How to make Flask-WTFoms update labels dynamically from list of label names?

Question

I use WTForms to define form for data filtering it is defined like this (My goal is to have user specified labels for BooleanFields set, I let each user to name labels for field

Answer 1

The approach that you have taken, calendar_colors_descriptions is assigned in the body of your form class.

This means that it is only evaluated once - when the forms module is first imported - and so the field label values are fixed until the server is restarted. In effect, the label values are part of the class definition, and so common across all instances of the class.

This example code is similar to yours;

import random
import wtforms


def get_labels(labels=None):
        if labels is None:
            labels = ['red', 'amber', 'green']
        # Simulate data changes by shuffling the list.
        random.shuffle(labels)
        return labels


class StaticLabelForm(wtforms.Form):

    # labels is set when the class is compiled at import time.
    labels = get_labels()

    foo = wtforms.BooleanField(labels[0], default=True)
    bar = wtforms.BooleanField(labels[1], default=True)
    baz = wtforms.BooleanField(labels[2], default=True)

Each time we instantiate a new StaticLabelForm, the labels are always the same, because the get_labels function is only called once.

>>> static1 = StaticLabelForm()
>>> for field in static1: print(field.label, field)
... 
amber 
green 
red 

>>> static2 = StaticLabelForm()
>>> for field in static2: print(field.label, field)
... 
amber 
green 
red 

We can fix this by passing the label values to the form's __init__ method, and setting them on the fields inside the __init__ method.

class DynamicLabelForm(wtforms.Form):

    # Don't set the labels here
    foo = wtforms.BooleanField(default=True)
    bar = wtforms.BooleanField(default=True)
    baz = wtforms.BooleanField(default=True)

    def __init__(self, labels=None, **kwargs):
        super().__init__(**kwargs)
        # super(DynamicLabelForm, self).__init__(**kwargs) for python2!
        if labels is None:
            labels = ['red', 'amber', 'green']
        self['foo'].label = wtforms.Label(self['foo'].id, labels[0])
        self['bar'].label = wtforms.Label(self['bar'].id, labels[1])
        self['baz'].label = wtforms.Label(self['baz'].id, labels[2])

Now the labels are reset on each new form:

>>> dynamic1 = DynamicLabelForm(labels=get_labels())
>>> for field in dynamic1: print(field.label, field)
... 
amber 
red 
green 

>>> dynamic2 = DynamicLabelForm(labels=get_labels())
>>> for field in dynamic2: print(field.label, field)
... 
amber 
green 
red