Unused-Private-Method.md

Introduction

Classes should use their private methods. Otherwise this is dead code which is confusing and bad for maintenance.

The Unused Private Method detector reports unused private instance methods and instance methods only - class methods are ignored.

Example

Given:

class Car
  private
  def drive; end
  def start; end
end

Reek would emit the following warning:

2 warnings:
  [3]:Car has the unused private instance method `drive` (UnusedPrivateMethod)
  [4]:Car has the unused private instance method `start` (UnusedPrivateMethod)

Configuration

Unused Private Method offers the Basic Smell Options.

Private methods that are called via dynamic dispatch will trigger a false alarm since detecting something like this is far out of scope for Reek. In this case you can disable this detector via the exclude configuration option (which is part of the Basic Smell Options) for instance like this (an example from Reek's own codebase):

# :reek:UnusedPrivateMethod { exclude: [ process_ ] }
class ContextBuilder
  def process_begin
    # ....
  end
end

Note that disabling this detector via comment works on a class scope, not a method scope (like you can see above).

Another simple example would be:

class Alfa
  private
  def bravo
  end
end

This would report:

ruby.rb -- 1 warning: [3]:UnusedPrivateMethod: Alfa has the unused private instance method 'bravo'

If you want to suppress this warning you can do this via source comment like this:

# :reek:UnusedPrivateMethod: { exclude: bravo }
class Alfa
  private
  def bravo
  end
end

Known limitations

• Method calls via dynamic dispatch (e.g. via send) is something Reek (or any other static tool for that matter) cannot detect.
• Method calls via callback like Rails filters will trigger this as well, e.g.:

  class BankController < ActionController::Base
    before_action :audit

    private
    def audit
      # ....
    end
  end

• Reek works on a per-file base. This means that using something like the template pattern with private methods will trigger this detector. We do believe though that using private methods to fill out a template in a superclass is not a good idea in general so this probably isn't really a problem but still worth mentioning it.