django-rest-framework/rest_framework/fields.py

"""
Serializer fields perform validation on incoming data.

They are very similar to Django's form fields.
"""
from __future__ import unicode_literals

import copy
import datetime
from decimal import Decimal, DecimalException
import inspect
import re
import warnings

from django.core import validators
from django.core.exceptions import ValidationError
from django.conf import settings
from django import forms
from django.forms import widgets
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 timezone, parse_date, parse_datetime, parse_time
from rest_framework.compat import BytesIO
from rest_framework.compat import six
from rest_framework.compat import smart_text
from rest_framework.settings import api_settings


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_component(obj, attr_name):
    """
    Given an object, and an attribute name,
    return that attribute on the object.
    """
    if isinstance(obj, dict):
        val = obj[attr_name]
    else:
        val = getattr(obj, attr_name)

    if is_simple_callable(val):
        return val()
    return val


def readable_datetime_formats(formats):
    format = ', '.join(formats).replace(ISO_8601, 'YYYY-MM-DDThh:mm[:ss[.uuuuuu]][+HHMM|-HHMM|Z]')
    return humanize_strptime(format)


def readable_date_formats(formats):
    format = ', '.join(formats).replace(ISO_8601, 'YYYY[-MM[-DD]]')
    return humanize_strptime(format)


def readable_time_formats(formats):
    format = ', '.join(formats).replace(ISO_8601, 'hh:mm[:ss[.uuuuuu]]')
    return humanize_strptime(format)


def humanize_strptime(format_string):
    # Note that we're missing some of the locale specific mappings that
    # don't really make sense.
    mapping = {
        "%Y": "YYYY",
        "%y": "YY",
        "%m": "MM",
        "%b": "[Jan-Dec]",
        "%B": "[January-December]",
        "%d": "DD",
        "%H": "hh",
        "%I": "hh",  # Requires '%p' to differentiate from '%H'.
        "%M": "mm",
        "%S": "ss",
        "%f": "uuuuuu",
        "%a": "[Mon-Sun]",
        "%A": "[Monday-Sunday]",
        "%p": "[AM|PM]",
        "%z": "[+HHMM|-HHMM]"
    }
    for key, val in mapping.items():
        format_string = format_string.replace(key, val)
    return format_string


class Field(object):
    read_only = True
    creation_counter = 0
    empty = ''
    type_name = None
    partial = False
    use_files = False
    form_field_class = forms.CharField

    def __init__(self, source=None):
        self.parent = None

        self.creation_counter = Field.creation_counter
        Field.creation_counter += 1

        self.source = source

    def initialize(self, parent, field_name):
        """
        Called to set up a field prior to field_to_native or field_from_native.

        parent - The parent serializer.
        model_field - The model field this field corresponds to, if one exists.
        """
        self.parent = parent
        self.root = parent.root or parent
        self.context = self.root.context
        self.partial = self.root.partial
        if self.partial:
            self.required = False

    def field_from_native(self, data, files, field_name, into):
        """
        Given a dictionary and a field name, updates the dictionary `into`,
        with the field and it's deserialized value.
        """
        return

    def field_to_native(self, obj, field_name):
        """
        Given and object and a field name, returns the value that should be
        serialized for that field.
        """
        if obj is None:
            return self.empty

        if self.source == '*':
            return self.to_native(obj)

        source = self.source or field_name
        value = obj

        for component in source.split('.'):
            value = get_component(value, component)
            if value is None:
                break

        return self.to_native(value)

    def to_native(self, value):
        """
        Converts the field's value into it's simple representation.
        """
        if is_simple_callable(value):
            value = value()

        if is_protected_type(value):
            return value
        elif hasattr(value, '__iter__') and not isinstance(value, (dict, six.string_types)):
            return [self.to_native(item) for item in value]
        elif isinstance(value, dict):
            return dict(map(self.to_native, (k, v)) for k, v in value.items())
        return smart_text(value)

    def attributes(self):
        """
        Returns a dictionary of attributes to be used when serializing to xml.
        """
        if self.type_name:
            return {'type': self.type_name}
        return {}


class WritableField(Field):
    """
    Base for read/write fields.
    """
    default_validators = []
    default_error_messages = {
        'required': _('This field is required.'),
        'invalid': _('Invalid value.'),
    }
    widget = widgets.TextInput
    default = None

    def __init__(self, source=None, read_only=False, required=None,
                 validators=[], error_messages=None, widget=None,
                 default=None, blank=None):

        # 'blank' is to be deprecated in favor of 'required'
        if blank is not None:
            warnings.warn('The `blank` keyword argument is deprecated. '
                          'Use the `required` keyword argument instead.',
                          DeprecationWarning, stacklevel=2)
            required = not(blank)

        super(WritableField, self).__init__(source=source)

        self.read_only = read_only
        if required is None:
            self.required = not(read_only)
        else:
            assert not (read_only and required), "Cannot set required=True and read_only=True"
            self.required = required

        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
        self.default = default if default is not None else self.default

        # Widgets are ony used for HTML forms.
        widget = widget or self.widget
        if isinstance(widget, type):
            widget = widget()
        self.widget = widget

    def validate(self, value):
        if value in validators.EMPTY_VALUES and self.required:
            raise ValidationError(self.error_messages['required'])

    def run_validators(self, value):
        if value in validators.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:
                    message = self.error_messages[e.code]
                    if e.params:
                        message = message % e.params
                    errors.append(message)
                else:
                    errors.extend(e.messages)
        if errors:
            raise ValidationError(errors)

    def field_from_native(self, data, files, field_name, into):
        """
        Given a dictionary and a field name, updates the dictionary `into`,
        with the field and it's deserialized value.
        """
        if self.read_only:
            return

        try:
            if self.use_files:
                files = files or {}
                native = files[field_name]
            else:
                native = data[field_name]
        except KeyError:
            if self.default is not None and not self.partial:
                # Note: partial updates shouldn't set defaults
                native = self.default
            else:
                if self.required:
                    raise ValidationError(self.error_messages['required'])
                return

        value = self.from_native(native)
        if self.source == '*':
            if value:
                into.update(value)
        else:
            self.validate(value)
            self.run_validators(value)
            into[self.source or field_name] = value

    def from_native(self, value):
        """
        Reverts a simple representation back to the field's value.
        """
        return value


class ModelField(WritableField):
    """
    A generic field that can be used against an arbitrary model field.
    """
    def __init__(self, *args, **kwargs):
        try:
            self.model_field = kwargs.pop('model_field')
        except KeyError:
            raise ValueError("ModelField requires 'model_field' kwarg")

        self.min_length = kwargs.pop('min_length',
                            getattr(self.model_field, 'min_length', None))
        self.max_length = kwargs.pop('max_length',
                            getattr(self.model_field, 'max_length', None))

        super(ModelField, self).__init__(*args, **kwargs)

        if self.min_length is not None:
            self.validators.append(validators.MinLengthValidator(self.min_length))
        if self.max_length is not None:
            self.validators.append(validators.MaxLengthValidator(self.max_length))

    def from_native(self, value):
        rel = getattr(self.model_field, "rel", None)
        if rel is not None:
            return rel.to._meta.get_field(rel.field_name).to_python(value)
        else:
            return self.model_field.to_python(value)

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

    def attributes(self):
        return {
            "type": self.model_field.get_internal_type()
        }


##### Typed Fields #####

class BooleanField(WritableField):
    type_name = 'BooleanField'
    form_field_class = forms.BooleanField
    widget = widgets.CheckboxInput
    default_error_messages = {
        'invalid': _("'%s' value must be either True or False."),
    }
    empty = False

    # Note: we set default to `False` in order to fill in missing value not
    # supplied by html form.  TODO: Fix so that only html form input gets
    # this behavior.
    default = False

    def from_native(self, value):
        if value in ('true', 't', 'True', '1'):
            return True
        if value in ('false', 'f', 'False', '0'):
            return False
        return bool(value)


class CharField(WritableField):
    type_name = 'CharField'
    form_field_class = forms.CharField

    def __init__(self, max_length=None, min_length=None, *args, **kwargs):
        self.max_length, self.min_length = max_length, min_length
        super(CharField, self).__init__(*args, **kwargs)
        if min_length is not None:
            self.validators.append(validators.MinLengthValidator(min_length))
        if max_length is not None:
            self.validators.append(validators.MaxLengthValidator(max_length))

    def from_native(self, value):
        if isinstance(value, six.string_types) or value is None:
            return value
        return smart_text(value)


class URLField(CharField):
    type_name = 'URLField'

    def __init__(self, **kwargs):
        kwargs['max_length'] = kwargs.get('max_length', 200)
        kwargs['validators'] = [validators.URLValidator()]
        super(URLField, self).__init__(**kwargs)


class SlugField(CharField):
    type_name = 'SlugField'

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = kwargs.get('max_length', 50)
        super(SlugField, self).__init__(*args, **kwargs)


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

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

    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.
        self._choices = self.widget.choices = list(value)

    choices = property(_get_choices, _set_choices)

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

    def valid_value(self, value):
        """
        Check to see if the provided value is a valid choice.
        """
        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 == smart_text(k2):
                        return True
            else:
                if value == smart_text(k) or value == k:
                    return True
        return False


class EmailField(CharField):
    type_name = 'EmailField'
    form_field_class = forms.EmailField

    default_error_messages = {
        'invalid': _('Enter a valid e-mail address.'),
    }
    default_validators = [validators.validate_email]

    def from_native(self, value):
        ret = super(EmailField, self).from_native(value)
        if ret is None:
            return None
        return ret.strip()

    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 RegexField(CharField):
    type_name = 'RegexField'
    form_field_class = forms.RegexField

    def __init__(self, regex, max_length=None, min_length=None, *args, **kwargs):
        super(RegexField, self).__init__(max_length, min_length, *args, **kwargs)
        self.regex = regex

    def _get_regex(self):
        return self._regex

    def _set_regex(self, regex):
        if isinstance(regex, six.string_types):
            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)

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


class DateField(WritableField):
    type_name = 'DateField'
    widget = widgets.DateInput
    form_field_class = forms.DateField

    default_error_messages = {
        'invalid': _("Date has wrong format. Use one of these formats instead: %s"),
    }
    empty = None
    input_formats = api_settings.DATE_INPUT_FORMATS
    format = api_settings.DATE_FORMAT

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

    def from_native(self, value):
        if value in validators.EMPTY_VALUES:
            return None

        if isinstance(value, datetime.datetime):
            if timezone and settings.USE_TZ and timezone.is_aware(value):
                # Convert aware datetimes to the default time zone
                # before casting them to dates (#17742).
                default_timezone = timezone.get_default_timezone()
                value = timezone.make_naive(value, default_timezone)
            return value.date()
        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()

        msg = self.error_messages['invalid'] % readable_date_formats(self.input_formats)
        raise ValidationError(msg)

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

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

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


class DateTimeField(WritableField):
    type_name = 'DateTimeField'
    widget = widgets.DateTimeInput
    form_field_class = forms.DateTimeField

    default_error_messages = {
        'invalid': _("Datetime has wrong format. Use one of these formats instead: %s"),
    }
    empty = None
    input_formats = api_settings.DATETIME_INPUT_FORMATS
    format = api_settings.DATETIME_FORMAT

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

    def from_native(self, value):
        if value in validators.EMPTY_VALUES:
            return None

        if isinstance(value, datetime.datetime):
            return value
        if isinstance(value, datetime.date):
            value = datetime.datetime(value.year, value.month, value.day)
            if settings.USE_TZ:
                # For backwards compatibility, interpret naive datetimes in
                # local time. This won't work during DST change, but we can't
                # do much about it, so we let the exceptions percolate up the
                # call stack.
                warnings.warn("DateTimeField received a naive datetime (%s)"
                              " while time zone support is active." % value,
                              RuntimeWarning)
                default_timezone = timezone.get_default_timezone()
                value = timezone.make_aware(value, default_timezone)
            return 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 parsed
            else:
                try:
                    parsed = datetime.datetime.strptime(value, format)
                except (ValueError, TypeError):
                    pass
                else:
                    return parsed

        msg = self.error_messages['invalid'] % readable_datetime_formats(self.input_formats)
        raise ValidationError(msg)

    def to_native(self, value):
        if value is None or 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 TimeField(WritableField):
    type_name = 'TimeField'
    widget = widgets.TimeInput
    form_field_class = forms.TimeField

    default_error_messages = {
        'invalid': _("Time has wrong format. Use one of these formats instead: %s"),
    }
    empty = None
    input_formats = api_settings.TIME_INPUT_FORMATS
    format = api_settings.TIME_FORMAT

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

    def from_native(self, value):
        if value in validators.EMPTY_VALUES:
            return None

        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()

        msg = self.error_messages['invalid'] % readable_time_formats(self.input_formats)
        raise ValidationError(msg)

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

        if isinstance(value, datetime.datetime):
            value = value.time()

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


class IntegerField(WritableField):
    type_name = 'IntegerField'
    form_field_class = forms.IntegerField

    default_error_messages = {
        'invalid': _('Enter a whole number.'),
        'max_value': _('Ensure this value is less than or equal to %(limit_value)s.'),
        'min_value': _('Ensure this value is greater than or equal to %(limit_value)s.'),
    }

    def __init__(self, max_value=None, min_value=None, *args, **kwargs):
        self.max_value, self.min_value = max_value, min_value
        super(IntegerField, self).__init__(*args, **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 from_native(self, value):
        if value in validators.EMPTY_VALUES:
            return None

        try:
            value = int(str(value))
        except (ValueError, TypeError):
            raise ValidationError(self.error_messages['invalid'])
        return value


class FloatField(WritableField):
    type_name = 'FloatField'
    form_field_class = forms.FloatField

    default_error_messages = {
        'invalid': _("'%s' value must be a float."),
    }

    def from_native(self, value):
        if value in validators.EMPTY_VALUES:
            return None

        try:
            return float(value)
        except (TypeError, ValueError):
            msg = self.error_messages['invalid'] % value
            raise ValidationError(msg)


class DecimalField(WritableField):
    type_name = 'DecimalField'
    form_field_class = forms.DecimalField

    default_error_messages = {
        'invalid': _('Enter a number.'),
        'max_value': _('Ensure this value is less than or equal to %(limit_value)s.'),
        'min_value': _('Ensure this value is greater than or equal to %(limit_value)s.'),
        'max_digits': _('Ensure that there are no more than %s digits in total.'),
        'max_decimal_places': _('Ensure that there are no more than %s decimal places.'),
        'max_whole_digits': _('Ensure that there are no more than %s digits before the decimal point.')
    }

    def __init__(self, max_value=None, min_value=None, max_digits=None, decimal_places=None, *args, **kwargs):
        self.max_value, self.min_value = max_value, min_value
        self.max_digits, self.decimal_places = max_digits, decimal_places
        super(DecimalField, self).__init__(*args, **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 from_native(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.
        """
        if value in validators.EMPTY_VALUES:
            return None
        value = smart_text(value).strip()
        try:
            value = Decimal(value)
        except DecimalException:
            raise ValidationError(self.error_messages['invalid'])
        return value

    def validate(self, value):
        super(DecimalField, self).validate(value)
        if value in validators.EMPTY_VALUES:
            return
        # Check for NaN, Inf and -Inf values. We can't compare directly for NaN,
        # since it is never equal to itself. However, NaN is the only value that
        # isn't equal to itself, so we can use this to identify NaN
        if value != value or value == Decimal("Inf") or value == Decimal("-Inf"):
            raise ValidationError(self.error_messages['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:
            raise ValidationError(self.error_messages['max_digits'] % self.max_digits)
        if self.decimal_places is not None and decimals > self.decimal_places:
            raise ValidationError(self.error_messages['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):
            raise ValidationError(self.error_messages['max_whole_digits'] % (self.max_digits - self.decimal_places))
        return value


class FileField(WritableField):
    use_files = True
    type_name = 'FileField'
    form_field_class = forms.FileField
    widget = widgets.FileInput

    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': _('Ensure this filename has at most %(max)d characters (it has %(length)d).'),
        'contradiction': _('Please either submit a file or check the clear checkbox, not both.')
    }

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

    def from_native(self, data):
        if data in validators.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'])

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

        return data

    def to_native(self, value):
        return value.name


class ImageField(FileField):
    use_files = True
    form_field_class = forms.ImageField

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

    def from_native(self, data):
        """
        Checks that the file-upload field data contains a valid image (GIF, JPG,
        PNG, possibly others -- whatever the Python Imaging Library supports).
        """
        f = super(ImageField, self).from_native(data)
        if f is None:
            return None

        from compat import Image
        assert Image is not None, 'PIL must be installed for ImageField support'

        # We need to get a file object for PIL. 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.
            # verify() must be called immediately after the constructor.
            Image.open(file).verify()
        except ImportError:
            # Under PyPy, it is possible to import PIL. However, the underlying
            # _imaging C module isn't available, so an ImportError will be
            # raised. Catch and re-raise.
            raise
        except Exception:  # Python Imaging Library doesn't recognize it as an image
            raise ValidationError(self.error_messages['invalid_image'])
        if hasattr(f, 'seek') and callable(f.seek):
            f.seek(0)
        return f


class SerializerMethodField(Field):
    """
    A field that gets its value by calling a method on the serializer it's attached to.
    """

    def __init__(self, method_name):
        self.method_name = method_name
        super(SerializerMethodField, self).__init__()

    def field_to_native(self, obj, field_name):
        value = getattr(self.parent, self.method_name)(obj)
        return self.to_native(value)