I m getting

'base': tax['base'],
KeyError: 'base'

my custom_tax.py file is

# -*- coding: utf-8 -*-
# Part of Odoo. See LICENSE file for full copyright and licensing details.


from datetime import datetime from dateutil.relativedelta import relativedelta

from openerp import api, fields, models, _ from openerp.tools import float_is_zero, float_compare from openerp.addons.base.res.res_partner import format_address from openerp.tools.misc import formatLang from openerp.tools import amount_to_text_en, float_round

from openerp.exceptions import UserError, RedirectWarning, ValidationError import openerp.addons.decimal_precision as dp

class PurchaseOrder(models.Model):
    _inherit = "purchase.order"
    
    purchase_tax_line_ids = fields.One2many('purchase.order.tax', 'purchase_id', string='Tax Lines', readonly=True, states={'draft': [('readonly', False)]}, copy=True)
    
    @api.onchange('order_line')
    def _onchange_order_line(self):
        taxes_grouped = self.get_taxes_values()
        tax_lines = self.purchase_tax_line_ids.browse([])
        for tax in taxes_grouped.values():
            tax_lines += tax_lines.new(tax)
        self.purchase_tax_line_ids = tax_lines
        return
    
    @api.multi
    def compute_taxes(self):
        """Function used in other module to compute the taxes on a fresh invoice created (onchanges did not applied)"""
        sale_tax = self.env['purchase.order.tax']
        ctx = dict(self._context)
        for purchase in self:
#             Delete non-manual tax lines
            self._cr.execute("DELETE FROM purchase_order_tax WHERE purchase_id=%s AND manual is False", (purchase.id,))
            self.invalidate_cache()
 
            # Generate one tax line per tax, however many invoice lines it's applied to
            tax_grouped = purchase.get_taxes_values()
            
            # Create new tax lines
            for tax in tax_grouped.values():
                sale_tax.create(tax)
 
        # dummy write on self to trigger recomputations
        return self.with_context(ctx).write({'order_line': []})
    
    @api.model
    def create(self, vals):
        if vals.get('name', 'New') == 'New':
            vals['name'] = self.env['ir.sequence'].next_by_code('purchase.order') or '/'
            result = super(PurchaseOrder, self).create(vals)
        if any(line.taxes_id for line in result.order_line) and not result.purchase_tax_line_ids:
            result.compute_taxes()
        return result
    
    @api.multi
    def button_dummy(self):
        self.compute_taxes()
        return True
    
    def _prepare_tax_line_vals(self, line, tax):
        """ Prepare values to create an account.invoice.tax line

        The line parameter is an account.invoice.line, and the
        tax parameter is the output of account.tax.compute_all().
        """
        tax_obj = self.env['account.tax']
        tax_br = tax_obj.browse(tax['id'])
        vals = {
            'purchase_id':self.id,
            'name': tax_br.description or tax['name'],
            'tax_id': tax['id'],
            'amount': tax['amount'],
            'sequence': tax['sequence'],
            'base': tax['base'],
            'manual': False,
            #'sequence': tax['sequence'],
            'account_analytic_id': tax['analytic'] or False,
            'account_id': tax['account_id'],
        }
        return vals
    
    @api.multi
    def get_taxes_values(self):
        tax_grouped = {}
        for line in self.order_line:
            price_unit = line.price_unit 
            taxes = line.taxes_id.compute_all(price_unit, self.currency_id, line.product_qty, line.product_id, self.partner_id)['taxes']
            for tax in taxes:
                val = self._prepare_tax_line_vals(line, tax)
                key = self.env['account.tax'].browse(tax['id']).get_grouping_key(val)

                if key not in tax_grouped:
                    tax_grouped[key] = val
                else:
                    tax_grouped[key]['amount'] += val['amount']
                    tax_grouped[key]['base'] += val['base']
        return tax_grouped
    
class PurchaseOrderTax(models.Model):
    _name = "purchase.order.tax"
    _description = "Purchase Tax"
    _order = 'sequence'

    def _compute_base_amount(self):
        tax_grouped = {}
        for purchase in self.mapped('purchase_id'):
            tax_grouped[purchase.id] = purchase.get_taxes_values()
        for tax in self:
            tax.base = 0.0
            if tax.tax_id:
                key = tax.tax_id.get_grouping_key({
                    'tax_id': tax.tax_id.id,
                    'account_id': tax.account_id.id,
                    'account_analytic_id': tax.account_analytic_id.id,
                })
                if tax.purchase_id and key in tax_grouped[tax.purchase_id.id]:
                    tax.base = tax_grouped[tax.purchase_id.id][key]['base']
#                 else:
#                     raise Warning('Tax Base Amount not computable probably due to a change in an underlying tax (%s).', tax.tax_id.name)

    purchase_id = fields.Many2one('purchase.order', string='Purchase', ondelete='cascade', index=True)
    name = fields.Char(string='Tax Description', required=True)
    tax_id = fields.Many2one('account.tax', string='Tax', ondelete='restrict')
    account_id = fields.Many2one('account.account', string='Tax Account', required=True, domain=[('deprecated', '=', False)])
    account_analytic_id = fields.Many2one('account.analytic.account', string='Analytic account')
    amount = fields.Monetary()
    manual = fields.Boolean(default=True)
    sequence = fields.Integer(help="Gives the sequence order when displaying a list of invoice tax.")
    company_id = fields.Many2one('res.company', string='Company', related='account_id.company_id', store=True, readonly=True)
    currency_id = fields.Many2one('res.currency', related='purchase_id.currency_id', store=True, readonly=True)
    base = fields.Monetary(string='Base', compute='_compute_base_amount')


where i am doing wrong for purchase order tax splitting...
error is..
 
Traceback (most recent call last):
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 643, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 680, in dispatch
    result = self._call_function(**self.params)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 316, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 309, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 959, in __call__
    return self.method(*args, **kw)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/http.py", line 509, in response_wrap
    response = f(*args, **kw)
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/web/controllers/main.py", line 896, in call_button
    action = self._call_kw(model, method, args, {})
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/web/controllers/main.py", line 884, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/api.py", line 250, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/api.py", line 381, in old_api
    result = method(recs, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/custom_tax/models/custom_tax.py", line 73, in button_dummy
    self.compute_taxes()
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/api.py", line 248, in wrapper
    return new_api(self, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/custom_tax/models/custom_tax.py", line 53, in compute_taxes
    tax_grouped = purchase.get_taxes_values()
  File "/home/kundan/Desktop/odoo_dev/9.0/openerp/api.py", line 248, in wrapper
    return new_api(self, *args, **kwargs)
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/custom_tax/models/custom_tax.py", line 105, in get_taxes_values
    val = self._prepare_tax_line_vals(line, tax)
  File "/home/kundan/Desktop/odoo_dev/9.0/addons/custom_tax/models/custom_tax.py", line 90, in _prepare_tax_line_vals
    'base': tax['base'],
KeyError: 'base'