除非重新启动flask服务器，否则条目上的默认datetime.now值不会改变。

Question

我注意到，当我在我的表单页面中提交一个条目时，除非重新开始，否则日期输入会在随后的提交中重复。为什么会出现这种情况？我发誓，重复的时间是相隔几分钟，但似乎提交的时间只是从服务器重新启动。我也使用flask-sqlalchemy。

models.py

class Entry(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(200), unique=True, nullable=False)
    date_input = db.Column(db.DateTime, nullable=False, default=datetime.now(tz=pytz.timezone('Asia/Manila')))

Entry.date_input timestamp results (copy-pasted from cli)

 1. 2020-05-09 00:59:07.269489
 2. 2020-05-09 01:08:56.676228
 3. 2020-05-09 01:10:44.032547
 4. 2020-05-09 01:10:44.032547
 5. 2020-05-09 01:10:44.032547
 6. 2020-05-09 01:10:44.032547
 7. 2020-05-09 01:23:28.636399
 8. 2020-05-09 01:23:28.636399
 9. 2020-05-09 01:23:28.636399

Actual POST logs (copy-pasted from cli)

 1. 127.0.0.1 - - [09/May/2020 01:00:55]
 2. 127.0.0.1 - - [09/May/2020 01:09:25]
 3. 127.0.0.1 - - [09/May/2020 01:14:49]
 4. 127.0.0.1 - - [09/May/2020 01:18:07]
 5. 127.0.0.1 - - [09/May/2020 01:21:39]
 6. 127.0.0.1 - - [09/May/2020 01:23:46]
 7. 127.0.0.1 - - [09/May/2020 01:25:14]
 8. 127.0.0.1 - - [09/May/2020 01:25:14]
 9. 127.0.0.1 - - [09/May/2020 01:25:56]

Server restarts
-I took the timestamps from the first GET result after running the server. 
 The initial running itself doesnt have time indicated

 1. 127.0.0.1 - - [09/May/2020 00:59:11]
 2. 127.0.0.1 - - [09/May/2020 01:09:25]
 3. 127.0.0.1 - - [09/May/2020 01:10:50]
 4. 127.0.0.1 - - [09/May/2020 01:23:31]

Answer 1

注意固定的默认值和默认函数（在需要时提供一个值）之间的区别。该 文件 显示了一个例子 datetime.now.

你现在的代码可以重写成。

dt = datetime.now(tz=pytz.timezone('Asia/Manila')) # evaluated once
date_input = db.Column(db.DateTime, nullable=False, default=dt)

为了修正代码，用一个函数代替表达式，例如： default=lambda: datetime.now(tz=...)