django-rest-framework/rest_framework/fields.py

from django import forms
from django.conf import settings
from django.core import validators
from django.core.exceptions import ValidationError
from django.utils import timezone
from django.utils.datastructures import SortedDict
from django.utils.dateparse import parse_date, parse_datetime, parse_time
from django.utils.encoding import is_protected_type
from django.utils.translation import ugettext_lazy as _
from rest_framework import ISO_8601
from rest_framework.compat import smart_text, EmailValidator, MinValueValidator, MaxValueValidator, URLValidator
from rest_framework.settings import api_settings
from rest_framework.utils import html, representation, humanize_datetime
import copy
import datetime
import decimal
import inspect
import re


class empty:
    """
    This class is used to represent no data being provided for a given input
    or output value.

    It is required because `None` may be a valid input or output value.
    """
    pass


def is_simple_callable(obj):
    """
    True if the object is a callable that takes no arguments.
    """
    function = inspect.isfunction(obj)
    method = inspect.ismethod(obj)

    if not (function or method):
        return False

    args, _, _, defaults = inspect.getargspec(obj)
    len_args = len(args) if function else len(args) - 1
    len_defaults = len(defaults) if defaults else 0
    return len_args <= len_defaults


def get_attribute(instance, attrs):
    """
    Similar to Python's built in `getattr(instance, attr)`,
    but takes a list of nested attributes, instead of a single attribute.

    Also accepts either attribute lookup on objects or dictionary lookups.
    """
    for attr in attrs:
        try:
            instance = getattr(instance, attr)
        except AttributeError as exc:
            try:
                return instance[attr]
            except (KeyError, TypeError, AttributeError):
                raise exc
    return instance


def set_value(dictionary, keys, value):
    """
    Similar to Python's built in `dictionary[key] = value`,
    but takes a list of nested keys instead of a single key.

    set_value({'a': 1}, [], {'b': 2}) -> {'a': 1, 'b': 2}
    set_value({'a': 1}, ['x'], 2) -> {'a': 1, 'x': 2}
    set_value({'a': 1}, ['x', 'y'], 2) -> {'a': 1, 'x': {'y': 2}}
    """
    if not keys:
        dictionary.update(value)
        return

    for key in keys[:-1]:
        if key not in dictionary:
            dictionary[key] = {}
        dictionary = dictionary[key]

    dictionary[keys[-1]] = value


class SkipField(Exception):
    pass


NOT_READ_ONLY_WRITE_ONLY = 'May not set both `read_only` and `write_only`'
NOT_READ_ONLY_REQUIRED = 'May not set both `read_only` and `required`'
NOT_READ_ONLY_DEFAULT = 'May not set both `read_only` and `default`'
NOT_REQUIRED_DEFAULT = 'May not set both `required` and `default`'
USE_READONLYFIELD = 'Field(read_only=True) should be ReadOnlyField'
MISSING_ERROR_MESSAGE = (
    'ValidationError raised by `{class_name}`, but error key `{key}` does '
    'not exist in the `error_messages` dictionary.'
)


class Field(object):
    _creation_counter = 0

    default_error_messages = {
        'required': _('This field is required.'),
        'null': _('This field may not be null.')
    }
    default_validators = []
    default_empty_html = None
    initial = None

    def __init__(self, read_only=False, write_only=False,
                 required=None, default=empty, initial=empty, source=None,
                 label=None, help_text=None, style=None,
                 error_messages=None, validators=[], allow_null=False):
        self._creation_counter = Field._creation_counter
        Field._creation_counter += 1

        # If `required` is unset, then use `True` unless a default is provided.
        if required is None:
            required = default is empty and not read_only

        # Some combinations of keyword arguments do not make sense.
        assert not (read_only and write_only), NOT_READ_ONLY_WRITE_ONLY
        assert not (read_only and required), NOT_READ_ONLY_REQUIRED
        assert not (read_only and default is not empty), NOT_READ_ONLY_DEFAULT
        assert not (required and default is not empty), NOT_REQUIRED_DEFAULT
        assert not (read_only and self.__class__ == Field), USE_READONLYFIELD

        self.read_only = read_only
        self.write_only = write_only
        self.required = required
        self.default = default
        self.source = source
        self.initial = self.initial if (initial is empty) else initial
        self.label = label
        self.help_text = help_text
        self.style = {} if style is None else style
        self.validators = validators or self.default_validators[:]
        self.allow_null = allow_null

        # These are set up by `.bind()` when the field is added to a serializer.
        self.field_name = None
        self.parent = None

        # Collect default error message from self and parent classes
        messages = {}
        for cls in reversed(self.__class__.__mro__):
            messages.update(getattr(cls, 'default_error_messages', {}))
        messages.update(error_messages or {})
        self.error_messages = messages

    def bind(self, field_name, parent):
        """
        Initializes the field name and parent for the field instance.
        Called when a field is added to the parent serializer instance.
        """

        # In order to enforce a consistent style, we error if a redundant
        # 'source' argument has been used. For example:
        # my_field = serializer.CharField(source='my_field')
        assert self.source != field_name, (
            "It is redundant to specify `source='%s'` on field '%s' in "
            "serializer '%s', because it is the same as the field name. "
            "Remove the `source` keyword argument." %
            (field_name, self.__class__.__name__, parent.__class__.__name__)
        )

        self.field_name = field_name
        self.parent = parent

        # `self.label` should default to being based on the field name.
        if self.label is None:
            self.label = field_name.replace('_', ' ').capitalize()

        # self.source should default to being the same as the field name.
        if self.source is None:
            self.source = field_name

        # self.source_attrs is a list of attributes that need to be looked up
        # when serializing the instance, or populating the validated data.
        if self.source == '*':
            self.source_attrs = []
        else:
            self.source_attrs = self.source.split('.')

    def get_initial(self):
        """
        Return a value to use when the field is being returned as a primative
        value, without any object instance.
        """
        return self.initial

    def get_value(self, dictionary):
        """
        Given the *incoming* primative data, return the value for this field
        that should be validated and transformed to a native value.
        """
        if html.is_html_input(dictionary):
            # HTML forms will represent empty fields as '', and cannot
            # represent None or False values directly.
            ret = dictionary.get(self.field_name, '')
            return self.default_empty_html if (ret == '') else ret
        return dictionary.get(self.field_name, empty)

    def get_field_representation(self, instance):
        """
        Given the outgoing object instance, return the primative value
        that should be used for this field.
        """
        attribute = get_attribute(instance, self.source_attrs)
        if attribute is None:
            return None
        return self.to_representation(attribute)

    def get_default(self):
        """
        Return the default value to use when validating data if no input
        is provided for this field.

        If a default has not been set for this field then this will simply
        return `empty`, indicating that no value should be set in the
        validated data for this field.
        """
        if self.default is empty:
            raise SkipField()
        return self.default

    def run_validation(self, data=empty):
        """
        Validate a simple representation and return the internal value.

        The provided data may be `empty` if no representation was included
        in the input.

        May raise `SkipField` if the field should not be included in the
        validated data.
        """
        if data is empty:
            if getattr(self.root, 'partial', False):
                raise SkipField()
            if self.required:
                self.fail('required')
            return self.get_default()

        if data is None:
            if not self.allow_null:
                self.fail('null')
            return None

        value = self.to_internal_value(data)
        self.run_validators(value)
        self.validate(value)
        return value

    def run_validators(self, value):
        """
        Test the given value against all the validators on the field,
        and either raise a `ValidationError` or simply return.
        """
        errors = []
        for validator in self.validators:
            if getattr(validator, 'requires_context', False):
                validator.serializer_field = self
            try:
                validator(value)
            except ValidationError as exc:
                errors.extend(exc.messages)
        if errors:
            raise ValidationError(errors)

    def validate(self, value):
        pass

    def to_internal_value(self, data):
        """
        Transform the *incoming* primative data into a native value.
        """
        raise NotImplementedError('to_internal_value() must be implemented.')

    def to_representation(self, value):
        """
        Transform the *outgoing* native value into primative data.
        """
        raise NotImplementedError('to_representation() must be implemented.')

    def fail(self, key, **kwargs):
        """
        A helper method that simply raises a validation error.
        """
        try:
            msg = self.error_messages[key]
        except KeyError:
            class_name = self.__class__.__name__
            msg = MISSING_ERROR_MESSAGE.format(class_name=class_name, key=key)
            raise AssertionError(msg)
        raise ValidationError(msg.format(**kwargs))

    @property
    def root(self):
        """
        Returns the top-level serializer for this field.
        """
        root = self
        while root.parent is not None:
            root = root.parent
        return root

    @property
    def context(self):
        """
        Returns the context as passed to the root serializer on initialization.
        """
        return getattr(self.root, '_context', {})

    def __new__(cls, *args, **kwargs):
        """
        When a field is instantiated, we store the arguments that were used,
        so that we can present a helpful representation of the object.
        """
        instance = super(Field, cls).__new__(cls)
        instance._args = args
        instance._kwargs = kwargs
        return instance

    def __deepcopy__(self, memo):
        """
        When cloning fields we instantiate using the arguments it was
        originally created with, rather than copying the complete state.
        """
        args = copy.deepcopy(self._args)
        kwargs = copy.deepcopy(self._kwargs)
        return self.__class__(*args, **kwargs)

    def __repr__(self):
        """
        Fields are represented using their initial calling arguments.
        This allows us to create descriptive representations for serializer
        instances that show all the declared fields on the serializer.
        """
        return representation.field_repr(self)


# Boolean types...

class BooleanField(Field):
    default_error_messages = {
        'invalid': _('`{input}` is not a valid boolean.')
    }
    default_empty_html = False
    initial = False
    TRUE_VALUES = set(('t', 'T', 'true', 'True', 'TRUE', '1', 1, True))
    FALSE_VALUES = set(('f', 'F', 'false', 'False', 'FALSE', '0', 0, 0.0, False))

    def __init__(self, **kwargs):
        assert 'allow_null' not in kwargs, '`allow_null` is not a valid option. Use `NullBooleanField` instead.'
        super(BooleanField, self).__init__(**kwargs)

    def to_internal_value(self, data):
        if data in self.TRUE_VALUES:
            return True
        elif data in self.FALSE_VALUES:
            return False
        self.fail('invalid', input=data)

    def to_representation(self, value):
        if value in self.TRUE_VALUES:
            return True
        elif value in self.FALSE_VALUES:
            return False
        return bool(value)


class NullBooleanField(Field):
    default_error_messages = {
        'invalid': _('`{input}` is not a valid boolean.')
    }
    default_empty_html = None
    initial = None
    TRUE_VALUES = set(('t', 'T', 'true', 'True', 'TRUE', '1', 1, True))
    FALSE_VALUES = set(('f', 'F', 'false', 'False', 'FALSE', '0', 0, 0.0, False))
    NULL_VALUES = set(('n', 'N', 'null', 'Null', 'NULL', '', None))

    def __init__(self, **kwargs):
        assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
        kwargs['allow_null'] = True
        super(NullBooleanField, self).__init__(**kwargs)

    def to_internal_value(self, data):
        if data in self.TRUE_VALUES:
            return True
        elif data in self.FALSE_VALUES:
            return False
        elif data in self.NULL_VALUES:
            return None
        self.fail('invalid', input=data)

    def to_representation(self, value):
        if value in self.NULL_VALUES:
            return None
        if value in self.TRUE_VALUES:
            return True
        elif value in self.FALSE_VALUES:
            return False
        return bool(value)


# String types...

class CharField(Field):
    default_error_messages = {
        'blank': _('This field may not be blank.')
    }
    default_empty_html = ''
    initial = ''

    def __init__(self, **kwargs):
        self.allow_blank = kwargs.pop('allow_blank', False)
        self.max_length = kwargs.pop('max_length', None)
        self.min_length = kwargs.pop('min_length', None)
        super(CharField, self).__init__(**kwargs)

    def run_validation(self, data=empty):
        # Test for the empty string here so that it does not get validated,
        # and so that subclasses do not need to handle it explicitly
        # inside the `to_internal_value()` method.
        if data == '':
            if not self.allow_blank:
                self.fail('blank')
            return ''
        return super(CharField, self).run_validation(data)

    def to_internal_value(self, data):
        return str(data)

    def to_representation(self, value):
        return str(value)


class EmailField(CharField):
    default_error_messages = {
        'invalid': _('Enter a valid email address.')
    }

    def __init__(self, **kwargs):
        super(EmailField, self).__init__(**kwargs)
        validator = EmailValidator(message=self.error_messages['invalid'])
        self.validators.append(validator)

    def to_internal_value(self, data):
        return str(data).strip()

    def to_representation(self, value):
        return str(value).strip()


class RegexField(CharField):
    default_error_messages = {
        'invalid': _('This value does not match the required pattern.')
    }

    def __init__(self, regex, **kwargs):
        super(RegexField, self).__init__(**kwargs)
        validator = validators.RegexValidator(regex, message=self.error_messages['invalid'])
        self.validators.append(validator)


class SlugField(CharField):
    default_error_messages = {
        'invalid': _("Enter a valid 'slug' consisting of letters, numbers, underscores or hyphens.")
    }

    def __init__(self, **kwargs):
        super(SlugField, self).__init__(**kwargs)
        slug_regex = re.compile(r'^[-a-zA-Z0-9_]+$')
        validator = validators.RegexValidator(slug_regex, message=self.error_messages['invalid'])
        self.validators.append(validator)


class URLField(CharField):
    default_error_messages = {
        'invalid': _("Enter a valid URL.")
    }

    def __init__(self, **kwargs):
        super(URLField, self).__init__(**kwargs)
        validator = URLValidator(message=self.error_messages['invalid'])
        self.validators.append(validator)


# Number types...

class IntegerField(Field):
    default_error_messages = {
        'invalid': _('A valid integer is required.'),
        'max_value': _('Ensure this value is less than or equal to {max_value}.'),
        'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
    }

    def __init__(self, **kwargs):
        max_value = kwargs.pop('max_value', None)
        min_value = kwargs.pop('min_value', None)
        super(IntegerField, self).__init__(**kwargs)
        if max_value is not None:
            message = self.error_messages['max_value'].format(max_value=max_value)
            self.validators.append(MaxValueValidator(max_value, message=message))
        if min_value is not None:
            message = self.error_messages['min_value'].format(min_value=min_value)
            self.validators.append(MinValueValidator(min_value, message=message))

    def to_internal_value(self, data):
        try:
            data = int(str(data))
        except (ValueError, TypeError):
            self.fail('invalid')
        return data

    def to_representation(self, value):
        return int(value)


class FloatField(Field):
    default_error_messages = {
        'invalid': _("A valid number is required."),
        'max_value': _('Ensure this value is less than or equal to {max_value}.'),
        'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
    }

    def __init__(self, **kwargs):
        max_value = kwargs.pop('max_value', None)
        min_value = kwargs.pop('min_value', None)
        super(FloatField, self).__init__(**kwargs)
        if max_value is not None:
            message = self.error_messages['max_value'].format(max_value=max_value)
            self.validators.append(MaxValueValidator(max_value, message=message))
        if min_value is not None:
            message = self.error_messages['min_value'].format(min_value=min_value)
            self.validators.append(MinValueValidator(min_value, message=message))

    def to_internal_value(self, value):
        try:
            return float(value)
        except (TypeError, ValueError):
            self.fail('invalid')

    def to_representation(self, value):
        return float(value)


class DecimalField(Field):
    default_error_messages = {
        'invalid': _('A valid number is required.'),
        'max_value': _('Ensure this value is less than or equal to {max_value}.'),
        'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
        'max_digits': _('Ensure that there are no more than {max_digits} digits in total.'),
        'max_decimal_places': _('Ensure that there are no more than {max_decimal_places} decimal places.'),
        'max_whole_digits': _('Ensure that there are no more than {max_whole_digits} digits before the decimal point.')
    }

    coerce_to_string = api_settings.COERCE_DECIMAL_TO_STRING

    def __init__(self, max_digits, decimal_places, coerce_to_string=None, max_value=None, min_value=None, **kwargs):
        self.max_digits = max_digits
        self.decimal_places = decimal_places
        self.coerce_to_string = coerce_to_string if (coerce_to_string is not None) else self.coerce_to_string
        super(DecimalField, self).__init__(**kwargs)
        if max_value is not None:
            message = self.error_messages['max_value'].format(max_value=max_value)
            self.validators.append(MaxValueValidator(max_value, message=message))
        if min_value is not None:
            message = self.error_messages['min_value'].format(min_value=min_value)
            self.validators.append(MinValueValidator(min_value, message=message))

    def to_internal_value(self, value):
        """
        Validates that the input is a decimal number. Returns a Decimal
        instance. Returns None for empty values. Ensures that there are no more
        than max_digits in the number, and no more than decimal_places digits
        after the decimal point.
        """
        value = smart_text(value).strip()
        try:
            value = decimal.Decimal(value)
        except decimal.DecimalException:
            self.fail('invalid')

        # Check for NaN. It is the only value that isn't equal to itself,
        # so we can use this to identify NaN values.
        if value != value:
            self.fail('invalid')

        # Check for infinity and negative infinity.
        if value in (decimal.Decimal('Inf'), decimal.Decimal('-Inf')):
            self.fail('invalid')

        sign, digittuple, exponent = value.as_tuple()
        decimals = abs(exponent)
        # digittuple doesn't include any leading zeros.
        digits = len(digittuple)
        if decimals > digits:
            # We have leading zeros up to or past the decimal point.  Count
            # everything past the decimal point as a digit.  We do not count
            # 0 before the decimal point as a digit since that would mean
            # we would not allow max_digits = decimal_places.
            digits = decimals
        whole_digits = digits - decimals

        if self.max_digits is not None and digits > self.max_digits:
            self.fail('max_digits', max_digits=self.max_digits)
        if self.decimal_places is not None and decimals > self.decimal_places:
            self.fail('max_decimal_places', max_decimal_places=self.decimal_places)
        if self.max_digits is not None and self.decimal_places is not None and whole_digits > (self.max_digits - self.decimal_places):
            self.fail('max_whole_digits', max_whole_digits=self.max_digits - self.decimal_places)

        return value

    def to_representation(self, value):
        if not isinstance(value, decimal.Decimal):
            value = decimal.Decimal(str(value).strip())

        context = decimal.getcontext().copy()
        context.prec = self.max_digits
        quantized = value.quantize(
            decimal.Decimal('.1') ** self.decimal_places,
            context=context
        )
        if not self.coerce_to_string:
            return quantized
        return '{0:f}'.format(quantized)


# Date & time fields...

class DateTimeField(Field):
    default_error_messages = {
        'invalid': _('Datetime has wrong format. Use one of these formats instead: {format}'),
        'date': _('Expected a datetime but got a date.'),
    }
    format = api_settings.DATETIME_FORMAT
    input_formats = api_settings.DATETIME_INPUT_FORMATS
    default_timezone = timezone.get_default_timezone() if settings.USE_TZ else None

    def __init__(self, format=empty, input_formats=None, default_timezone=None, *args, **kwargs):
        self.format = format if format is not empty else self.format
        self.input_formats = input_formats if input_formats is not None else self.input_formats
        self.default_timezone = default_timezone if default_timezone is not None else self.default_timezone
        super(DateTimeField, self).__init__(*args, **kwargs)

    def enforce_timezone(self, value):
        """
        When `self.default_timezone` is `None`, always return naive datetimes.
        When `self.default_timezone` is not `None`, always return aware datetimes.
        """
        if (self.default_timezone is not None) and not timezone.is_aware(value):
            return timezone.make_aware(value, self.default_timezone)
        elif (self.default_timezone is None) and timezone.is_aware(value):
            return timezone.make_naive(value, timezone.UTC())
        return value

    def to_internal_value(self, value):
        if isinstance(value, datetime.date) and not isinstance(value, datetime.datetime):
            self.fail('date')

        if isinstance(value, datetime.datetime):
            return self.enforce_timezone(value)

        for format in self.input_formats:
            if format.lower() == ISO_8601:
                try:
                    parsed = parse_datetime(value)
                except (ValueError, TypeError):
                    pass
                else:
                    if parsed is not None:
                        return self.enforce_timezone(parsed)
            else:
                try:
                    parsed = datetime.datetime.strptime(value, format)
                except (ValueError, TypeError):
                    pass
                else:
                    return self.enforce_timezone(parsed)

        humanized_format = humanize_datetime.datetime_formats(self.input_formats)
        self.fail('invalid', format=humanized_format)

    def to_representation(self, value):
        if self.format is None:
            return value

        if self.format.lower() == ISO_8601:
            ret = value.isoformat()
            if ret.endswith('+00:00'):
                ret = ret[:-6] + 'Z'
            return ret
        return value.strftime(self.format)


class DateField(Field):
    default_error_messages = {
        'invalid': _('Date has wrong format. Use one of these formats instead: {format}'),
        'datetime': _('Expected a date but got a datetime.'),
    }
    format = api_settings.DATE_FORMAT
    input_formats = api_settings.DATE_INPUT_FORMATS

    def __init__(self, format=empty, input_formats=None, *args, **kwargs):
        self.format = format if format is not empty else self.format
        self.input_formats = input_formats if input_formats is not None else self.input_formats
        super(DateField, self).__init__(*args, **kwargs)

    def to_internal_value(self, value):
        if isinstance(value, datetime.datetime):
            self.fail('datetime')

        if isinstance(value, datetime.date):
            return value

        for format in self.input_formats:
            if format.lower() == ISO_8601:
                try:
                    parsed = parse_date(value)
                except (ValueError, TypeError):
                    pass
                else:
                    if parsed is not None:
                        return parsed
            else:
                try:
                    parsed = datetime.datetime.strptime(value, format)
                except (ValueError, TypeError):
                    pass
                else:
                    return parsed.date()

        humanized_format = humanize_datetime.date_formats(self.input_formats)
        self.fail('invalid', format=humanized_format)

    def to_representation(self, value):
        if self.format is None:
            return value

        # Applying a `DateField` to a datetime value is almost always
        # not a sensible thing to do, as it means naively dropping
        # any explicit or implicit timezone info.
        assert not isinstance(value, datetime.datetime), (
            'Expected a `date`, but got a `datetime`. Refusing to coerce, '
            'as this may mean losing timezone information. Use a custom '
            'read-only field and deal with timezone issues explicitly.'
        )

        if self.format.lower() == ISO_8601:
            return value.isoformat()
        return value.strftime(self.format)


class TimeField(Field):
    default_error_messages = {
        'invalid': _('Time has wrong format. Use one of these formats instead: {format}'),
    }
    format = api_settings.TIME_FORMAT
    input_formats = api_settings.TIME_INPUT_FORMATS

    def __init__(self, format=empty, input_formats=None, *args, **kwargs):
        self.format = format if format is not empty else self.format
        self.input_formats = input_formats if input_formats is not None else self.input_formats
        super(TimeField, self).__init__(*args, **kwargs)

    def to_internal_value(self, value):
        if isinstance(value, datetime.time):
            return value

        for format in self.input_formats:
            if format.lower() == ISO_8601:
                try:
                    parsed = parse_time(value)
                except (ValueError, TypeError):
                    pass
                else:
                    if parsed is not None:
                        return parsed
            else:
                try:
                    parsed = datetime.datetime.strptime(value, format)
                except (ValueError, TypeError):
                    pass
                else:
                    return parsed.time()

        humanized_format = humanize_datetime.time_formats(self.input_formats)
        self.fail('invalid', format=humanized_format)

    def to_representation(self, value):
        if self.format is None:
            return value

        # Applying a `TimeField` to a datetime value is almost always
        # not a sensible thing to do, as it means naively dropping
        # any explicit or implicit timezone info.
        assert not isinstance(value, datetime.datetime), (
            'Expected a `time`, but got a `datetime`. Refusing to coerce, '
            'as this may mean losing timezone information. Use a custom '
            'read-only field and deal with timezone issues explicitly.'
        )

        if self.format.lower() == ISO_8601:
            return value.isoformat()
        return value.strftime(self.format)


# Choice types...

class ChoiceField(Field):
    default_error_messages = {
        'invalid_choice': _('`{input}` is not a valid choice.')
    }

    def __init__(self, choices, **kwargs):
        # Allow either single or paired choices style:
        # choices = [1, 2, 3]
        # choices = [(1, 'First'), (2, 'Second'), (3, 'Third')]
        pairs = [
            isinstance(item, (list, tuple)) and len(item) == 2
            for item in choices
        ]
        if all(pairs):
            self.choices = SortedDict([(key, display_value) for key, display_value in choices])
        else:
            self.choices = SortedDict([(item, item) for item in choices])

        # Map the string representation of choices to the underlying value.
        # Allows us to deal with eg. integer choices while supporting either
        # integer or string input, but still get the correct datatype out.
        self.choice_strings_to_values = dict([
            (str(key), key) for key in self.choices.keys()
        ])

        super(ChoiceField, self).__init__(**kwargs)

    def to_internal_value(self, data):
        try:
            return self.choice_strings_to_values[str(data)]
        except KeyError:
            self.fail('invalid_choice', input=data)

    def to_representation(self, value):
        return self.choice_strings_to_values[str(value)]


class MultipleChoiceField(ChoiceField):
    default_error_messages = {
        'invalid_choice': _('`{input}` is not a valid choice.'),
        'not_a_list': _('Expected a list of items but got type `{input_type}`')
    }

    def to_internal_value(self, data):
        if isinstance(data, type('')) or not hasattr(data, '__iter__'):
            self.fail('not_a_list', input_type=type(data).__name__)

        return set([
            super(MultipleChoiceField, self).to_internal_value(item)
            for item in data
        ])

    def to_representation(self, value):
        return set([
            self.choice_strings_to_values[str(item)] for item in value
        ])


# File types...

class FileField(Field):
    default_error_messages = {
        'required': _("No file was submitted."),
        'invalid': _("The submitted data was not a file. Check the encoding type on the form."),
        'no_name': _("No filename could be determined."),
        'empty': _("The submitted file is empty."),
        'max_length': _('Ensure this filename has at most {max_length} characters (it has {length}).'),
    }
    use_url = api_settings.UPLOADED_FILES_USE_URL

    def __init__(self, *args, **kwargs):
        self.max_length = kwargs.pop('max_length', None)
        self.allow_empty_file = kwargs.pop('allow_empty_file', False)
        self.use_url = kwargs.pop('use_url', self.use_url)
        super(FileField, self).__init__(*args, **kwargs)

    def to_internal_value(self, data):
        try:
            # `UploadedFile` objects should have name and size attributes.
            file_name = data.name
            file_size = data.size
        except AttributeError:
            self.fail('invalid')

        if not file_name:
            self.fail('no_name')
        if not self.allow_empty_file and not file_size:
            self.fail('empty')
        if self.max_length and len(file_name) > self.max_length:
            self.fail('max_length', max_length=self.max_length, length=len(file_name))

        return data

    def to_representation(self, value):
        if self.use_url:
            url = settings.MEDIA_URL + value.url
            request = self.context.get('request', None)
            if request is not None:
                return request.build_absolute_uri(url)
            return url
        return value.name


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

    def __init__(self, *args, **kwargs):
        self._DjangoImageField = kwargs.pop('_DjangoImageField', forms.ImageField)
        super(ImageField, self).__init__(*args, **kwargs)

    def to_internal_value(self, data):
        # Image validation is a bit grungy, so we'll just outright
        # defer to Django's implementation so we don't need to
        # consider it, or treat PIL as a test dependancy.
        file_object = super(ImageField, self).to_internal_value(data)
        django_field = self._DjangoImageField()
        django_field.error_messages = self.error_messages
        django_field.to_python(file_object)
        return file_object


# Composite field types...

class ListField(Field):
    child = None
    initial = []
    default_error_messages = {
        'not_a_list': _('Expected a list of items but got type `{input_type}`')
    }

    def __init__(self, *args, **kwargs):
        self.child = kwargs.pop('child', copy.deepcopy(self.child))
        assert self.child is not None, '`child` is a required argument.'
        assert not inspect.isclass(self.child), '`child` has not been instantiated.'
        super(ListField, self).__init__(*args, **kwargs)
        self.child.bind(field_name='', parent=self)

    def get_value(self, dictionary):
        # We override the default field access in order to support
        # lists in HTML forms.
        if html.is_html_input(dictionary):
            return html.parse_html_list(dictionary, prefix=self.field_name)
        return dictionary.get(self.field_name, empty)

    def to_internal_value(self, data):
        """
        List of dicts of native values <- List of dicts of primitive datatypes.
        """
        if html.is_html_input(data):
            data = html.parse_html_list(data)
        if isinstance(data, type('')) or not hasattr(data, '__iter__'):
            self.fail('not_a_list', input_type=type(data).__name__)
        return [self.child.run_validation(item) for item in data]

    def to_representation(self, data):
        """
        List of object instances -> List of dicts of primitive datatypes.
        """
        return [self.child.to_representation(item) for item in data]


# Miscellaneous field types...

class ReadOnlyField(Field):
    """
    A read-only field that simply returns the field value.

    If the field is a method with no parameters, the method will be called
    and it's return value used as the representation.

    For example, the following would call `get_expiry_date()` on the object:

    class ExampleSerializer(self):
        expiry_date = ReadOnlyField(source='get_expiry_date')
    """

    def __init__(self, **kwargs):
        kwargs['read_only'] = True
        super(ReadOnlyField, self).__init__(**kwargs)

    def to_representation(self, value):
        if is_simple_callable(value):
            return value()
        return value


class SerializerMethodField(Field):
    """
    A read-only field that get its representation from calling a method on the
    parent serializer class. The method called will be of the form
    "get_{field_name}", and should take a single argument, which is the
    object being serialized.

    For example:

    class ExampleSerializer(self):
        extra_info = SerializerMethodField()

        def get_extra_info(self, obj):
            return ...  # Calculate some data to return.
    """
    def __init__(self, method_name=None, **kwargs):
        self.method_name = method_name
        kwargs['source'] = '*'
        kwargs['read_only'] = True
        super(SerializerMethodField, self).__init__(**kwargs)

    def bind(self, field_name, parent):
        # In order to enforce a consistent style, we error if a redundant
        # 'method_name' argument has been used. For example:
        # my_field = serializer.CharField(source='my_field')
        default_method_name = 'get_{field_name}'.format(field_name=field_name)
        assert self.method_name != default_method_name, (
            "It is redundant to specify `%s` on SerializerMethodField '%s' in "
            "serializer '%s', because it is the same as the default method name. "
            "Remove the `method_name` argument." %
            (self.method_name, field_name, parent.__class__.__name__)
        )

        # The method name should default to `get_{field_name}`.
        if self.method_name is None:
            self.method_name = default_method_name

        super(SerializerMethodField, self).bind(field_name, parent)

    def to_representation(self, value):
        method = getattr(self.parent, self.method_name)
        return method(value)


class ModelField(Field):
    """
    A generic field that can be used against an arbitrary model field.

    This is used by `ModelSerializer` when dealing with custom model fields,
    that do not have a serializer field to be mapped to.
    """
    def __init__(self, model_field, **kwargs):
        self.model_field = model_field
        kwargs['source'] = '*'
        super(ModelField, self).__init__(**kwargs)

    def to_internal_value(self, data):
        rel = getattr(self.model_field, 'rel', None)
        if rel is not None:
            return rel.to._meta.get_field(rel.field_name).to_python(data)
        return self.model_field.to_python(data)

    def to_representation(self, obj):
        value = self.model_field._get_val_from_obj(obj)
        if is_protected_type(value):
            return value
        return self.model_field.value_to_string(obj)