visibility_test.rb

require 'minitest/autorun'

class Foo
  attr_accessor :a

  def initialize
    @a = :unchanged
  end

  # call 'a' with explicit 'self' as receiver
  def b; self.a; end
  def b=(v); self.a = v; end

  # call 'a' with implicit 'self' as receiver
  def c; a; end
  def c=(v); a = v; end

  def value
    @a
  end
end

class ProtectedFoo < Foo
  protected :a, :a=
end

class PrivateFoo < Foo
  private :a, :a=
end

class PublicVisibilityTest < Minitest::Test
  def setup
    @foo = Foo.new
  end

  def test_reader_explicit_receiver_succeeds
    assert_equal :unchanged, @foo.a
  end

  def test_writer_explicit_receiver_succeeds
    @foo.a = 4
    assert_equal 4, @foo.value
  end

  def test_reader_explicit_self_receiver_succeeds
    assert_equal :unchanged, @foo.b
  end

  def test_writer_explicit_self_receiver_succeeds
    @foo.b = 4
    assert_equal 4, @foo.value
  end

  def test_reader_implicit_self_receiver_succeeds
    assert_equal :unchanged, @foo.c
  end

  def test_writer_implicit_self_receiver_succeeds_but_does_nothing
    @foo.c = 4
    assert_equal :unchanged, @foo.value
  end
end

class ProtectedVisibilityTest < Minitest::Test
  def setup
    @foo = ProtectedFoo.new
  end

  def test_reader_explicit_receiver_fails
    assert_raises(NoMethodError) do
      @foo.a
    end
  end

  def test_writer_explicit_receiver_fails
    assert_raises(NoMethodError) do
      @foo.a = 4
    end
    assert_equal :unchanged, @foo.value
  end

  def test_reader_explicit_self_receiver_succeeds
    assert_equal :unchanged, @foo.b
  end

  def test_writer_explicit_self_receiver_succeeds
    @foo.b = 4
    assert_equal 4, @foo.value
  end

  def test_reader_implicit_self_receiver_succeeds
    assert_equal :unchanged, @foo.c
  end

  def test_writer_implicit_self_receiver_succeeds_but_does_nothing
    @foo.c = 4
    assert_equal :unchanged, @foo.value
  end
end

class PrivateVisibilityTest < Minitest::Test
  def setup
    @foo = PrivateFoo.new
  end

  def test_reader_explicit_receiver_fails
    assert_raises(NoMethodError) do
      @foo.a
    end
  end

  def test_writer_explicit_receiver_fails
    assert_raises(NoMethodError) do
      @foo.a = 4
    end
    assert_equal :unchanged, @foo.value
  end

  def test_reader_explicit_self_receiver_fails
    assert_raises(NoMethodError) do
      @foo.b
    end
  end

  def test_writer_explicit_self_receiver_fails
    assert_raises(NoMethodError) do
      @foo.b = 4
    end
    assert_equal :unchanged, @foo.value
  end

  def test_reader_implicit_self_receiver_succeeds
    assert_equal :unchanged, @foo.c
  end

  def test_writer_implicit_self_receiver_succeeds_but_does_nothing
    @foo.c = 4
    assert_equal :unchanged, @foo.value
  end
end

# Run options: --seed 18945
# 
# Running:
# 
# ................F.
# 
# Finished in 0.002823s, 6376.1955 runs/s, 7084.6617 assertions/s.
# 
#   1) Failure:
# PrivateVisibilityTest#test_writer_explicit_self_receiver_fails [visibility_test.rb:126]:
# NoMethodError expected but nothing was raised.
# 
# 18 runs, 20 assertions, 1 failures, 0 errors, 0 skips

self, and only self, is allowed on private writer methods. This is a needed special exception to the rules because just a = 42 is a local variable assignment, not a method call. Line 16 of your example does not call a=().

@JEG2 thanks.

It started as a discussion about why ruby issues warnings for private attr_*.

We'd assumed that the only way to send a private writer message would be using #send.

Of course it makes sense to make an exception for the = operator – which then leaves us back with the question about the warnings!

I'm not a fan of that warning. I avoid it by doing what you showed in the thread and specifically naming what I want to be private. I'm not sure why the warning exists, sorry.