Daniel Voigt Godoy - Deep Learning with PyTorch Step-by-Step A Beginner’s Guide-leanpub

Methods, _methods, and __methods
Some programming languages, like Java, have three kinds of methods: public,
protected, and private. Public methods are the kind you’re most familiar
with: They can be called by the user.
Protected methods, on the other hand, shouldn’t be called by the user—they
are supposed to be called either internally or by the child class (the child
class can call a protected method from its parent class).
Finally, private methods are supposed to be called exclusively internally.
They should be invisible even to a child class.
These rules are strictly enforced in Java, but Python takes a more relaxed
approach: All methods are public, meaning you can call whatever method
you want. But you can suggest the appropriate usage by prefixing the
method name with a single underscore (for protected methods) or a double
underscore (for private methods). This way, the user is aware of the
programmer’s intention.
In our example, both _make_train_step_fn() and _make_val_step_fn() are
defined as protected methods. I expect users not to call them directly, but if
someone decides to define a class that inherits from StepByStep, they should feel
entitled to do so.
In order to make the additions to our code visually simpler; that is, without having
to replicate the full class every time I introduce a new method, I am resorting to
something that shouldn’t be used in regular circumstances: setattr. [65]
# ATTENTION! Using SETATTR for educational purposes only :-)
setattr(StepByStep, '_make_train_step_fn', _make_train_step_fn)
setattr(StepByStep, '_make_val_step_fn', _make_val_step_fn)
Using setattr is a hack, I can’t stress this enough! Please don’t
use setattr in your regular code.
184 | Chapter 2.1: Going Classy