"""
Mixin to dynamically select only a subset of fields per DRF resource.
"""
import warnings

from django.conf import settings
from django.utils.functional import cached_property


class DynamicFieldsMixin(object):
    """
    A serializer mixin that takes an additional `fields` argument that controls
    which fields should be displayed.
    """

    @cached_property
    def fields(self):
        """
        Filters the fields according to the `fields` query parameter.

        A blank `fields` parameter (?fields) will remove all fields. Not
        passing `fields` will pass all fields individual fields are comma
        separated (?fields=id,name,url,email).

        """
        fields = super(DynamicFieldsMixin, self).fields

        if not hasattr(self, "_context"):
            # We are being called before a request cycle
            return fields

        # Only filter if this is the root serializer, or if the parent is the
        # root serializer with many=True
        is_root = self.root == self
        parent_is_list_root = self.parent == self.root and getattr(
            self.parent, "many", False
        )
        if not (is_root or parent_is_list_root):
            return fields

        try:
            request = self.context["request"]
        except KeyError:
            conf = getattr(settings, "DRF_DYNAMIC_FIELDS", {})
            if not conf.get("SUPPRESS_CONTEXT_WARNING", False) is True:
                warnings.warn(
                    "Context does not have access to request. "
                    "See README for more information."
                )
            return fields

        # NOTE: drf test framework builds a request object where the query
        # parameters are found under the GET attribute.
        params = getattr(request, "query_params", getattr(request, "GET", None))
        if params is None:
            warnings.warn("Request object does not contain query parameters")

        try:
            filter_fields = params.get("fields", None).split(",")
        except AttributeError:
            filter_fields = None

        try:
            omit_fields = params.get("omit", None).split(",")
        except AttributeError:
            omit_fields = []

        # Drop any fields that are not specified in the `fields` argument.
        existing = set(fields.keys())
        if filter_fields is None:
            # no fields param given, don't filter.
            allowed = existing
        else:
            allowed = set(filter(None, filter_fields))

        # omit fields in the `omit` argument.
        omitted = set(filter(None, omit_fields))

        for field in existing:

            if field not in allowed:
                fields.pop(field, None)

            if field in omitted:
                fields.pop(field, None)

        return fields