How do you set a default value for a WTForms SelectField?

I believe this problem is caused by the Field‘s data attribute overriding the default with something that WTForms doesn’t understand (e.g. a DB model object — it expects an int). This would happen if you have populated your form in the constructor like so:

form = PostForm(obj=post)

the solution is to manually set the data attribute after the form has been populated:

form = PostForm(obj=post)
form.category.data = (post.category.id
                      if page.category
                      else 0) # I make 0 my default

There are a few ways to do this. Your first code snippet is indeed correct.

If you want to do this in a View dynamically though, you can also do:

form = TestForm()
form.test_field.default = some_default_id
form.process()

This is a choices settings with SelectField when you use an int, it works like this:

test_select = SelectField("Test", coerce=int, choices=[(0, "test0"), (1, "test1")], default=1)

or:

class TestForm(Form):
    test_select = SelectField("Test", coerce=int)

@app.route("/test")
def view_function():
    form = TestForm()
    form.test_select.choices = [(0, "test0"), (1, "test1")]
    form.test_select.default = 1
    form.process()

Flask-WTF 0.14.2 user here. So this answer is for anyone who have similar problem with me.

Basically, none of the previous solutions function properly with form.validate_on_submit().

Setting form.test_field.data will indeed change the default value when the page is loaded, but the data stays the same after validate_on_submit (user changes in browser has no effect).

Setting form.test_field.default then call form.process() also changes the value when the page is loaded, but validate_on_submit will fail.

Here is the new way to do it:

class TestForm(Form):
    test_field = SelectField("Test", choices=[(0, "test0"), (1, "test1")])

@app.route("/test")
def view_function():
    form = TestForm(test_field=1)
    if form.validate_on_submit():
        ...

In case you’re using flask_wtf and would like to set a default value for different SelectFields nested in a FieldList like this

from flask_wtf import FlaskForm


class FormWithSelect(FlaskForm):
    my_select_field = SelectField(validators=[DataRequired()], coerce=int)


class FormWithMultipleSelects(FlaskForm):
    select_fields = FieldList(FormField(FormWithSelect), min_entries=1)
    submit = SubmitField(label="save all the select field values")

the solution is to overwrite data, not default as you would expect.

def generate_my_form(my_data, overwrite_data=True):
    form = FormWithMultipleSelects(select_fields=[{
        "my_select_field": {
             "name": d["name"],
             # ...
        }
    } for d in my_data]))
    for n range(len(form.select_fields)):
        form.select_fields[n].my_select_field.choices = [(1,'foo'), (2,'bar')]
        # I wan't 'bar' to be the option selected by default
        # form.select_fields[n].my_select_field.default = 2  # has no effect!
        if overwrite_data:
            form.select_fields[n].my_select_field.data = 2 #  works

Basically this solution is similar to Elliots answer above; I just wanted to provide a solution for the more complex case of a FieldList. nikitz mentioned in a comment, that there is a side effect: form.validate_on_submit() does not work if you’re overwriting data! You can build a simple workaround where to disable the overwriting of data when calling validate_on_submit().

 form_for_validation = generate_my_form(my_data, False)
 if form_for_validation.validate_on_submit():
      # do something with form_for_validation.data
 else:
     form_with_default_data = generate_my_form(my_data, True)
     render_template(..., form=form_with_default_data)

It’s not super elegant, but it works.

PS: My reputation is too low to directly comment on the correct answer by Elliot.

I have some problem with dynamic set default selectfield thati think can be useful.
the model was like this:

class State(db.Model):
    __tablename__ = 'states'
    id = db.Column(db.Integer, primary_key=True)   
    name = db.Column(db.String(128), nullable=False)


class City(db.Model):
    __tablename__ = 'cities'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64))
    state_id = db.Column(db.Integer, db.ForeignKey('states.id'))   
    state = db.relationship(State, backref='state')


class User(UserMixin, db.Model):
    ...
    city_id = db.Column(db.Integer, db.ForeignKey('cities.id'))
    city = db.relationship('City', backref='city')

the form was like this:

class UserEditForm(FlaskForm):
    ...
    state = SelectField("state", coerce=int)
    city = SelectField("city", coerce=int)
    ....

  def __init__(self, state_id, *args, **kwargs):
       super(UserEditForm, self).__init__(*args, **kwargs)
       self.state.choices =  [(state.id, state.name)
                        for state in State.query.order_by('name').all()]
       self.city.choices = [(city.id, city.name)
                            for city in   City.query.filter_by(state_id=state_id).order_by('name').all()]   

and the view was like this:

@dashboard.route('/useredit', methods=['GET', 'POST'])
@login_required
def useredit():
    ....
    user = current_user._get_current_object()       
    form = OrganEditForm(state_id=user.city.state_id, state=user.city.state_id, city=user.city_id)
     ....

it works good and set Selectfield default value correctly. but i changed the code like this:

in view:

@dashboard.route('/useredit', methods=['GET', 'POST'])
    @login_required
    def useredit():
        ....
        user = current_user._get_current_object()       
        form = OrganEditForm(obj=user)
         ....

in forms

def __init__(self, *args, **kwargs):
       super(UserEditForm, self).__init__(*args, **kwargs)
       self.state.choices =  [(state.id, state.name)
                        for state in State.query.order_by('name').all()]
       self.city.choices = [(city.id, city.name)
                            for city in   City.query.filter_by(state_id=kwargs['obj'].city.state_id).order_by('name').all()] 

but this time the city default value doesn’t set. i changed form like this:

def __init__(self, *args, **kwargs):
           kwargs['city'] = kwargs['obj'].city_id
           super(UserEditForm, self).__init__(*args, **kwargs)
           self.state.choices =  [(state.id, state.name)
                            for state in State.query.order_by('name').all()]
           self.city.choices = [(city.id, city.name)
                                for city in   City.query.filter_by(state_id=kwargs['obj'].city.state_id).order_by('name').all()] 

but it didn’t work. I tried many solution and in the last I changed the city variable name to usercity:

usercity= SelectField("city", coerce=int)

and kwargs['city'] = kwargs['obj'].city_id to kwargs['usercity'] = kwargs['obj'].city_id. affter that it worked correctly.

the problem was that when I set obj=user, default value for city selectfield read from kwargs['obj'].city that I defined in user model.

Python Flask SelectField choices 

    # method 1 - routes.py
     form.proj_cat_id.default    = table1.proj_cat_id
     form.process()  # for preset 

    # method 2 - routes.py
    form.proj_cat_id.process_formdata([table1.proj_cat_id])



# method 3  - routes.py
      proj_cat_id_default    = table1.proj_cat_id
      return render_template('abc.html',proj_cat_id_default=proj_cat_id_default)

# method 3 - abc.html
        {% set z = form.proj_cat_id.process_data(proj_cat_id_default) %}
        {{ form.proj_cat_id(class="form-control form-control-lg") }}