I have two classes that refer to each other, but obviously the compiler complains. Is there any way around this?
Actually my code is slightly different than what Hank Gay uses. So python can definitely deal with some kinds of circular references, but it tosses an error in the following situation. Below is what I've got and I get an 'name Y not defined error'
creator = Registry()
a = models.ForeignKey(X)
b = models.CharField(max_length=200)
In python, the code in a class is run when the class is loaded.
Now, what the hell does that mean? ;-)
Consider the following code:
class x: print "hello" def __init__(self): print "hello again"
When you load the module that contains the code, python will print
hello. Whenever you create an
x, python will print
You can think of
def __init__(self): ... as equivalent with
__init__ = lambda self: ..., except none of the python lambda restrictions apply. That is,
def is an assignment, which might explain why code outside methods but not inside methods is run.
When your code says
class X(models.Model): creator = Registry() creator.register(Y)
You refer to
Y when the module is loaded, before
Y has a value. You can think of
class X as an assignment (but I can't remember the syntax for creating anonymous classes off-hand; maybe it's an invocation of
What you may want to do is this:
class X(models.Model): pass class Y(models.Model): foo = something_that_uses_(X) X.bar = something_which_uses(Y)
That is, create the class attributes of
X which reference
Y is created. Or vice versa: create
Y first, then
X, then the attributes of
Y which depend on
X, if that's easier.
Hope this helps :)