Hi,

You can do this with an XML record defined as a ir.actions.server and then binding it to an ir.values record. Example in XML:

<record model="ir.actions.server" id="your_custom_action_id">
    <field name="name">Your description here</field>
    <field name="model_id" ref="your_module.model_your_model_name"/>
    <field name="code">
        for item in records:
        item.your_function()
        </field>
</record>
<record model="ir.values" id="ir_value_your_custom">
    <field name="model_id" ref="your_module.model_your_model_name"/>
    <field name="name">Your title here (shown to user)</field>
    <field name="key2">client_action_multi</field>
    <field name="value" eval="'ir.actions.server,' +str(ref('your_custom_action_id'))"/>
    <field name="key">action</field>
    <field name="model">your.model.name</field>
</record>

You can then create a Python function to execute code when the user clicks on this action:

@api.multi
def your_function(self):   

  print('Your custom Python function')

Regards