one step closer to the behavior we want.

$ rake pry
[1] pry(main)> _pry_.config.requires
=> []
[2] pry(main)> _pry_.config.requires << 1
=> [1]
[3] pry(main)> Pry.config.requires = [1,2,3]
=> [1, 2, 3]
[4] pry(main)> _pry_.config.requires
=> [1, 2, 3]
[5] pry(main)> _pry_.config.requires = [42]
=> [42]
[6] pry(main)> Pry.config.requires = [5667]
=> [5667]
[7] pry(main)> _pry_.config.requires
=> [42]
[8] pry(main)> exit

Gemfile

@@ -8,6 +8,8 @@ group :development do
   gem 'rb-fsevent', :require => 'false'
 end
 
+gem 'binding.repl'
+
 if RbConfig::CONFIG['ruby_install_name'] == 'rbx'
   gem 'rubysl-singleton'
   gem 'rubysl-prettyprint'

lib/pry/config/behavior.rb

@@ -1,9 +1,10 @@
 module Pry::Config::Behavior
   ASSIGNMENT = "=".freeze
-  NODUP = [TrueClass, FalseClass, NilClass, Module, Proc, Numeric].freeze
+  NODUP = [TrueClass, FalseClass, NilClass, String, Module, Proc, Numeric].freeze
 
   def initialize(default = Pry.config)
     @default = default
+    @default._register(self) if @default
     @lookup = {}
     @read_lookup = {}
   end
@@ -20,6 +21,7 @@ module Pry::Config::Behavior
     key = name.to_s
     if key[-1] == ASSIGNMENT
       short_key = key[0..-2]
+      @default._forget(short_key) if @default
       self[short_key] = args[0]
     elsif @lookup.has_key?(key)
       self[key]

lib/pry/config/default.rb

@@ -51,6 +51,14 @@ class Pry::Config::Default
     configure_history
   end
 
+  def _register(other)
+    @child = other
+  end
+
+  def _forget(key)
+    @child.instance_variable_get(:@read_lookup).delete(key.to_s)
+  end
+
   default.each do |key, value|
     define_method(key) do
       if default[key].equal?(value)