, initial, data):
            if self.disabled:
                return False
            # Sometimes data or initial may be a string equivalent of a boolean
            # so we should run it through to_python first to get a boolean value
            return self.to_python(initial) != self.to_python(data)

    class NullBooleanField(BooleanField):
        """
        A field whose valid values are None, True, and False. Clean invalid values
        to None.
        """
        widget = NullBooleanSelect

        def to_python(self, value):
            """
            Explicitly check for the string 'True' and 'False', which is what a
            hidden field will submit for True and False, for 'true' and 'false',
            which are likely to be returned by JavaScript serializations of forms,
            and for '1' and '0', which is what a RadioField will submit. Unlike
            the Booleanfield, this field must check for True because it doesn't
            use the bool() function.
            """
            if value in (True, 'True', 'true', '1'):
                return True
            elif value in (False, 'False', 'false', '0'):
                return False
            else:
                return None

        def validate(self, value):
            pass

    class CallableChoiceIterator:
        def __init__(self, choices_func):
            self.choices_func = choices_func

        def __iter__(self):
            yield from self.choices_func()

    class ChoiceField(Field):
        widget = Select
        default_error_messages = {
            'invalid_choice': _('Select a valid choice. %(value)s is not one of the available choices.'),
        }

        def __init__(self, *, choices=(), **kwargs):
            super().__init__(**kwargs)
            self.choices = choices

        def __deepcopy__(self, memo):
            result = super().__deepcopy__(memo)
            result._choices = copy.deepcopy(self._choices, memo)
            return result

        def _get_choices(self):
            return self._choices

        def _set_choices(self, value):
            # Setting choices also sets the choices on the widget.
            # choices can be any iterable, but we call list() on it because
            # it will be consumed more than once.
            if callable(value):
                value = CallableChoiceIterator(value)
            else:
                value = list(value)

            self._choices = self.widget.choices = value

        choices = property(_get_choices, _set_choices)

        def to_python(self, value):
            """Return a string."""
            if value in self.empty_values:
                return ''
            return str(value)

        def validate(self, value):
            """Validate that the input is in self.choices."""
            super().validate(value)
            if value and not self.valid_value(value):
                raise ValidationError(
                    self.error_messages['invalid_choice'],
                    code='invalid_choice',
                    params={'value': value},
                )

        def valid_value(self, value):
            """Check to see if the provided value is a valid choice."""
            text_value = str(value)
            for k, v in self.choices:
                if isinstance(v, (list, tuple)):
                    # This is an optgroup, so look inside the group for options
                    for k2, v2 in v:
                        if value == k2 or text_value == str(k2):
                            return True
                else:
                    if value == k or text_value == str(k):
                        return True
            return False

    class TypedChoiceField(ChoiceField):
        def __init__(self, *, coerce=lambda val: val, empty_value='', **kwargs):
            self.coerce = coerce
            self.empty_value = empty_value
            super().__init__(**kwargs)

        def _coerce(self, value):
            """
            Validate that the value can be coerced to the right type (if not empty).
            """
            if value == self.empty_value or value in self.empty_values:
                return self.empty_value
            try:
                value = self.coerce(value)
            except (ValueError, TypeError, ValidationError):
                raise ValidationError(
                    self.error_messages['invalid_choice'],
                    code='invalid_choice',
                    params={'value': value},
                )
            return value

        def clean(self, value):
            value = super().clean(value)
            return self._coerce(value)

    class MultipleChoiceField(ChoiceField):
        hidden_widget = MultipleHiddenInput
        widget = SelectMultiple
        default_error_messages = {
            'invalid_choice': _('Select a valid choice. %(value)s is not one of the available choices.'),
            'invalid_list': _('Enter a list of values.'),
        }

        def to_python(self, value):
            if not value:
                return []
            elif not isinstance(value, (list, tuple)):
                raise ValidationError(self.error_messages['invalid_list'], code='invalid_list')
            return [str(val) for val in value]

        def validate(self, value):
            """Validate that the input is a list or tuple."""
            if self.required and not value:
                raise ValidationError(self.error_messages['required'], code='required')
            # Validate that each value in the value list is in self.choices.
            for val in value:
                if not self.valid_value(val):
                    raise ValidationError(
                        self.error_messages['invalid_choice'],
                        code='invalid_choice',
                        params={'value': val},
                    )

        def has_changed(self, initial, data):
            if self.disabled:
                return False
            if initial is None:
                initial = []
            if data is None:
                data = []
            if len(initial) != len(data):
                return True
            initial_set = {str(value) for value in initial}
            data_set = {str(value) for value in data}
            return data_set != initial_set

    class TypedMultipleChoiceField(MultipleChoiceField):
        def __init__(self, *, coerce=lambda val: val, **kwargs):
            self.coerce = coerce
            self.empty_value = kwargs.pop('empty_value', [])
            super().__init__(**kwargs)

        def _coerce(self, value):
            """
            Validate that the values are in self.choices and can be coerced to the
            right type.
            """
            if value == self.empty_value or value in self.empty_values:
                return self.empty_value
            new_value = []
            for choice in value:
                try:
                    new_value.append(self.coerce(choice))
                except (ValueError, TypeError, ValidationError):
                    raise ValidationError(
                        self.error_messages['invalid_choice'],
                        code='invalid_choice',
                        params={'value': choice},
                    )
            return new_value

        def clean(self, value):
            value = super().clean(value)
            return self._coerce(value)

        def validate(self, value):
            if value != self.empty_value:
                super().validate(value)
            elif self.required:
                raise ValidationError(self.error_messages['required'], code='required')

    class ComboField(Field):
        """
        A Field whose clean() method calls multiple Field clean() methods.
        """
        def __init__(self, fields, **kwargs):
            super().__init__(**kwargs)
            # Set 'required' to False on the individual fields, because the
            # required validation will be handled by ComboField, not by those
            # individual fields.
            for f in fields:
                f.required = False
            self.fields = fields

        def clean(self, value):
            """
            Validate the given value against all of self.fields, which is a
            list of Field instances.
            """
            super().clean(value)
            for field in self.fields:
                value = field.clean(value)
            return value

    class MultiValueField(Field):
        """
        Aggregate the logic of multiple Fields.

        Its clean() method takes a "decompressed" list of values, which are then
        cleaned into a single value according to self.fields. Each value in
        this list is cleaned by the corresponding field -- the first value is
        cleaned by the first field, the second value is cleaned by the second
        field, etc. Once all fields are cleaned, the list of clean values is
        "compressed" into a single value.

        Subclasses should not have to implement clean(). Instead, they must
        implement compress(), which takes a list of valid values and returns a
        "compressed" version of those values -- a single value.

        You'll probably want to use this with MultiWidget.
        """
        default_error_messages = {
            'invalid': _('Enter a list of values.'),
            'incomplete': _('Enter a complete value.'),
        }

        def __init__(self, fields, *, require_all_fields=True, **kwargs):
            self.require_all_fields = require_all_fields
            super().__init__(**kwargs)
            for f in fields:
                f.error_messages.setdefault('incomplete',
                                            self.error_messages['incomplete'])
                if self.disabled:
                    f.disabled = True
                if self.require_all_fields:
                    # Set 'required' to False on the individual fields, because the
                    # required validation will be handled by MultiValueField, not
                    # by those individual fields.
                    f.required = False
            self.fields = fields

        def __deepcopy__(self, memo):
            result = super().__deepcopy__(memo)
            result.fields = tuple(x.__deepcopy__(memo) for x in self.fields)
            return result

        def validate(self, value):
            pass

        def clean(self, value):
            """
            Validate every value in the given list. A value is validated against
            the corresponding Field in self.fields.

            For example, if this MultiValueField was instantiated with
            fields=(DateField(), TimeField()), clean() would call
            DateField.clean(value[0]) and TimeField.clean(value[1]).
            """
            clean_data = []
            errors = []
            if self.disabled and not isinstance(value, list):
                value = self.widget.decompress(value)
            if not value or isinstance(value, (list, tuple)):
                if not value or not [v for v in value if v not in self.empty_values]:
                    if self.required:
                        raise ValidationError(self.error_messages['required'], code='required')
                    else:
                        return self.compress([])
            else:
                raise ValidationError(self.error_messages['invalid'], code='invalid')
            for i, field in enumerate(self.fields):
                try:
                    field_value = value[i]
                except IndexError:
                    field_value = None
                if field_value in self.empty_values:
                    if self.require_all_fields:
                        # Raise a 'required' error if the MultiValueField is
                        # required and any field is empty.
                        if self.required:
                            raise ValidationError(self.error_messages['required'], code='required')
                    elif field.required:
                        # Otherwise, add an 'incomplete' error to the list of
                        # collected errors and skip field cleaning, if a required
                        # field is empty.
                        if field.error_messages['incomplete'] not in errors:
                            errors.append(field.error_messages['incomplete'])
                        continue
                try:
                    clean_data.append(field.clean(field_value))
                except ValidationError as e:
                    # Collect all validation errors in a single list, which we'll
                    # raise at the end of clean(), rather than raising a single
                    # exception for the first error we encounter. Skip duplicates.
                    errors.extend(m for m in e.error_list if m not in errors)
            if errors:
                raise ValidationError(errors)

            out = self.compress(clean_data)
            self.validate(out)
            self.run_validators(out)
            return out

        def compress(self, data_list):
            """
            Return a single value for the given list of values. The values can be
            assumed to be valid.

            For example, if this MultiValueField was instantiated with
            fields=(DateField(), TimeField()), this might return a datetime
            object created by combining the date and time in data_list.
            """
            raise NotImplementedError('Subclasses must implement this method.')

        def has_changed(self, initial, data):
            if self.disabled:
                return False
            if initial is None:
                initial = ['' for x in range(0, len(data))]
            else:
                if not isinstance(initial, list):
                    initial = self.widget.decompress(initial)
            for field, initial, data in zip(self.fields, initial, data):
                try:
                    initial = field.to_python(initial)
                except ValidationError:
                    return True
                if field.has_changed(initial, data):
                    return True
            return False

    class FilePathField(ChoiceField):
        def __init__(self, path, *, match=None, recursive=False, allow_files=True,
                     allow_folders=False, **kwargs):
            self.path, self.match, self.recursive = path, match, recursive
            self.allow_files, self.allow_folders = allow_files, allow_folders
            super().__init__(choices=(), **kwargs)

            if self.required:
                self.choices = []
            else:
                self.choices = [("", "---------")]

            if self.match is not None:
                self.match_re = re.compile(self.match)

            if recursive:
                for root, dirs, files in sorted(os.walk(self.path)):
                    if self.allow_files:
                        for f in sorted(files):
                            if self.match is None or self.match_re.search(f):
                                f = os.path.join(root, f)
                                self.choices.append((f, f.replace(path, "", 1)))
                    if self.allow_folders:
                        for f in sorted(dirs):
                            if f == '__pycache__':
                                continue
                            if self.match is None or self.match_re.search(f):
                                f = os.path.join(root, f)
                                self.choices.append((f, f.replace(path, "", 1)))
            else:
                choices = []
                for f in os.scandir(self.path):
                    if f.name == '__pycache__':
                        continue
                    if (((self.allow_files and f.is_file()) or
                            (self.allow_folders and f.is_dir())) and
                            (self.match is None or self.match_re.search(f.name))):
                        choices.append((f.path, f.name))
                choices.sort(key=operator.itemgetter(1))
                self.choices.extend(choices)

            self.widget.choices = self.choices

    class SplitDateTimeField(MultiValueField):
        widget = SplitDateTimeWidget
        hidden_widget = SplitHiddenDateTimeWidget
        default_error_messages = {
            'invalid_date': _('Enter a valid date.'),
            'invalid_time': _('Enter a valid time.'),
        }

        def __init__(self, *, input_date_formats=None, input_time_formats=None, **kwargs):
            errors = self.default_error_messages.copy()
            if 'error_messages' in kwargs:
                errors.update(kwargs['error_messages'])
            localize = kwargs.get('localize', False)
            fields = (
                DateField(input_formats=input_date_formats,
                          error_messages={'invalid': errors['invalid_date']},
                          localize=localize),
                TimeField(input_formats=input_time_formats,
                          error_messages={'invalid': errors['invalid_time']},
                          localize=localize),
            )
            super().__init__(fields, **kwargs)

        def compress(self, data_list):
            if data_list:
                # Raise a validation error if time or date is empty
                # (possible if SplitDateTimeField has required=False).
                if data_list[0] in self.empty_values:
                    raise ValidationError(self.error_messages['invalid_date'], code='invalid_date')
                if data_list[1] in self.empty_values:
                    raise ValidationError(self.error_messages['invalid_time'], code='invalid_time')
                result = datetime.datetime.combine(*data_list)
                return from_current_timezone(result)
            return None

    class GenericIPAddressField(CharField):
        def __init__(self, *, protocol='both', unpack_ipv4=False, **kwargs):
            self.unpack_ipv4 = unpack_ipv4
            self.default_validators = validators.ip_address_validators(protocol, unpack_ipv4)[0]
            super().__init__(**kwargs)

        def to_python(self, value):
            if value in self.empty_values:
                return ''
            value = value.strip()
            if value and ':' in value:
                return clean_ipv6_address(value, self.unpack_ipv4)
            return value

    class SlugField(CharField):
        default_validators = [validators.validate_slug]

        def __init__(self, *, allow_unicode=False, **kwargs):
            self.allow_unicode = allow_unicode
            if self.allow_unicode:
                self.default_validators = [validators.validate_unicode_slug]
            super().__init__(**kwargs)

    class UUIDField(CharField):
        default_error_messages = {
            'invalid': _('Enter a valid UUID.'),
        }

        def prepare_value(self, value):
            if isinstance(value, uuid.UUID):
                return str(value)
            return value

        def to_python(self, value):
            value = super().to_python(value)
            if value in self.empty_values:
                return None
            if not isinstance(value, uuid.UUID):
                try:
                    value = uuid.UUID(value)
                except ValueError:
                    raise ValidationError(self.error_messages['invalid'], code='invalid')
            return value