Fingerprint the filenames in site-packages to decide when to install requirements.

rakefiles/prereqs.rake

 require './rakefiles/helpers.rb'
-
+require 'tempfile'
 
 PREREQS_MD5_DIR = ENV["PREREQ_CACHE_DIR"] || File.join(REPO_ROOT, '.prereqs_cache')
 
@@ -28,15 +28,21 @@ end
 
 desc "Install all python prerequisites for the lms and cms"
 task :install_python_prereqs => "ws:migrate" do
-    unchanged = 'Python requirements unchanged, nothing to install'
-    when_changed(unchanged, 'requirements/**/*') do
-        ENV['PIP_DOWNLOAD_CACHE'] ||= '.pip_download_cache'
-        sh('pip install --exists-action w -r requirements/edx/base.txt')
-        sh('pip install --exists-action w -r requirements/edx/post.txt')
-        # Check for private-requirements.txt: used to install our libs as working dirs,
-        # or personal-use tools.
-        if File.file?("requirements/private.txt")
-            sh('pip install -r requirements/private.txt')
+    if !ENV['NO_PREREQ_INSTALL']
+        Tempfile.open('pyinstalled') do |pyinstalled|
+            # Read the names of everything in site-packages, and include them in the fingerprint.
+            sh("python -c 'import os; import distutils.sysconfig as dusc; print sorted(os.listdir(dusc.get_python_lib()))' > #{pyinstalled.path}")
+            unchanged = 'Python requirements unchanged, nothing to install'
+            when_changed(unchanged, 'requirements/**/*', pyinstalled.path) do
+                ENV['PIP_DOWNLOAD_CACHE'] ||= '.pip_download_cache'
+                sh('pip install --exists-action w -r requirements/edx/base.txt')
+                sh('pip install --exists-action w -r requirements/edx/post.txt')
+                # requirements/private.txt is used to install our libs as
+                # working dirs, or for personal-use tools.
+                if File.file?("requirements/private.txt")
+                    sh('pip install -r requirements/private.txt')
+                end
+            end
         end
-    end unless ENV['NO_PREREQ_INSTALL']
+    end
 end