using names before they're defined

[Bruno Desthuilliers]

davehowey at f2s.com a écrit :
> Hiya
> 
> Could you just talk me through this... is it:
> 
> 
>>schema = {'turbine1': {'class': 'Turbine',
>>                       'upstream' : ('frobnicator2',),
>>                       'downstream' : () # nothing,
>>                       },
>>          'frobnicator2' : {'class' : 'Frobnicator',
>>                            'upstream' : (),
>>                            'downstream' : ('frobnicator2',),
>>                           },
>>         }
>>
> 
> 
> ok, so schema is a dictionary of different components, defining what
> class they are and what they're connected to.

Yeps.

Note that it could as well be a list of tuple or anything like this, ie:
schema = [
   ('turbine1', 'Turbine', ('frobnicator2',), ()),
   ('frobnicator2', 'Frobnicator', (), ('frobnicator2',),
   ]

I choose a dict for readability.

Also, I gave the example using Python code as 'config' format, but any 
structured enough text format could do, ie JSON, XML, or even ini-like:

# schema.ini
objects = turbine1, frobnicator2

[turbine1]
class=Turbine
upstream=frobnicator2
downstream=

[frobnicator2]
class=Frobnicator
upstream=
downstream=turbine


Now you just read the file with the standard config parser and build 
your chain from it... (implementation left as an exercice...)


>>def get_class_by_name(name):
>>  return globals()[name]
> 
> 
> what does this function do exactly?

Q&D way to retrieve the class object (Python's classes are themselves 
objects) known by it's name (as a string).

globals() returns the module's namespace as a dict object with 
'name':object pairs. This code assume that the name is available in the 
current module's namespace, but could be improved to handle needed 
imports etc.

>>def chain_from_schema(schema):
>>  objects = {}   # so objects is our list of objects ?
>>  for name, desc in schema:   
# can you step through a dictionary like this?

Nope, sorry, it's a mistake I do over and over. It's of course:;
     for name, desc in schema.items():

(note that you can iterate over a dict's keys with 'for k in thedict:' )

>>     klass =  get_class_by_name(desc['class'])   
# does this create an object called klass?

Nope, it binds the class object (retrieved by get_class_by_name()) to 
the local name 'klass' ('class' is a reserved name).

>>     objects[name] = klass()
> 
> sorry for being dim..

Where ?