How to redefine a Ruby constant without warning?

If you want to redefine a value then don't use constants, use a global variable instead ($tau = 2 * Pi), but that's not a good practice too. You should make it an instance variable of a suitable class.

For the other case, Tau = 2 * Pi unless defined?(Tau) is perfectly alright and the most readable, therefore the most elegant solution.

The following module may do what you want. If not it may provide some pointers to your solution

module RemovableConstants

  def def_if_not_defined(const, value)
    self.class.const_set(const, value) unless self.class.const_defined?(const)
  end

  def redef_without_warning(const, value)
    self.class.send(:remove_const, const) if self.class.const_defined?(const)
    self.class.const_set(const, value)
  end
end

And as an example of using it

class A
  include RemovableConstants

  def initialize
    def_if_not_defined("Foo", "ABC")
    def_if_not_defined("Bar", "DEF")
  end

  def show_constants
    puts "Foo is #{Foo}"
    puts "Bar is #{Bar}"
  end

  def reload
    redef_without_warning("Foo", "GHI")
    redef_without_warning("Bar", "JKL")
  end

end

a = A.new
a.show_constants
a.reload
a.show_constants

Gives the following output

Foo is ABC
Bar is DEF
Foo is GHI
Bar is JKL

Forgive me if i've broken any ruby taboos here as I am still getting my head around some of the Module:Class:Eigenclass structure within Ruby

Another approach, using $VERBOSE, to suppress warnings, is discussed here: http://mentalized.net/journal/2010/04/02/suppress_warnings_from_ruby/

Update 2020/5/6: In response to the comment that the link is now dead, I am pasting an example here from my old project, though I can't say whether and in what circumstances it is a good approach:

original_verbose = $VERBOSE
$VERBOSE = nil # suppress warnings
# do stuff that raises warnings you don't care about
$VERBOSE = original_verbose