Declarative styling inspired by django-crispy-forms

gistfile1.py

import toga


f_input = toga.TextInput()
c_input = toga.TextInput(readonly=True)

c_label = toga.Label('Celcius', alignment=toga.LEFT_ALIGNED)
f_label = toga.Label('Fahrenheit', alignment=toga.LEFT_ALIGNED)
join_label = toga.Label('is equivalent to', alignment=toga.RIGHT_ALIGNED)


def calculate(widget):
    try:
        c_input.value = (float(f_input.value) - 32.0) * 5.0 / 9.0
    except Exception:
        c_input.value = '???'


# RowContainer is a Container subclass that automatically sets
# flexDirection='row'; ColumnContainer likewise.

container = toga.ColumnContainer(
    # Positional arguments are child containers/widgets of a container. Keyword
    # arguments provide extra information, including a style object.
    toga.RowContainer(
        # Supply a tuple of (widget, style) is auto possible. Parameters of the
        # Style object will be applied automatically to the widget. This avoids
        # the need of setting f_input.style somewhere else, which can be easy to
        # overlook.
        (
            f_input,
            toga.Style(
                flex=1,
                marginLeft=10,
            ),
        ),
        f_label,    # Add a widget directly.
        style=toga.Style(
            margin=5,
        ),
    ),
    toga.RowContainer(      # c_container
        join_label,
        (
            c_input,
            {
                'flex': 1,
                'marginLeft': 10,
            },
        ),
        c_label,
        # Maybe a dict is enough for style parameters?
        style={'margin': 5},
    ),
    toga.Button(
        'Calculate',
        on_press=calculate,
        # Widget classes should take a style parameter, too.
        style=toga.Style(margin=5),
    ),
    style=toga.Style(paddingTop=10),
)

# Setting styles later should work, too.
join_label.style(width=150, marginRight=10)