language/python: add helper methods for pth detection

Library/Homebrew/language/python.rb

@@ -8,6 +8,10 @@ module Language
       Version.new(version.to_s)
     end
 
+    def self.homebrew_site_packages(version="2.7")
+      HOMEBREW_PREFIX/"lib/python#{version}/site-packages"
+    end
+
     def self.each_python build, &block
       original_pythonpath = ENV["PYTHONPATH"]
       ["python", "python3"].each do |python|
@@ -16,11 +20,33 @@ module Language
         ENV["PYTHONPATH"] = if Formulary.factory(python).installed?
           nil
         else
-          "#{HOMEBREW_PREFIX}/lib/python#{version}/site-packages"
+          homebrew_site_packages(version)
         end
         block.call python, version if block
       end
       ENV["PYTHONPATH"] = original_pythonpath
     end
+
+    def self.reads_brewed_pth_files? python
+      version = major_minor_version python
+      return unless homebrew_site_packages(version).directory?
+      probe_file = homebrew_site_packages(version)/"homebrew-pth-probe.pth"
+      probe_file.atomic_write("import site; site.homebrew_was_here = True")
+      result = quiet_system python, "-c", "import site; assert(site.homebrew_was_here)"
+      probe_file.unlink
+      result
+    end
+
+    def self.user_site_packages python
+      Pathname.new(`#{python} -c "import site; print(site.getusersitepackages())"`.chomp)
+    end
+
+    def self.in_sys_path? python, path
+      script = <<-EOS.undent
+        import os, sys
+        [os.path.realpath(p) for p in sys.path].index(os.path.realpath("#{path}"))
+      EOS
+      quiet_system python, "-c", script
+    end
   end
 end