using names before they're defined

[Iain King]

daveho... at f2s.com wrote:
> I have a problem. I'm writing a simulation program with a number of
> mechanical components represented as objects. When I create instances
> of objects, I need to reference (link) each object to the objects
> upstream and downstream of it, i.e.
>
> supply = supply()
> compressor = compressor(downstream=combustor, upstream=supply)
> combuster = combuster(downstream=turbine, upstream=compressor)
> etc.
>
> the problem with this is that I reference 'combustor' before is it
> created. If I swap the 2nd and 3rd lines I get the same problem
> (compressor is referenced before creation).
>
>
> aargh!!! any ideas on getting around this?
>
> Dave

At the top of your code you could put:

supply = None
compressor = None
combuster = None
turbine = None

It might be better, though, to arrange your code like:
supply = Supply()
compressor = Compressor()
combuster = Combuster()
turbine = Turbine()
compressor.setStreams(down=combuster, up=supply)
combuster.setStreams(down=turbine, up=compressor)

Do the streams reflect each other?  That is, if supply.down is
compressor, is compressor.up supply?  In that case you probably want to
do something like:

class Component():

    upstream = None
    downstream = None

    def setUpstream(self, c):
        self.upstream = c
        if c.downstream != self:
            c.setDownstream(self)

    def setDownstream(self, c):
        self.downstream = c
        if c.upstream != self:
            c.setUpstream(self)

class Supply(Component):
    pass

etc.

Iain