WTForms Can I add a placeholder attribute when I init a field?

Updated for WTForms 2.1

You can now as of WTForms 2.1 (December 2015) set rendering keywords by using the render_kw= parameter to the field constructor.

So the field would look like:

abc = StringField('abc', [InputRequired()], render_kw={"placeholder": "test"})

Note while this is possible; it does start to bridge the line between code and presentation; so use it wisely!

---

(Old answer, still true for versions older than WTForms 2.1)

placeholder is not supported in the Python constructor in WTforms 2.0.x and below.

However, you can do this easily in your template:

{{ form.abc(placeholder="test") }}

Correct answer is as follows:

abc = TextField('abc', validators=[Required(), Length(min=3, max=30)], description="test")

As one can read in documenatation:

description – A description for the field, typically used for help text.

Then in your template:

{% import 'forms.html' as forms %}

{% for field in form %}
    {{ forms.render_field(field) }}
{% endfor %}

Where render_field is a macro that is defined in forms.html:

{% macro render_field(field) -%}

{% if field.type == 'CSRFTokenField' %}
    {{ field }}

    {% if field.errors %}
        <div class="warning">You have submitted an invalid CSRF token</div>
    {% endif %}
{% elif field.type == 'HiddenField' %}
    {{ field }}
{# any other special case you may need #}
{% else %}
    <div class="form-group">
        <label for="{{ field.label.field_id }}" class="col-sm-2 control-label">{{ field.label.text }}</label>
        <div class="col-sm-10">
            {{ field(placeholder=field.description) }}
            {% if field.errors %}
                <div class="alert alert-danger" role="alert">
                {% for err in field.errors %}
                    <p>{{ err|e }}</p>
                {% endfor %}
                </div>
            {% endif %}
        </div>
    </div>
{% endif %}

{%- endmacro %}