example1.py

from math import inf

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import NumericProperty, StringProperty


class Field(BoxLayout):
    label_text = StringProperty()


class Example1App(App):
    smallest_positive = NumericProperty(inf)

    def add_field(self):
        container = self.root.ids.fields
        container.add_widget(Field(label_text=f'Field #{len(container.children)}: '))

    def remove_field(self):
        container = self.root.ids.fields
        if len(container.children) > 0:
            container.remove_widget(container.children[0])

    @staticmethod
    def _find_smallest_positive(values):
        smallest_positive = inf
        for value in values:
            if 0 < value < smallest_positive:
                smallest_positive = value
        return smallest_positive

    def update(self):
        container = self.root.ids.fields
        values = []
        for field in container.children:
            try:
                values.append(float(field.ids.input.text))
            except ValueError:
                pass
        self.smallest_positive = Example1App._find_smallest_positive(values)


if __name__ == '__main__':
    app = Example1App()
    app.run()