Python callbacks using classes and methods

By Vasudev Ram

Hi readers,

I had written this post a few days ago:

Implementing and using callbacks in Python

In it, I had shown how to create simple callbacks using just plain Python functions, and said I would write a bit more about callbacks in my next post.

This is that next post. It discusses how to create callbacks in Python using classes and methods.

Here is an example program, callback_demo2.py, that shows how to do that:

'''
File: callback_demo2.py
To demonstrate implementation and use of callbacks in Python, 
using classes with methods as the callbacks.
Author: Vasudev Ram
Copyright 2017 Vasudev Ram
Web site: https://vasudevram.github.io
Blog: https://jugad2.blogspot.com
Product store: https://gumroad.com/vasudevram
'''

from __future__ import print_function
import sys
from time import sleep

class FileReporter(object):

    def __init__(self, filename):
        self._filename = filename
        try:
            self._fil = open(self._filename, "w")
        except IOError as ioe:
            sys.stderr.write("While opening {}, caught IOError: {}\n".format(
                self._filename, repr(ioe)))
            sys.exit(1)

    def report(self, message):
        self._fil.write(message)

class ScreenReporter(object):

    def __init__(self, dest):
        self._dest = dest

    def report(self, message):
        self._dest.write(message)

def square(i):
    return i * i

def cube(i):
    return i * i * i

def processor(process, times, report_interval, reporter):
    result = 0
    for i in range(1, times + 1):
        result += process(i)
        sleep(0.1)
        if i % report_interval == 0:
            # This is the call to the callback method 
            # that was passed to this function.
            reporter.report("Items processed: {}. Running result: {}.\n".format(i, result))

file_reporter = FileReporter("processor_report.txt")
processor(square, 20, 5, file_reporter)

stdout_reporter = ScreenReporter(sys.stdout)
processor(square, 20, 5, stdout_reporter)

stderr_reporter = ScreenReporter(sys.stderr)
processor(square, 20, 5, stderr_reporter)

I ran it with:

$ python callback_demo2.py >out 2>err

The above command creates 3 files, processor_report.txt, out and err.
Running fc /l on those 3 files, pairwise, shows that all three have the same content, but the output has gone to 3 different destinations (a specified file, standard output, and standard error output, based on which callback was passed to the processor function, in the 3 calls to it.

These two lines from the program:

file_reporter = FileReporter("processor_report.txt")
processor(square, 20, 5, file_reporter)

send output to the file processor_report.txt.

These two lines:

stdout_reporter = ScreenReporter(sys.stdout)
processor(square, 20, 5, stdout_reporter)

send output to the standard output (stdout), which is redirected to the file out.

These two lines:

stderr_reporter = ScreenReporter(sys.stderr)
processor(square, 20, 5, stderr_reporter)

send output to the standard error output (stderr), which is redirected to the file err.

The difference between this program (callback_demo2.py) and the one in the previous post (callback_demo.py), is that in this one, I pass an instance of some class to the processor function, as its last argument. This argument is the callback. And this time, rather than treating it as a function, processor treats it as an object, and invokes the report method on it, giving us much the same output as before (I just made minor cosmetic changes in the output). This same thing is done in all the 3 calls to processor. The difference is that different types of objects are passed each time, for the callback argument.

[ An interesting side note here is that in some other languages, for example, Java, at least in earlier Java versions (I'm not sure about recent ones), we cannot just pass different types of objects for the same callback parameter, unless they are all derived from some common base class (via inheritance). (Can it be done using Java interfaces? Need to check - been a while.) While that is also possible in Python, it is not necessary, as we see here, due to Python's more dynamic nature. Just passing any object that implements a report method is enough, as the program shows. The FileReporter and ScreenReporter classes do not (need to) descend from some common base class (other than object, but that is just the syntax for new-style classes; the object class not provide the report method). Python's smooth polymorphism, a.k.a. duck typing, takes care of it. ]

The first time, a FileReporter object is passed, so the output goes to a text file.

The second and third times, a ScreenReporter object is passed, initializing the _dest field to stdout and stderr respectively, so the output goes to those two destinations respectively. And the command line redirects those outputs to the files out and err.

Although I didn't say so in the previous post, the first argument to processor (square), is also a callback, since it, in turn, is called from processor, via the process parameter. So I can pass some other argument in place of it, like the cube function defined in the program, to get different computations done and hence different results.

---

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- Vasudev Ram - Online Python training and consulting

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