Form Validation with WTForms¶
When you have to work with form data submitted by a browser view, code quickly becomes very hard to read. There are libraries out there designed to make this process easier to manage. One of them is WTForms which we will handle here. If you find yourself in the situation of having many forms, you might want to give it a try.
When you are working with WTForms you have to define your forms as classes first. I recommend breaking up the application into multiple modules (Large Applications as Packages) for that and adding a separate module for the forms.
Getting the most out of WTForms with an Extension
The Flask-WTF extension expands on this pattern and adds a few little helpers that make working with forms and Flask more fun. You can get it from PyPI.
The Forms¶
This is an example form for a typical registration page:
from wtforms import Form, BooleanField, StringField, PasswordField, validators
class RegistrationForm(Form):
username = StringField('Username', [validators.Length(min=4, max=25)])
email = StringField('Email Address', [validators.Length(min=6, max=35)])
password = PasswordField('New Password', [
validators.DataRequired(),
validators.EqualTo('confirm', message='Passwords must match')
])
confirm = PasswordField('Repeat Password')
accept_tos = BooleanField('I accept the TOS', [validators.DataRequired()])
In the View¶
In the view function, the usage of this form looks like this:
@app.route('/register', methods=['GET', 'POST'])
def register():
form = RegistrationForm(request.form)
if request.method == 'POST' and form.validate():
user = User(form.username.data, form.email.data,
form.password.data)
db_session.add(user)
flash('Thanks for registering')
return redirect(url_for('login'))
return render_template('register.html', form=form)
Notice we’re implying that the view is using SQLAlchemy here (SQLAlchemy in Flask), but that’s not a requirement, of course. Adapt the code as necessary.
Things to remember:
create the form from the request
form
value if the data is submitted via the HTTPPOST
method andargs
if the data is submitted asGET
.to validate the data, call the
validate()
method, which will returnTrue
if the data validates,False
otherwise.to access individual values from the form, access
form.<NAME>.data
.
Forms in Templates¶
Now to the template side. When you pass the form to the templates, you can easily render them there. Look at the following example template to see how easy this is. WTForms does half the form generation for us already. To make it even nicer, we can write a macro that renders a field with label and a list of errors if there are any.
Here’s an example _formhelpers.html
template with such a macro:
{% macro render_field(field) %}
<dt>{{ field.label }}
<dd>{{ field(**kwargs)|safe }}
{% if field.errors %}
<ul class=errors>
{% for error in field.errors %}
<li>{{ error }}</li>
{% endfor %}
</ul>
{% endif %}
</dd>
{% endmacro %}
This macro accepts a couple of keyword arguments that are forwarded to
WTForm’s field function, which renders the field for us. The keyword
arguments will be inserted as HTML attributes. So, for example, you can
call render_field(form.username, class='username')
to add a class to
the input element. Note that WTForms returns standard Python strings,
so we have to tell Jinja2 that this data is already HTML-escaped with
the |safe
filter.
Here is the register.html
template for the function we used above, which
takes advantage of the _formhelpers.html
template:
{% from "_formhelpers.html" import render_field %}
<form method=post>
<dl>
{{ render_field(form.username) }}
{{ render_field(form.email) }}
{{ render_field(form.password) }}
{{ render_field(form.confirm) }}
{{ render_field(form.accept_tos) }}
</dl>
<p><input type=submit value=Register>
</form>
For more information about WTForms, head over to the WTForms website.