I try to insert a call sign before the entered text.

if I enter a text in the field after saving! are displayed

 def _warnings(self, cr, uid, context=None):
        mytext = textwrap.dedent('warning')
        wrapped = textwrap.fill(mytext)
        result = textwrap.indent(wrapped, '! ')
        return result

'warning': fields.char(compute='_warnings',readonly=True, states={'Draft': [('readonly', False)]}),
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XML
<tree string='Test'>
<field name="warning"/>