general class functions

[Kent Johnson]

syd wrote:
> In my project, I've got dozens of similar classes with hundreds of
> description variables in each.  In my illustrative example below, I
> have a Library class that contains a list of Nation classes.  In one
> case, I might want a Library class with only Nations of with the
> continent variable "Europe", and so I'll do something like
> library.getContinent('Europe') which will return a Library() instance
> with only European nations.  Similarly, I could also want a Library()
> instance with only 'small' size nations.
> 
> My question follows: is there a way I can make a general function for
> getting a subclass with my specific criteria?  For instance, can I
> pass in an equivalence test or something like that?  I have hundreds
> of variables like "nation", "continent", and so forth, and do want to
> write a separate "get______" function for each.

You can use getattr to read the Nation attributes by name. Add this 
method to Library:
   def getMatches(self,attribute, value):
     library=Library()
     for nation in self.list:
       if getattr(nation, attribute) == value:
         library.add(nation)
     return library

Now you can call
print library.getMatches('continent', 'Europe').getNameList()
print library.getMatches('continent', 'Europe').getMatches('size', 
'small').getNameList()

Is that what you want?
Kent

> 
> This stumped everyone I work with.  Help would be greatly
> appreciated!!
> 
> #!/usr/bin/python
> class Library:
>   def __init__(self,list=None):
>     self.list=list
>   def add(self,nation):
>     try: self.list.append(nation)
>     except: self.list=[nation]
>   def defineNation(self,name,continent,size):
>     nation=Library.Nation()
>     nation.name=name
>     nation.continent=continent
>     nation.size=size
>     self.add(nation)
>   def getContinent(self,continent):
>     library=Library()
>     for nation in self.list:
>       if nation.isContinent(continent)==True:
>         library.add(nation)
>     return library
>   def getSize(self,size):
>     library=Library()
>     for nation in self.list:
>       if nation.isSize(size)==True:
>         library.add(nation)
>     return library
>   def getNameList(self):
>     outList=[]
>     for nation in self.list:
>       outList.append(nation.name)
>     return outList
>   class Nation:
>     def __init__(self,name=None,continent=None,size=None):
>       self.name=name # eg, Spain
>       self.continent=continent # eg, Europe
>       self.size=size # eg, medium
>     def isContinent(self,continent):
>       if self.continent==continent: return True
>       else: return False    def isSize(self,size):
>       if self.size==size: return True
>       else: return False
> 
> library=Library()
> library.defineNation('Spain','Europe','medium')
> library.defineNation('USA','NorthAmerica','large')
> library.defineNation('Luxembourg','Europe','small')
> library.defineNation('Portugal','Europe','medium')
> 
> print library.getNameList()
> print library.getContinent('Europe').getNameList()
> print library.getContinent('Europe').getSize('small').getNameList()