caveats: show python caveats if modules installed.

Library/Homebrew/caveats.rb

@@ -7,11 +7,12 @@ class Caveats
 
   def caveats
     caveats = []
-    caveats << f.caveats
+    caveats << f.caveats if f.caveats.to_s.length > 0
     caveats << f.keg_only_text if f.keg_only? && f.respond_to?(:keg_only_text)
     caveats << bash_completion_caveats
     caveats << zsh_completion_caveats
     caveats << plist_caveats
+    caveats << python_caveats
     caveats.compact.join("\n")
   end
 
@@ -43,6 +44,21 @@ class Caveats
     end
   end
 
+  def python_caveats
+    site_packages = if f.keg_only?
+      "#{f.opt_prefix}/lib/python2.7/site-packages"
+    else
+      "#{HOMEBREW_PREFIX}/lib/python2.7/site-packages"
+    end
+    if keg and keg.python_site_packages_installed? \
+      and !ENV['PYTHONPATH'].to_s.include? site_packages
+      <<-EOS.undent
+        Set PYTHONPATH if you want Python to find your site-packages:
+          export PYTHONPATH=#{site_packages}:$PYTHONPATH
+      EOS
+    end
+  end
+
   def plist_caveats
     s = []
     if f.plist or (keg and keg.plist_installed?)

Library/Homebrew/keg.rb

@@ -92,6 +92,10 @@ class Keg < Pathname
     end
   end
 
+  def python_site_packages_installed?
+    (self/'lib/python2.7/site-packages').directory?
+  end
+
   def version
     require 'version'
     Version.new(basename.to_s)