# Solving Specific Problems

What follows is a collection of recipes that will help you tackle specific challenges that may crop up when using WTForms along with various other python frameworks.

## Prelude: Poke it with a Stick!

The aim of WTForms is not to do it all, but rather to stick to the basics, while being compatible with as many frameworks as possible. We attempt to place useful things in the API so that developers can get what they want out of it, if the default behaviour is not desired.

For example, many fields in WTForms are iterable to allow you to access enclosed fields inside them, providing you another way to customize their rendering. Many attributes on the fields are readily available for you to use in your templates. We encourage you to use the introspection abilities of the python interpreter to find new ways to manipulate fields. When introspection fails, you should try reading the source for insight into how things work and how you can use things to your advantage.

If you come up with a solution that you feel is useful to others and wish to share it, please let us know via email or the mailing list, and we'll add it to this document.

## Removing Fields Per-instance

Sometimes, you create a form which has fields that aren't useful in all circumstances or to all users. While it is indeed possible with form inheritance to define a form with exactly the fields you need, sometimes it is necessary to just tweak an existing form. Luckily, forms can have fields removed post-instantiation by using the del keyword:

class MagazineIssueForm(Form):
title  = StringField()
year   = IntegerField('Year')
month  = SelectField(choices=MONTHS)

def edit_issue():
publication = get_something_from_db()
form = MagazineIssueForm(...)

if publication.frequency == 'annual':
del form.month

# render our form


Removing a field from a form will cause it to not be validated, and it will not show up when iterating the form. It's as if the field was never defined to begin with. Note that you cannot add fields in this way, as all fields must exist on the form when processing input data.

## Dynamic Form Composition

This is a rare occurrence, but sometimes it's necessary to create or modify a form dynamically in your view. This is possible by creating internal subclasses:

def my_view():
class F(MyBaseForm):
pass

for name in iterate_some_model_dynamically():
setattr(F, name, StringField(name.title()))

form = F(request.POST, ...)
# do view stuff


For more form composition tricks, refer to this mailing list post

## Rendering Errors

In your template, you will often find yourself faced with the repetitive task of rendering errors for a form field. Here's a Jinja2 macro that may save you time:

{% macro with_errors(field) %}
<div class="form_field">
{% if field.errors %}
{% set css_class = 'has_error ' + kwargs.pop('class', '') %}
{{ field(class=css_class, **kwargs) }}
<ul class="errors">{% for error in field.errors %}<li>{{ error|e }}</li>{% endfor %}</ul>
{% else %}
{{ field(**kwargs) }}
{% endif %}
</div>
{% endmacro %}

Usage: {{ with_errors(form.field, style='font-weight: bold') }}


## Specialty Field Tricks

By using widget and field combinations, it is possible to create new behaviours and entirely new ways of displaying a form input to the user.

A classic example is easily supported using the widget= keyword arg, such as making a hidden field which stores and coerces integer data:

user_id = IntegerField(widget=HiddenInput())


Alternatively, you can create a field which does this by subclassing:

class HiddenInteger(IntegerField):
widget = HiddenInput()


Some fields support even more sophisticated customization.For example, what if a multiple-select was desired where instead of using a multi-row <select>, a series of checkboxes was used? By using widgets, one can get that behavior very easily:

class MultiCheckboxField(SelectMultipleField):
"""
A multiple-select, except displays a list of checkboxes.

Iterating the field will produce subfields, allowing custom rendering of
the enclosed checkbox fields.
"""
widget = widgets.ListWidget(prefix_label=False)
option_widget = widgets.CheckboxInput()


By overriding option_widget, our new multiple-select when iterated will now produce fields that render as checkboxes.