关于python:openerp7：如何在创建表单中设置默认日期？

OpenERP 7 : How can I set a default date in a create form?

我不熟悉openerp和python，我试图在创建表单中设置一个默认日期，该日期必须是用户使用创建表单后28天。

我试过的最后一件事是：

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from datetime import datetime
from datetime import date
from datetime import timedelta
from dateutil.relativedelta import relativedelta

from openerp.osv import fields, osv

class sale_order_dates(osv.osv):
_inherit = 'sale.order'

_columns = {
'requested_date': fields.date('Requested Date', help="Date requested by the customer for the sale."),
}

_defaults = {
'requested_date': date.today() + timedelta(days=28),
}

sale_order_dates()

但是如果我打开创建表单，就会得到这个错误：

"服务器遇到内部错误，无法完成您的请求。服务器过载或应用程序出错。"

这可能是因为我不能在默认情况下进行操作，但是我不知道该怎么做，我试图在函数中进行操作，但是我对函数还不是很满意。你知道我该怎么做吗？提前谢谢

编辑：这是计算机终端上的错误消息

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2015-04-30 19:50:40,217 8666 ERROR Armand werkzeug: Error on request:
Traceback (most recent call last):
File"/usr/local/lib/python2.7/dist-packages/werkzeug/serving.py", line 180, in run_wsgi
execute(self.server.app)
File"/usr/local/lib/python2.7/dist-packages/werkzeug/serving.py", line 168, in execute
application_iter = app(environ, start_response)
File"/home/odoo/server/7.0/openerp/service/wsgi_server.py", line 417, in application
return application_unproxied(environ, start_response)
File"/home/odoo/server/7.0/openerp/service/wsgi_server.py", line 403, in application_unproxied
result = handler(environ, start_response)
File"/home/odoo/web/7.0/addons/web/http.py", line 528, in __call__
return self.dispatch(environ, start_response)
File"/home/odoo/web/7.0/addons/web/http.py", line 487, in __call__
return self.app(environ, start_wrapped)
File"/usr/local/lib/python2.7/dist-packages/werkzeug/wsgi.py", line 591, in __call__
return self.app(environ, start_response)
File"/home/odoo/web/7.0/addons/web/http.py", line 487, in __call__
return self.app(environ, start_wrapped)
File"/usr/local/lib/python2.7/dist-packages/werkzeug/wsgi.py", line 591, in __call__
return self.app(environ, start_response)
File"/home/odoo/web/7.0/addons/web/http.py", line 553, in dispatch
result = handler(request)
File"/home/odoo/web/7.0/addons/web/http.py", line 618, in <lambda>
return lambda request: JsonRequest(request).dispatch(method)
File"/home/odoo/web/7.0/addons/web/http.py", line 251, in dispatch
body = simplejson.dumps(response)
File"/usr/local/lib/python2.7/dist-packages/simplejson/__init__.py", line 370, in dumps
return _default_encoder.encode(obj)
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 271, in encode
chunks = list(chunks)
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 632, in _iterencode
for chunk in _iterencode_dict(o, _current_indent_level):
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 591, in _iterencode_dict
for chunk in chunks:
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 591, in _iterencode_dict
for chunk in chunks:
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 642, in _iterencode
o = _default(o)
File"/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 246, in default
raise TypeError(repr(o) +" is not JSON serializable")
TypeError: datetime.date(2015, 5, 28) is not JSON serializable

所以看起来是默认值中的操作不正确，可能这两个字段也不兼容，但我不知道应该使用什么。