Warn when max_length is used with IntegerField

[David Eyk]

An IntegerField with a max_length silently truncates integers. Observe:

>>> class MyModel(Model):
>>>    num = IntegerField(max_length=2)

>>> m = MyModel.objects.create(num=42)
>>> m.num
42
>>> m.num = 427
>>> m.num
427
>>> m.save()
>>> m.num
42

This violates the Zen of Python: "Errors should never pass silently." It has also has been a reliable source of fun bugs over the years. This is long-standing Procrustean behavior, since at least v1.0 I think, if not before, but it bit me for the nth time yesterday, so I'm filing a bug.

[Tim Graham]

I am not sure how to proceed given in #22637 "the opinion was that the max_length option should not be forbidden in order to keep Django upward compatible, but that it also does not make sense for the IntegerField. I suppose we could add a warning using the checks framework (which could be silenced by those intentionally using max_length on IntegerField for some reason). Do you have any suggestions?

[David Eyk]

Even a warning would be splendid. Silence is the killer here.

[Mattia Procopio]

What backend are you using? With SQLite and postgresql we cannot observe this behaviour (truncating numbers)

[David Eyk]

I'm using django.db.backends.postgresql_psycopg2, and I've seen the behavior with Postgres 8.4 and 9.0. I should mention that we're on Django 1.4--I haven't tested this with a newer Django yet, but as I said, I've been aware of the behavior since 1.0 or so. My code session above is lightly edited from an actual session (obviously the model def needs to be in a file and there's a syncdb or migration implicit as well).

I'll try reproducing the behavior in Django 1.7 later, just to be sure.

[David Eyk]

Well bless me, now I can't reproduce this either, even on Django 1.1 and Postgres 8.4. Perhaps I've been taking crazy pills all this time? max_length appears to have no effect whatsoever. Please feel free to invalidate this ticket, and I apologize for wasting your time!

[Burhan Khalid]

Can't reproduce this against sqlite or postgresql (9.3) with django 1.7.1 final

[Mattia Procopio]

Adding a warning could be still maybe ok to just let people know about that. Thoughts?

[Mattia Procopio]

I wrote a small patch for this, you can find it here. ​https://github.com/django/django/pull/3620
Is it ok for the intent?

[Collin Anderson]

Hi, Do you want to hit the "Create pull request" button?

[Tim Graham <timograham@…>]

In e9d1f1182aaccaa8b60cf6a3491f0103d2bb0229:

Fixed #23801 -- Added warning when max_length is used with IntegerField

[Tim Graham <timograham@…>]

In c5129124:

Refs #23801 -- Made integer field max_length warning show correct field type.