[Tutor] creating variables at runtime

[Lloyd Kvam]

An interesting thread.

I've been using globals() in kind of the reverse way to create class instances based
on the desired class name, sort of a quick Class Factory Method.
	instance = globals()['Classname']()
I am enclosing a python snippet that (I think) shows why this makes sense.  I have not
found this in the text books.

class Site(Table):
	fk_obj_list = ['Reservation','Siteprice']
class Siteprice(Table):
class Reservation(Table):

class Table(Record,DataSet):
	def __getattr__(self, key):
		''' trap attempts to fetch related Table objects
		'''
		if key in self.fk_obj_list:
			obj = globals()[key]()	# <== Factory method ?????
			...
			# run SQL to get relevant data into obj
			...
			obj.Parent = self		# remember where we came from
			return obj
		else:
			return Record.__getattr__(self, key)	

site = Site()
# run SQL to choose a site
reservations = site.Reservation
# reservations contains DataSet of all reservations for the site


The general approach was inspired by "Python Programming on Win32", but the
	obj = globals()[key]()
must be blamed on me.  It sort of seems like cheating, but has worked nicely for us.

Andy W wrote:

> Sorry for butting in and all, but I couldn't help but throw my two cents in
> :o)
> (A solution)
> 
> 
>>Note shell prompts:
>>
>> >>> def shell():
>>         while 1:
>>            i = raw_input("Name: ")
>>            if not i:
>>                break
>>            names.append(i)
>>            exec(i + " = C()")
>>
>>
>> >>> class C:
>>         pass
>>
>> >>> names = []
>> >>> shell()
>>Name: A
>>Name: B
>>Name: C
>>Name:
>>
>> >>> dir()
>>['C', '__builtins__', '__doc__', '__name__', 'names', 'shell']
>>
>>A,B and C are gone -- the ones I created in the loop (the
>>C here is the original class definition).
>>
>>Because the exec(i + " = C()") statement creates new variables
>>*locally* to the shell() function -- then they get tossed.
>>
>>So that was my challenge:  how do you write a raw_input loop
>>function *in the shell* that creates new variables at the
>>top level?
>>
> 
>>>>def shell():
>>>>
>              while 1:
>                 i = raw_input("Name: ")
>                 if not i:
>                     break
>                 globals()[i]=C()
> 
> 
>>>>class C:
>>>>
>              pass
> 
> 
>>>>shell()
>>>>
> Name: a
> Name: b
> Name: c
> Name:
> 
> 
>>>>dir()
>>>>
> ['C', '__builtins__', '__doc__', '__name__', 'a', 'b', 'c', 'shell']
> 
> Andy
> 
> 
>>Sorry again for not being clear.
>>
>>Kirby
>>
>>
>>
>>_______________________________________________
>>Tutor maillist  -  Tutor@python.org
>>http://mail.python.org/mailman/listinfo/tutor
>>
>>
> 
> 
> _______________________________________________
> Tutor maillist  -  Tutor@python.org
> http://mail.python.org/mailman/listinfo/tutor
> 
> 


-- 
Lloyd Kvam
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