Add helper class for Python virtualenvs

Library/Homebrew/language/python.rb

@@ -1,4 +1,5 @@
 require "utils"
+require "language/python_virtualenv_constants"
 
 module Language
   module Python
@@ -96,5 +97,135 @@ module Language
     def self.package_available?(python, module_name)
       quiet_system python, "-c", "import #{module_name}"
     end
-  end
-end
+
+    # Mixin module for {Formula} adding virtualenv support features.
+    module Virtualenv
+      def self.included(base)
+        base.class_eval do
+          resource "homebrew-virtualenv" do
+            url PYTHON_VIRTUALENV_URL
+            sha256 PYTHON_VIRTUALENV_SHA256
+          end
+        end
+      end
+
+      # Instantiates, creates, and yields a {Virtualenv} object for use from
+      # Formula#install, which provides helper methods for instantiating and
+      # installing packages into a Python virtualenv.
+      # @param venv_root [Pathname, String] the path to the root of the virtualenv
+      #   (often `libexec/"venv"`)
+      # @param python [String] which interpreter to use (e.g. "python"
+      #   or "python3")
+      # @param formula [Formula] the active Formula
+      # @return [Virtualenv] a {Virtualenv} instance
+      def virtualenv_create(venv_root, python = "python", formula = self)
+        venv = Virtualenv.new formula, venv_root, python
+        venv.create
+        venv
+      end
+
+      # Helper method for the common case of installing a Python application.
+      # Creates a virtualenv in `libexec`, installs all `resource`s defined
+      # on the formula, and then installs the formula.
+      def virtualenv_install_with_resources
+        venv = virtualenv_create(libexec)
+        venv.pip_install resources
+        venv.link_scripts(bin) { venv.pip_install buildpath }
+        venv
+      end
+
+      # Convenience wrapper for creating and installing packages into Python
+      # virtualenvs.
+      class Virtualenv
+        # Initializes a Virtualenv instance. This does not create the virtualenv
+        # on disk; {#create} does that.
+        # @param formula [Formula] the active Formula
+        # @param venv_root [Pathname, String] the path to the root of the
+        #   virtualenv
+        # @param python [String] which interpreter to use; i.e. "python" or
+        #   "python3"
+        def initialize(formula, venv_root, python)
+          @formula = formula
+          @venv_root = Pathname.new(venv_root)
+          @python = python
+        end
+
+        # Obtains a copy of the virtualenv library and creates a new virtualenv
+        # on disk.
+        # @return [void]
+        def create
+          return if (@venv_root/"bin/python").exist?
+
+          @formula.resource("homebrew-virtualenv").stage do |stage|
+            old_pythonpath = ENV.delete "PYTHONPATH"
+            begin
+              xy = Language::Python.major_minor_version(@python)
+              staging = Pathname.new(stage.staging.tmpdir)
+              ENV.prepend_create_path "PYTHONPATH", staging/"target/lib/python#{xy}/site-packages"
+              @formula.system @python, *Language::Python.setup_install_args(staging/"target")
+              @formula.system @python, "-s", staging/"target/bin/virtualenv", "-p", @python, @venv_root
+            ensure
+              ENV["PYTHONPATH"] = old_pythonpath
+            end
+          end
+
+          # Robustify symlinks to survive python3 patch upgrades
+          @venv_root.find do |f|
+            next unless f.symlink?
+            if (rp = f.realpath.to_s).start_with? HOMEBREW_CELLAR
+              python = rp.include?("python3") ? "python3" : "python"
+              new_target = rp.sub %r{#{HOMEBREW_CELLAR}/#{python}/[^/]+}, Formula[python].opt_prefix
+              f.unlink
+              f.make_symlink new_target
+            end
+          end
+        end
+
+        # Installs packages represented by `targets` into the virtualenv.
+        # @param targets [String, Pathname, Resource,
+        #   Array<String, Pathname, Resource>] (A) token(s) passed to pip
+        #   representing the object to be installed. This can be a directory
+        #   containing a setup.py, a {Resource} which will be staged and
+        #   installed, or a package identifier to be fetched from PyPI.
+        #   Multiline strings are allowed and treated as though they represent
+        #   the contents of a `requirements.txt`.
+        # @return [void]
+        def pip_install(targets)
+          targets = [targets] unless targets.is_a? Array
+          targets.each do |t|
+            if t.respond_to? :stage
+              next if t.name == "homebrew-virtualenv"
+              t.stage { do_install Pathname.pwd }
+            else
+              t = t.lines.map(&:strip) if t.respond_to?(:lines) && t =~ /\n/
+              do_install t
+            end
+          end
+        end
+
+        # Compares the venv bin directory before and after executing a block,
+        # and symlinks any new scripts into `destination`.
+        # Use like: venv.link_scripts(bin) { venv.pip_install my_package }
+        # @param destination [Pathname, String] Destination into which new
+        #   scripts should be linked.
+        # @return [void]
+        def link_scripts(destination)
+          bin_before = Dir[@venv_root/"bin/*"].to_set
+          yield
+          bin_after = Dir[@venv_root/"bin/*"].to_set
+          destination = Pathname.new(destination)
+          destination.install_symlink((bin_after - bin_before).to_a)
+        end
+
+        private
+
+        def do_install(targets)
+          targets = [targets] unless targets.is_a? Array
+          @formula.system @venv_root/"bin/pip", "install",
+                 "-v", "--no-deps", "--no-binary", ":all:",
+                 *targets
+        end
+      end # class Virtualenv
+    end # module Virtualenv
+  end # module Python
+end # module Language

Library/Homebrew/language/python_virtualenv_constants.rb

@@ -0,0 +1,2 @@
+PYTHON_VIRTUALENV_URL = "https://files.pythonhosted.org/packages/5c/79/5dae7494b9f5ed061cff9a8ab8d6e1f02db352f3facf907d9eb614fb80e9/virtualenv-15.0.2.tar.gz"
+PYTHON_VIRTUALENV_SHA256 = "fab40f32d9ad298fba04a260f3073505a16d52539a84843cf8c8369d4fd17167"