You can make a record for sequence. here is example to make sequence. Create new xml file.

<openerp> <data noupdate="1">

    <!-- Sequences for sale.order -->
    <record id="seq_unique_id" model="ir.sequence.type">
        <field name="name">my_sequence</field>
        <field name="code">sequence_code</field>
    </record>

    <record id="seq_unique_id2" model="ir.sequence">
        <field name="name">my_sequence</field>
        <field name="code">sequence_code</field>
        <field name="prefix">prefix</field>
        <field name="padding">3</field>
    </record>

</data>

</openerp>

after creating the file you need to initialise it in the __openerp__,py file.

Then in your model.py file where your field is defined in _columns. you need to set default value for that using.

_columns = {
    'new_field': fields.char('New Field', size=16),
}
_defaults = {
        'new_field': lambda self,cr,uid,context={}: self.pool.get('ir.sequence').get(cr, uid, 'sequence_code'),
}

update the module, Now whenever you will create a record it will get a sequence from the database.

It will help you.

In your .xml file :

<?xml version="1.0"?>

<openerp>
<data>
<record id="view_mutante_form" model="ir.ui.view">
        <field name="name">res.partner.form.inherit</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
    <xpath expr="/form/sheet/group/group/label[@for='street']" position="before">
        <field name="n_client" attrs="{'invisible':[('customer','!=',True)]}"/>
        <field name="n_supplier" attrs="{'invisible':[('supplier','!=',True)]}"/>
    </xpath>
    </field>
    </record>
</data>
    <data noupdate="1">
        <record model="ir.sequence.type" id="seq_type_res_partner">
            <field name="name">number_client_sequence</field>
            <field name="code">res.partner.customer</field>
        </record>
        <record model="ir.sequence" id="seq_res_partner">
            <field name="name">number_client_sequence</field>
            <field name="code">res.partner.customer</field>
            <field name="prefix">C</field>
            <field name="padding">6</field>
        </record>
        <record model="ir.sequence.type" id="seq_type_res_supplier">
            <field name="name">number_supplier_sequence</field>
            <field name="code">res.partner.supplier</field>
        </record>
        <record model="ir.sequence" id="seq_res_supplier">
            <field name="name">number_supplier_sequence</field>
            <field name="code">res.partner.supplier</field>
            <field name="prefix">S</field>
            <field name="padding">6</field>
        </record>
</data>
</openerp>

In your .py file :

# -*- coding: utf-8 -*-

from openerp.osv import fields, osv

import openerp.addons.decimal_precision as dp

class res_partner(osv.osv):
    _inherit = 'res.partner'
    _name = 'res.partner'

    _columns = {
        'n_client' : fields.char('Client Number', size=64, readonly=True),
        'n_supplier' : fields.char('Supplier Number', size=64, readonly=True),
    }
    _sql_constraints = [
        ('name_uniq_1', 'unique(n_client)', 'Number of client must be unique!'),
        ('n_supplier_uniq', 'unique(n_supplier)', 'Number of supplier must be unique!'),        
    ]

    def create(self, cr, uid, vals, context=None):   

        if vals.get('n_client') == None:
            vals['n_client'] = self.pool.get('ir.sequence').get(cr, uid, 'res.partner.customer')

        if vals.get('n_supplier') == None:
            vals['n_supplier'] = self.pool.get('ir.sequence').get(cr, uid, 'res.partner.supplier')
        return super(res_partner,self).create(cr, uid, vals, context)

    def copy(self, cr, uid, id, default=None, context=None):
        default.update({
            'n_client': self.pool.get('ir.sequence').get(cr, uid, 'res.partner.customer'),
            'n_supplier': self.pool.get('ir.sequence').get(cr, uid, 'res.partner.supplier')
        })
        return super(res_partner, self).copy(cr, uid, id, default, context)

res_partner()