| Wait. How do I define the user-friendly stuff that will show up in the select box for the admin interface?
Todd On Mar 29, 2006, at 9:59 PM, Ned Batchelder wrote: What I've done in these cases is to define a Constants class:
class Constants:
""" Construct one of these with keyword arguments, and you can use the
attributes.
"""
def __init__(self, **kwargs):
for k, v in kwargs.items():
setattr(self, k, v)
def choices(self):
return list(self.__dict__.items()) (I guess Enumeration would be a better name), then I can define a list of constants:
kBranchKind = Constants(
main = 1,
aux = 2,
dead = 3
) Then in the code, you can use kBranchKind.dead, and in your model, you can use:
class Branch(meta.Model):
trunk = meta.ForeignKey(Trunk)
kind = meta.IntegerField(choices=kBranchKind.choices()) It keeps the list of choices in one place, gives you run-time errors if you mistype the constant name (string literals would not), and it works just as well with strings for the values.
--Ned.
Ivan Sagalaev wrote: Todd O'Bryan wrote:
Your comment at the end got me thinking, though. Writing
trunk.get_branch(kind__exact=2)
is not very illuminating, but you're correct that the value 'Dead'
could get changed later. In Java, I'd use constants for the integer
values
public static final int DEAD = 2;
but that seems to violate DRY, because the semantics is already
listed in the choices list. I like using integers for what end up
being enumerated types because they don't take much space in the
database and, as you mentioned, it's easy to change the English
version without having to do anything to the db representation.
Is there a better way to do this kind of thing?
This got me thinking too :-)
Generally when I need a constant in Python I don't hesitate to use
string values for constants which are both values and names. So I'd have
BRANCH_KINDS = (('main', 'Main'), ('aux', 'Auxiliary'), ('dead',
'Dead'),)
I think it won't even hurt performance in DB lookups if you create index
for this field. However this implies changing the field to CharField
which won't become a box in admin and in automatic manipulators
(if I'm not mistaken). A lookup table for branch kinds would solve this:
class BranchKind(meta.Model):
id = meta.SlugField(primary_key=True)
title = meta.CharField(maxlength=50)
You can then do something like
trunk.get_branch(pk='dead')
.
--
Ned Batchelder, http://nedbatchelder.com
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