root/patterns/proxy.py

'''
    Quick implmentation of the Proxy (aka Surrogate) pattern [GoF94]
    
    [Gof94] Erich Gamma, Richard Helm, Ralph E. Johnson, John Vlissides (the Gang of Four)
        Design Patterns - Elements of Object-Oriented Reusable Software Addison-Wesley
        Reading, Massachusetts, 1994
'''

class File(object):
    def load(self):
        pass
        
class LargeFile(File):
    def __init__(self, name):
        self.name = name
        self.load()
        
    def load(self):
        print "Loading %s..." % (self.name)
    def process1(self):
        print "[phase1] Processing %s..." % (self.name)
    def process2(self):
        print "[phase2] Processing %s..." % (self.name)
        
class ProxyFile(File):
    def __init__(self, name):
        self.name = name
        self.file = None
        
    def process1(self):
        if not self.file:
            self.file = LargeFile(self.name)
        self.file.process1()

    def process2(self):
        if not self.file:
            self.file = LargeFile(self.name)
        self.file.process2()
         
def main():
    f1 = ProxyFile("bigdb01.csv")
    f2 = ProxyFile("bigdb02.csv")
    f3 = ProxyFile("bigdb03.csv")
    
    f1.process1()
    # some busines logic
    f1.process2()
    # more BL
    f2.process2()
    # more BL
    f2.process1()
    # Hey, we found what we needed, skipped f3
    #f3.process()
    
if __name__ == '__main__':
    main()