CODEWITHSUNDEEP
pythondjango
Shamanu4
Shamanu4

Reputation: 5406

Django datetime issues (default=datetime.now())

I have the below db model:

from datetime import datetime    

class TermPayment(models.Model):
    # I have excluded fields that are irrelevant to the question
    date = models.DateTimeField(default=datetime.now(), blank=True)

I add a new instance by using the below:

tp = TermPayment.objects.create(**kwargs)

My issue: all records in database have the same value in date field, which is the date of the first payment. After the server restarts, one record has the new date and the other records have the same as the first. It looks as if some data is cached, but I can't find where.

database: mysql 5.1.25

django v1.1.1

Upvotes: 391

Views: 489581

Answers (10)

Manoj Parmar
Manoj Parmar

Reputation: 79

if you need only DateField try this

date = models.DateField(auto_now=False, auto_now_add=False, null=True, blank=True)

if you need Both Date and Time try this

date = models.DateTimeField(auto_now_add=True, null=True, blank=True)

Upvotes: 3

Bosco Oludhe
Bosco Oludhe

Reputation: 31

In Django 3.0 auto_now_add seems to work with auto_now

reg_date=models.DateField(auto_now=True,blank=True)

Upvotes: 3

andilabs
andilabs

Reputation: 23282

Instead of using datetime.now you should be really using from django.utils.timezone import now

Reference:

so go for something like this:

from django.utils.timezone import now


created_date = models.DateTimeField(default=now, editable=False)

Upvotes: 209

David
David

Reputation: 247

datetime.now() is being evaluated once, when your class is instantiated. Try removing the parenthesis so that the function datetime.now is returned and THEN evaluated. I had the same issue with setting default values for my DateTimeFields and wrote up my solution here.

Upvotes: 9

MicahT
MicahT

Reputation: 383

David had the right answer. The parenthesis () makes it so that the callable timezone.now() is called every time the model is evaluated. If you remove the () from timezone.now() (or datetime.now(), if using the naive datetime object) to make it just this:

default=timezone.now

Then it will work as you expect:
New objects will receive the current date when they are created, but the date won't be overridden every time you do manage.py makemigrations/migrate.

I just encountered this. Much thanks to David.

Upvotes: 22

Andrew Harris
Andrew Harris

Reputation: 2095

The answer to this one is actually wrong.

Auto filling in the value (auto_now/auto_now_add isn't the same as default). The default value will actually be what the user sees if its a brand new object. What I typically do is: 

date = models.DateTimeField(default=datetime.now, editable=False,)

Make sure, if your trying to represent this in an Admin page, that you list it as 'read_only' and reference the field name

read_only = 'date'

Again, I do this since my default value isn't typically editable, and Admin pages ignore non-editables unless specified otherwise. There is certainly a difference however between setting a default value and implementing the auto_add which is key here. Test it out!

Upvotes: 7

ars
ars

Reputation: 123468

From the Python language reference, under Function definitions:

Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that that same “pre-computed” value is used for each call.

Fortunately, Django has a way to do what you want, if you use the auto_now argument for the DateTimeField:

date = models.DateTimeField(auto_now=True)

See the Django docs for DateTimeField.

Upvotes: 7

Bartosz
Bartosz

Reputation: 6193

The datetime.now() is evaluated when the class is created, not when new record is being added to the database.

To achieve what you want define this field as:

date = models.DateTimeField(auto_now_add=True)

This way the date field will be set to current date for each new record.

Upvotes: 13

Carson Myers
Carson Myers

Reputation: 38564

it looks like datetime.now() is being evaluated when the model is defined, and not each time you add a record.

Django has a feature to accomplish what you are trying to do already:

date = models.DateTimeField(auto_now_add=True, blank=True)

or

date = models.DateTimeField(default=datetime.now, blank=True)

The difference between the second example and what you currently have is the lack of parentheses. By passing datetime.now without the parentheses, you are passing the actual function, which will be called each time a record is added. If you pass it datetime.now(), then you are just evaluating the function and passing it the return value.

More information is available at Django's model field reference

Upvotes: 774

mykhal
mykhal

Reputation: 19895

From the documentation on the django model default field:

The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.

Therefore following should work:

date = models.DateTimeField(default=datetime.now,blank=True)

Upvotes: 34

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