Factory Method Pattern in Python

Ok, here we go again. This time, I will demonstrate Factory Method Design Pattern  


Let's say we need to design a module to retrieve 4 statistics counters, 

1. Bytes Transmitted 
2. Bytes Received
3. Packet Transmitted
4. Packets Received

So, we start with creating a base class and 4 inherited classes:

1:  class StatsClass(object):  
2:    
3:    def __init__(self):  
4:      self._counter = 0  
5:      self._name ="Abtract Statistics Class"  
6:        
7:    def Get_Name(self):  
8:      return self._name  
9:      
10:    def Get(self):  
11:      return self._counter;  
12:      
13:    def Reset(self):  
14:      self._counter = 0;  
15:      
16:    def Set(self,counter):  
17:      self._counter = counter;  
18:        
19:    def PollFromHw(self):  
20:      print(self._name, "is poll statistics counter from hardware register")  
21:      
22:    
23:  class StatsBytesTransmitted(StatsClass):  
24:    def __init__(self):  
25:      self._name = "Bytes Transmitted"  
26:      self._counter = 0    
27:    
28:  class StatsBytesReceived(StatsClass):  
29:    def __init__(self):  
30:      self._name = "Bytes Received"  
31:      self._counter = 0  
32:    
33:  class StatsPacketsTransmitted(StatsClass):  
34:    def __init__(self):  
35:      self._name = "Packets Transmitted"  
36:      self._counter = 0  
37:      
38:  class StatsPacketsReceived(StatsClass):  
39:    def __init__(self):  
40:      self._name = "Packets Received"  
41:      self._counter = 0  
42:        

Then, create a Factory Class

1:  # Factory Class  
2:  class AbstractStatsFactory():  
3:    def CreateStats(self,strStats):  
4:      pass  
5:    
6:  class StatsFactory(AbstractStatsFactory):  
7:    def CreateStats(self,strStats):  
8:      if( strStats == "Bytes Received"):  
9:        StatsObj = StatsBytesReceived()  
10:      elif ( strStats == "Bytes Transmitted"):  
11:        StatsObj = StatsBytesTransmitted()  
12:      elif ( strStats == "Packets Transmitted"):  
13:        StatsObj = StatsPacketsTransmitted()    
14:      elif ( strStats == "Packets Received"):  
15:        StatsObj = StatsPacketsReceived()    
16:      else:  
17:        StatsObj = None    
18:    
19:      return StatsObj  

Now, let's put them into use

1:  def Use_StatsObj(StatsObj):   
2:    StatsObj.PollFromHw()  
3:    StatsObj.Set(100)  
4:    print(StatsObj.Get_Name(), "counter is ", StatsObj.Get())    
5:    StatsObj.Reset()  
6:    print(StatsObj.Get_Name(), "counter Reset. Now counter is ", StatsObj.Get())    
7:    
8:    
9:    
10:  strStats = "Bytes Received"  
11:  StatsObj = StatsFactory().CreateStats(strStats)  
12:  Use_StatsObj(StatsObj)  
13:    
14:  strStats = "Bytes Transmitted"  
15:  StatsObj = StatsFactory().CreateStats(strStats)  
16:  Use_StatsObj(StatsObj)  

The output is 

 Bytes Received is poll statistics counter from hardware register  
 Bytes Received counter is 100  
 Bytes Received counter Reset. Now counter is 0  
 Bytes Transmitted is poll statistics counter from hardware register  
 Bytes Transmitted counter is 100  
 Bytes Transmitted counter Reset. Now counter is 0  

No bad so far, though, I am not satisfied.

There is still an if-elif-else statement in the StatsFactory Class. It will be nice if the Factory class knows how many Statistics Objects are there, without explicitly instantiated these concrete objects .


In OO theme, you can use Reflection to do so.  In Python, this is how I implement it

1:  class StatsFactory2(AbstractStatsFactory):  
2:    def CreateStats(self,strStats):  
3:      # Get all subclass  
4:      for childClass in StatsClass.__subclasses__() :  
5:        obj = childClass()  
6:        if( strStats == obj.Get_Name()):  
7:          return obj  
8:    def ListStatsObjNames(self):  
9:      objList = list()  
10:      for childClass in StatsClass.__subclasses__() :  
11:        # print(childClass)  
12:        obj = childClass()  
13:        objList.append(obj.Get_Name())  
14:      return objList   
15:        
16:    

Now, it's time to put the code into test

1:  print("--- now use New Factory --- ")  
2:  strStats = "Bytes Received"  
3:  StatsObj = StatsFactory2().CreateStats(strStats)  
4:  Use_StatsObj(StatsObj)  
5:    
6:  print("Counter can be read: ", StatsFactory2().ListStatsObjNames())  
7:  while(True):  
8:    strStats = input("Which counter do you want to know ?");  
9:    StatsObj = StatsFactory2().CreateStats(strStats)  
10:    if( StatsObj == None ):  
11:      break  
12:    Use_StatsObj(StatsObj)  
13:    

This is the Output

 --- now use New Factory ---   
 Bytes Received is poll statistics counter from hardware register  
 Bytes Received counter is 100  
 Bytes Received counter Reset. Now counter is 0  
 Counter can be read: ['Bytes Transmitted', 'Bytes Received', 'Packets Transmitted', 'Packets Received']  
 Which counter do you want to know ?Bytes Transmitted  
 Bytes Transmitted is poll statistics counter from hardware register  
 Bytes Transmitted counter is 100  
 Bytes Transmitted counter Reset. Now counter is 0  
   

Now, this is beauty part -- let's add two new statistics counters right after the  StatsPacketsReceived 

1:  class StatsFcsError(StatsClass):  
2:    def __init__(self):  
3:      self._name = "Fcs Error"  
4:      self._counter = 0  
5:    
6:  class StatsDiscardPacket(StatsClass):  
7:    def __init__(self):  
8:      self._name = "Discard Packets"  
9:      self._counter = 0    
10:    

Then, run my test code without any change. Voila, the two new classes are automatically recognized and can be used. 

 --- now use New Factory ---   
 Bytes Received is poll statistics counter from hardware register  
 Bytes Received counter is 100  
 Bytes Received counter Reset. Now counter is 0  
 Counter can be read: ['Bytes Transmitted', 'Bytes Received', 'Packets Transmitted', 'Packets Received', 'Fcs Error', 'Discard Packets']  
 Which counter do you want to know ?Discard Packets  
 Discard Packets is poll statistics counter from hardware register  
 Discard Packets counter is 100  
 Discard Packets counter Reset. Now counter is 0  
 Which counter do you want to know ?  

Beautiful, isn't it ?