[Tutor] Declaration order of classes... why it is important?

[Kent Johnson]

On Wed, Aug 26, 2009 at 12:25 PM, Mac Ryan<quasipedia at gmail.com> wrote:
> Hello everybody,
>
>        I am using "storm" (https://storm.canonical.com/) to manage my
> database. In storm, relationships between tables (each table is
> represented by a class) are expressed like this (line #4):
>
> 1 >>> class Employee(Person):
> 2 ...     __storm_table__ = "employee"
> 3 ...     company_id = Int()
> 4 ...     company = Reference(company_id, Company.id)
>
> where Company is another class. Now, what I noticed is that Company must
> be declared as a class before Employee, or python will throw an
> exception (Company is not defined).
>
> I would be interested in understanding why this is so designed. I
> expected that the exception would not be thrown at all, as I imagined
> that the interpreter simply kept track of where classes were declared
> and would try to evaluate the code only once an actual object would be
> instantiated (at that point the interpreter would know where to look for
> each class code).

The body of a class definition is executed when it is encountered. The
result is a class object (an instance of 'type', usually). Any names
defined at class scope become attributes of the class. The class name
becomes a reference to the class object.

> BTW, the behaviour I am describing is exactly what happens with function
> declaration: the following code evaluates as expected, indeed.
>
> def fone():
>  ftwo()
> def ftwo():
>  print "hello"
> fone()

Yes, functions are different than classes. The body of a function is
not executed until it is called.

Note that class methods behave like functions (well, they are
functions) - they are not executed until called. But statements at
class scope are executed to create the class.
>
> I would also be interested in knowing if there is a way around this or
> if I simply have to live with it.

You have to live with it unless you can put the attributes inside a
method. In this case, I don't think that will work.

Kent