Only enable virtualenvwrapper, if WORKON_HOME is set

Makes sure that virtualenvwrapper is only enabled for those that really
intend to use it.

modules/python/README.md

@@ -9,9 +9,9 @@ This module will prepend the per user site packages directory as defined in [PEP
 man pages installed into the per user site.
 
 It also sources the [_virtualenvwrapper_][2] initialization script, if
-[_virtualenvwrapper_][2] is installed.  [_virtualenvwrapper_][2] is a frontend
-to [_virtualenv_][3] which provides convenient shell functions to create, switch
-and manage virtualenvs.
+[_virtualenvwrapper_][2] is installed and `$WORKON_HOME` is set.
+[_virtualenvwrapper_][2] is a frontend to [_virtualenv_][3] which provides
+convenient shell functions to create, switch and manage virtualenvs.
 
 Authors
 -------

modules/python/init.zsh

@@ -21,6 +21,8 @@ else
 fi
 
 # source virtualenvwrapper if available
-if (( $+commands[virtualenvwrapper.sh] )); then
-    source virtualenvwrapper.sh
+if (( $+WORKON_HOME )); then
+    if (( $+commands[virtualenvwrapper.sh] )); then
+        source virtualenvwrapper.sh
+    fi
 fi