How to set default values with methods in Odoo?

You can use a lambda function like this:

name = fields.Char(
    string='Name',
    default=lambda self: self._get_default_name(),
)

@api.model
def _get_default_name(self):
    return "test"

A simpler version for the @ChesuCR answer:

def _get_default_name(self):
    return "test"

name = fields.Char(
    string='Name',
    default=_get_default_name,
)