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Item 39: Use @classmethod Polymorphism to Construct Objects Generically

In Python, not only do objects support polymorphism, but classes do as well. What does that mean, and what is it good for?

Polymorphism enables multiple classes in a hierarchy to implement their own unique versions of a method. This means that many classes can fulfill the same interface or abstract base class while providing different functionality (see Item 43: “Inherit from collections.abc for Custom Container Types”).

For example, say that I’m writing a MapReduce implementation, and I want a common class to represent the input data. Here, I define such a class with a read method that must be defined by subclasses:

class InputData:
    def read(self):
        raise NotImplementedError

I also have a concrete subclass of InputData that reads data from a file on disk:

class PathInputData(InputData):
    def __init__(self, path):
        self.path = path

     def read(self):
         with open(self.path) as f:
             return f.read()

I could have any number of InputData subclasses, like PathInputData, and each of them could implement the standard interface for read to return the data to process. Other InputData subclasses could read from the network, decompress data transparently, and so on.

I’d want a similar abstract interface for the MapReduce worker that consumes the input data in a standard way:

class Worker:
    def __init__(self, input_data):
        self.input_data = input_data
        self.result = None

    def map(self):
        raise NotImplementedError

    def reduce(self, other):
        raise NotImplementedError

Here, I define a concrete subclass of Worker to implement the specific MapReduce function I want to apply—a simple newline counter:

class LineCountWorker(Worker):
    def map(self):
        data = self.input_data.read()
        self.result = data.count('\n')
    def reduce(self, other):
        self.result += other.result

It may look like this implementation is going great, but I’ve reached the biggest hurdle in all of this. What connects all of these pieces? I have a nice set of classes with reasonable interfaces and abstractions, but that’s only useful once the objects are constructed. What’s responsible for building the objects and orchestrating the MapReduce?

The simplest approach is to manually build and connect the objects with some helper functions. Here, I list the contents of a directory and construct a PathInputData instance for each file it contains:

import os

def generate_inputs(data_dir):
    for name in os.listdir(data_dir):
        yield PathInputData(os.path.join(data_dir, name))

Next, I create the LineCountWorker instances by using the InputData instances returned by generate_inputs:

def create_workers(input_list):
    workers = []
    for input_data in input_list:
    return workers

I execute these Worker instances by fanning out the map step to multiple threads (see Item 53: “Use Threads for Blocking I/O, Avoid for Parallelism” for background). Then, I call reduce repeatedly to combine the results into one final value:

from threading import Thread

def execute(workers):
    threads = [Thread(target=w.map) for w in workers]
    for thread in threads: thread.start()
    for thread in threads: thread.join()

    first, *rest = workers
    for worker in rest:
    return first.result

Finally, I connect all the pieces together in a function to run each step:

def mapreduce(data_dir):
    inputs = generate_inputs(data_dir)
    workers = create_workers(inputs)
    return execute(workers)

Running this function on a set of test input files works great:

import os
import random

def write_test_files(tmpdir):
    for i in range(100):
        with open(os.path.join(tmpdir, str(i)), 'w') as f:
            f.write('\n' * random.randint(0, 100))

tmpdir = 'test_inputs'

result = mapreduce(tmpdir)
print(f'There are {result} lines')

There are 4360 lines

What’s the problem? The huge issue is that the mapreduce function is not generic at all. If I wanted to write another InputData or Worker subclass, I would also have to rewrite the generate_inputs, create_workers, and mapreduce functions to match.

This problem boils down to needing a generic way to construct objects. In other languages, you’d solve this problem with constructor polymorphism, requiring that each InputData subclass provides a special constructor that can be used generically by the helper methods that orchestrate the MapReduce (similar to the factory pattern). The trouble is that Python only allows for the single constructor method __init__. It’s unreasonable to require every InputData subclass to have a compatible constructor.

The best way to solve this problem is with class method polymorphism. This is exactly like the instance method polymorphism I used for InputData.read, except that it’s for whole classes instead of their constructed objects.

Let me apply this idea to the MapReduce classes. Here, I extend the InputData class with a generic @classmethod that’s responsible for creating new InputData instances using a common interface:

class GenericInputData:
    def read(self):
        raise NotImplementedError

    def generate_inputs(cls, config):
        raise NotImplementedError

I have generate_inputs take a dictionary with a set of configuration parameters that the GenericInputData concrete subclass needs to interpret. Here, I use the config to find the directory to list for input files:

class PathInputData(GenericInputData):

   def generate_inputs(cls, config):
        data_dir = config['data_dir']
        for name in os.listdir(data_dir):
             yield cls(os.path.join(data_dir, name))

Similarly, I can make the create_workers helper part of the GenericWorker class. Here, I use the input_class parameter, which must be a subclass of GenericInputData, to generate the necessary inputs. I construct instances of the GenericWorker concrete subclass by using cls() as a generic constructor:

class GenericWorker:
    def __init__(self, input_data):
        self.input_data = input_data
        self.result = None

   def map(self):
       raise NotImplementedError

   def reduce(self, other):
       raise NotImplementedError

   def create_workers(cls, input_class, config):
       workers = []
       for input_data in input_class.generate_inputs(config):
       return workers

Note that the call to input_class.generate_inputs above is the class polymorphism that I’m trying to show. You can also see how create_workers calling cls() provides an alternative way to construct GenericWorker objects besides using the __init__ method directly.

The effect on my concrete GenericWorker subclass is nothing more than changing its parent class:

class LineCountWorker(GenericWorker):

Finally, I can rewrite the mapreduce function to be completely generic by calling create_workers:

def mapreduce(worker_class, input_class, config):
     workers = worker_class.create_workers(input_class, config)
     return execute(workers)

Running the new worker on a set of test files produces the same result as the old implementation. The difference is that the mapreduce function requires more parameters so that it can operate generically:

config = {'data_dir': tmpdir}
result = mapreduce(LineCountWorker, PathInputData, config)
print(f'There are {result} lines')

There are 4360 lines

Now, I can write other GenericInputData and GenericWorker subclasses as I wish, without having to rewrite any of the glue code.

Things to Remember

  • Python only supports a single constructor per class: the __init__ method.

  • Use @classmethod to define alternative constructors for your classes.

  • Use class method polymorphism to provide generic ways to build and connect many concrete subclasses.

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