Python: Implementing Factory Method Design Pattern

Ideally, program should be closed for modifications, but open for extensions and hard-coded stuff should be avoided like plague. This is the point, where Design Patterns are usually stepping into picture. When the code base will grow enough in size and we are in charge of production-level programs, it is relatively easy to justify the usage of patterns. In this post, Python mutation of Factory Method Pattern is introduced for creating Task class objects, based on argument feed received from different types of sources. Complete program can be downloaded from my github page.

Imaginary scheme

Third-party analytical software is used for processing different types of complex calculations. One calculation request requires a set of specific arguments from the client program. Moreover, different types of requests will require different set of arguments (number of arguments, types of arguments). Before sending calculation request for analytical software, client program must collect arguments for all tasks to be processed from somewhere and we would like to leave the source for such task arguments open (json, text, console, xml, etc.).

Json source file

{
  "taskName": "ANZ.XVA",
  "taskType": "XVA",
  "paths": 1000,
  "storeExposures": "True" 
}

Text source file

taskName,RBC.PV
taskType,PV

Program

import sys, json, csv

# class for hosting calculation task-related information
# NOTE: the actual method for creating 'calculation request' is currently not implemented
class Task(object):
    def __init__(self, arguments):
        self.arguments = arguments
        
    # non-relevant dummy method, which just writes out given arguments
    def print(self):
        for k, v in self.arguments.items():
            print(k, 'is', v)
        
    @classmethod
    # factory: create task object from json file
    def from_json(cls, path)->'Task':
        arguments = json.load(open(path, 'r'))
        return cls(arguments)
    
    @classmethod
    # factory: create task object from text file
    def from_text(cls, path)->'Task':
        arguments = dict((k, v) for k, v in csv.reader(open(path, 'r')))
        return cls(arguments)
    
    @classmethod
    # factory: create task object from console input
    def from_console(cls)->'Task':
        arguments = json.loads(input())
        return cls(arguments)        

path = '/home/mikejuniperhill/json.feed/task.json'
task = Task.from_json(path)
task.print()

path = '/home/mikejuniperhill/json.feed/task.txt'
task = Task.from_text(path)
task.print()

task = Task.from_console()
# console feed string: {"taskName":"ANZ.WHATIF.XVA","taskType":"WHATIF","referenceTask":"ANZ.XVA"}
task.print()

Program output

Thanks for reading this blog.
-Mike