Question

I have the following Django model:

   created=models.DateTimeField(auto_now_add=True)

I now need a model object method which shows the # of hours since creation. I tried the following:

  def hours_live(self):
    diff=((datetime.datetime.now - self.created).seconds)/3600
    return diff   

but it threw a TypeError

unsupported operand type(s) for -: 'builtin_function_or_method' and 'datetime.datetime'

I then went with

 def hours_live(self):
    diff=((datetime.datetime.now() - self.created).seconds)/3600
    return diff   

My question is whether the datetime.datetime.now() expression is executed only one time here or whether it executes each time hours_live is called.

Will this work?

Answer 1

In your example, it will be executed every time the function is called. The gotcha is when datetime.datetime.now() is used as a default value for a parameter in a function definition. In that case, it is executed just once when the module is loaded.

For example: (this is dangerous because since is calculated only once when the module is loaded):

def hours_live(since=datetime.datetime.now()):
    return since - self.created

You should rather do:

def hours_live(since=None):
    if not since:
        since = datetime.datetime.now()
    return since - self.created