example2.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 Example2App(App):
    smallest_positive = NumericProperty(inf)

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.values = []

    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])

    def _find_smallest_positive(self):
        result = inf
        for value in self.values:
            if 0 < value < result:
                result = value
        return result

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


if __name__ == '__main__':
    app = Example2App()
    app.run()