web/web_widget_dropdown_dynamic/readme/USAGE.md

@api.model
def method_name(self):
    values = [
        ('value_a', 'Title A'),
    ]
    if self.env.context.get('depending_on') == True:
        values += [
            ('value_b', 'Title B'),
        ]
    return values

<field
    name="other_field"
/>
<field
    name="char_field"
    widget="dynamic_dropdown"
    options="{'values':'method_name'}"
    context="{'depending_on': other_field}"
/>