Django: Is it reasonable to use objects as dictionary keys?

Question

Is it reasonable to use objects as keys to a dictionary in django? I have done so and it works. But I am wondering if this is best practice, or if it is going to make difficulties I don't foresee right now.

I am working on a project which deals with educational standards. I have dictionaries with a structure along the lines of {Subject:[Standards]}. The model for subject looks something like:

class Subject(models.Model):
  subject = models.CharField(max_length=255, unique=True)

  def __unicode__(self):
    return self.subject

Is it okay to use the objects from this model as keys to my dictionaries, or should I be using a string represenation, such as Subject.subject instead?

If so, does the unicode method affect this? When I tried using Subject.subject as the key, I got things like {u'Math': [<Subject: Students can perform calculations.>]} Using objects as keys, it looks like {<Subject: Math>: [<Standard: Students can perform calculations.>]}

This is a followup to a question I asked yesterday about using None as a dictionary key.

score 13 · Accepted Answer · answered Aug 12 '11 at 15:38

13

Mutable objects shouldn't really be used as dictionary keys. That said, this works because the base model class defines __hash__ in terms of the model's primary key, which is unlikely to change. But I would prefer to use the pk directly as the key.

answered Aug 12 '11 at 15:38

Daniel Roseman

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Is this as simple as `def __hash__(self): return self.id`? If so, why does that need to be made explicit? Or do you mean I should define my dictionaries as `{subject.id:[standard.id]}`? – japhyr Aug 12 '11 at 16:29
This has the same idea as Alexandru's response, but is a little more specific. – japhyr Aug 12 '11 at 18:49

Alex Plugaru · Answer 2 · 2011-08-12T16:24:14.930

3

It depends how you want to use them. I'd suggest a simpler approach though:

The keys of the dictionaries could be the Model's primary key.

edited Aug 12 '11 at 16:24

answered Aug 12 '11 at 15:39

Alex Plugaru

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score -1 · Answer 3 · answered Aug 12 '11 at 15:36

-1

Assuming the objects implement a good hash function I'd say that there's nothing wrong with using objects as keys, but that is just my personal opinion.

answered Aug 12 '11 at 15:36

Exelian

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score -1 · Answer 4 · answered Aug 12 '11 at 15:37

It would be better to use string representation since, when you will need to do looks up you will need to write out all that stuff, which will be a pain, plus if in the future you want to change your unicode representation you will have to find a way of rewriting the old unicode to do look ups.

Goodluck.

Django: Is it reasonable to use objects as dictionary keys?

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