Thursday, August 31, 2017

Re: Weird date math bug?

On Aug 31, 2017 9:44 PM, "Lachlan Musicman" <> wrote:
Sorry, let me start again:

Yay, I'm not the only one that does it! Lol...


class ChemoRegime(models.Model):
    patient = models.ForeignKey(Patient, on_delete=models.CASCADE)
    chemotype = models.ForeignKey(ChemoType, on_delete=models.CASCADE)
    start_date = models.DateField(null=True, blank=True)
    stop_date = models.DateField(null=True, blank=True)

    def regime_age(self):
        age = - self.stop_date
        return age.days


But in the browser:

Exception Type:     TypeError
Exception Value:    

unsupported operand type(s) for -: '' and 'NoneType'

Exception Location:     ./patients/ in regime_age, line 95

line 95 is         age = - self.stop_date

What am I doing wrong?


I'm guessing that ChemoRegime.regime_age() contains line 95.

ChemoRegime defines the stop_date field with null=True and blank=True. I'm guessing this is because patients may not have yet stopped treatment, so they have no stop_date.

With that in mind, self.stop_date may be None in Python (and Null in the DB). Subtracting None from a real object is not supported.

Ensure that stop_date has a date value before doing date math with it.


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