A sample Wtforms field for creating a list of tags with custom separators, e.g. "," or " ".

render.html

  <form action="" method="POST" role="form" class="form">
            {{ form.hidden_tag() }}
            <!--Other fields-->
            {{ wtf.form_field(form.tags, placeholder='audio, hardware, chip') }}
            <button class="btn btn-success" type="submit">submit</button>
  </form>

tag_list_field.py

from wtforms.fields import StringField


class TagListField(StringField):
    """Stringfield for a list of separated tags"""

    def __init__(self, label='', validators=None, remove_duplicates=True, to_lowercase=True, separator=' ', **kwargs):
        """
        Construct a new field.
        :param label: The label of the field.
        :param validators: A sequence of validators to call when validate is called.
        :param remove_duplicates: Remove duplicates in a case insensitive manner.
        :param to_lowercase: Cast all values to lowercase.
        :param separator: The separator that splits the individual tags.
        """
        super(TagListField, self).__init__(label, validators, **kwargs)
        self.remove_duplicates = remove_duplicates
        self.to_lowercase = to_lowercase
        self.separator = separator
        self.data = []

    def _value(self):
        if self.data:
            return u', '.join(self.data)
        else:
            return u''

    def process_formdata(self, valuelist):
        if valuelist:
            self.data = [x.strip() for x in valuelist[0].split(self.separator)]
            if self.remove_duplicates:
                self.data = list(self._remove_duplicates(self.data))
            if self.to_lowercase:
                self.data = [x.lower() for x in self.data]

    @classmethod
    def _remove_duplicates(cls, seq):
        """Remove duplicates in a case insensitive, but case preserving manner"""
        d = {}
        for item in seq:
            if item.lower() not in d:
                d[item.lower()] = True
                yield item

usage.py

from flask_wtf import FlaskForm
from tag_list_field import TagListField

class Post(FlaskForm):
    # other fields

    tags = TagListField(
        "Tags",
        separator=",",
        validators=[Length(max=8, message="You can only use up to 8 tags.")]
    )