From 39c81b8a8807b368154a9364efc335944ddf1e9a Mon Sep 17 00:00:00 2001
From: Frank Sachsenheim <funkyfuture@riseup.net>
Date: Mon, 18 Sep 2017 09:36:07 +0200
Subject: [PATCH] Ensures mutable config defaults are copied

---
 mkdocs/config/config_options.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mkdocs/config/config_options.py b/mkdocs/config/config_options.py
index 5723f90b..48cc6caf 100644
--- a/mkdocs/config/config_options.py
+++ b/mkdocs/config/config_options.py
@@ -119,7 +119,11 @@ class OptionallyRequired(BaseConfigOption):
 
         if value is None:
             if self.default is not None:
-                value = self.default
+                if hasattr(self.default, 'copy'):
+                    # ensure no mutable values are assigned
+                    value = self.default.copy()
+                else:
+                    value = self.default
             elif not self.required:
                 return
             elif self.required: