Django:timezone.now与timezone.now() [英] Django: timezone.now vs timezone.now()
问题描述
我正在将项目从Django 1.8.2升级到1.9.7,并且收到此警告:
警告:
my_app.my_model.date_available:(fields.W161)提供了固定的默认值。
提示:您似乎将此字段的默认日期/时间/日期时间值设置为默认值。这可能不是您想要的。
如果您想将当前日期作为默认日期,请使用django.utils.timezone.now
这是my_app / models.py中的行:
来自django.utils导入时区
...
类my_model(models.Model):
...
datetime_released = models.DateTimeField(default = timezone.now())
如果我删除括号而改为使用:
datetime_released = models.DateTimeField(default = timezone.now)
Django警告消失了。两者之间有什么区别?
在我项目的另一个区域中,我在查询集中使用timezone.now()过滤器:
def date_available(self):
return self.filter(date_available__lte = timezone.now())
在这里,如果我删除括号,则会引发错误:
TypeError:预期的字符串或缓冲区
我可以通过按需添加/删除括号来完成这两项工作,但是 timezone.now()
和 timezone有什么区别.now
以及为什么它们会在这些情况下引起警告/错误?
在python中,一切都是一个对象,包括功能。这意味着您可以将函数影响到变量:
>>从django.utils导入时区
>> foo = timezone.now
>> foo
< function django.utils.timezone.now>
>> foo()
datetime.datetime(2016、7、7、9、11、6、489063)
一个函数是一个可调用对象:
>> callable(foo)
真
>>> callable(foo())
假
当默认时
接收一个可调用对象,每次请求默认值时都会调用该可调用对象。
另一方面,当您调用 timezone.now()
设置 default
之前,该值已指定并固定。提醒一下,由于这是类属性,因此以下行仅在服务器启动时执行一次:
datetime_released = models .DateTimeField(default = timezone.now())
并因此时区。 now()
仅执行一次。传递一个可调用的 timezone.now
可以在需要时重新计算该值。
I am in the process of upgrading my project from Django 1.8.2 to 1.9.7 and I'm getting this warning:
WARNINGS:
my_app.my_model.date_available: (fields.W161) Fixed default value provided.
HINT: It seems you set a fixed date / time / datetime value as default for this field. This may not be what you want.
If you want to have the current date as default, use `django.utils.timezone.now
Here's the line from my_app/models.py:
from django.utils import timezone
...
class my_model(models.Model):
...
datetime_released = models.DateTimeField(default=timezone.now() )
If I remove the parentheses and instead use:
datetime_released = models.DateTimeField(default=timezone.now )
The Django warning goes away. What's the difference between the two?
In another area of my project I am using timezone.now() in a queryset filter:
def date_available(self):
return self.filter(date_available__lte = timezone.now())
Here, if I remove the parentheses, an error is thrown:
TypeError: expected string or buffer
I can get both of these two work by adding/removing the parenthesis as required, but what is the difference between timezone.now()
and timezone.now
and why do they cause warnings/errors in these cases?
In python everything is an object, including functions. This means you can affect a function to a variable:
>>> from django.utils import timezone
>>> foo = timezone.now
>>> foo
<function django.utils.timezone.now>
>>> foo()
datetime.datetime(2016, 7, 7, 9, 11, 6, 489063)
A function is a callable object:
>>> callable(foo)
True
>>> callable(foo())
False
When default
receives a callable, the callable is called each time a default value is requested.
On the other hand, when you call timezone.now()
prior to setting default
, the value is given and fixed. As a reminder, the following line is executed only once at server start up, since it is a class attribute:
datetime_released = models.DateTimeField(default=timezone.now())
and thus timezone.now()
is executed only once. Passing a callable timezone.now
makes it possible to recalculate the value whenever it needs to be.
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