Flask-WTF - validate_on_submit() is never executed

I'm using Flask-WTF:

Here is my form:

from flask.ext.wtf import Form, TextField

class BookNewForm(Form):
    name = TextField('Name')

Here is the controller:

@book.route('/book/new', methods=['GET', 'POST'])
def customers_new():
    form = BookNewForm()
    if form.is_submitted():
        print "submitted"
    if form.validate():
        print "valid"
    if form.validate_on_submit():
        flash("Successfully created a new book")
        return redirect(url_for('.books_show'))
    return render_template('views/books_new.html', form=form)

Now the problem is, if you look at my print statements, it always prints submitted, but it NEVER prints valid and validate_on_submit() is never executed. Why?

You're not inserting the CSRF field in the HTML form.

<form method=post>
    {{ form.csrf_token }}
    {{ form.name }}
    <input type=submit>
</form>

After adding form.csrf_token to the template (docs), the form will validate as expected.

Add print(form.errors) after validating the form to see the errors that were raised. errors will be empty before validation. In this case, there is an error about missing

@book.route('/book/new_no_csrf', methods=['GET', 'POST'])
def customers_new_no_csrf():
    form = BookNewForm()
    print(form.errors)

    if form.is_submitted():
        print("submitted")

    if form.validate():
        print("valid")

    print(form.errors)

    if form.validate_on_submit():
        flash("Successfully created a new book")
        return redirect(url_for('.books_show'))

    return render_template('books_new.html', form=form)

{}
submitted
{'csrf_token': [u'CSRF token missing']}
127.0.0.1 - - [29/May/2012 02:01:08] "POST /book/new_no_csrf HTTP/1.1" 200 -
127.0.0.1 - - [29/May/2012 02:01:08] "GET /favicon.ico HTTP/1.1" 404 -

I created an example on GitHub.