python3: version detection for --HEAD packages (#7654)

Formula/python3.rb

@@ -57,19 +57,6 @@ class Python3 < Formula
   # X11.
   patch :DATA if build.with? "tcl-tk"
 
-  def lib_cellar
-    prefix/"Frameworks/Python.framework/Versions/#{xy}/lib/python#{xy}"
-  end
-
-  def site_packages_cellar
-    lib_cellar/"site-packages"
-  end
-
-  # The HOMEBREW_PREFIX location of site-packages.
-  def site_packages
-    HOMEBREW_PREFIX/"lib/python#{xy}/site-packages"
-  end
-
   # setuptools remembers the build flags python is built with and uses them to
   # build packages later. Xcode-only systems need different flags.
   pour_bottle? do
@@ -89,6 +76,9 @@ class Python3 < Formula
     ENV["PYTHONHOME"] = nil
     ENV["PYTHONPATH"] = nil
 
+    xy = (buildpath/"configure.ac").read.slice(/PYTHON_VERSION, (3\.\d)/, 1)
+    lib_cellar = prefix/"Frameworks/Python.framework/Versions/#{xy}/lib/python#{xy}"
+
     args = %W[
       --prefix=#{prefix}
       --enable-ipv6
@@ -198,7 +188,7 @@ class Python3 < Formula
     rm bin/"2to3"
 
     # Remove the site-packages that Python created in its Cellar.
-    site_packages_cellar.rmtree
+    (prefix/"Frameworks/Python.framework/Versions/#{xy}/lib/python#{xy}/site-packages").rmtree
 
     %w[setuptools pip wheel].each do |r|
       (libexec/r).install resource(r)
@@ -213,6 +203,10 @@ class Python3 < Formula
   end
 
   def post_install
+    xy = (prefix/"Frameworks/Python.framework/Versions").children.first.basename.to_s
+    site_packages = HOMEBREW_PREFIX/"lib/python#{xy}/site-packages"
+    site_packages_cellar = prefix/"Frameworks/Python.framework/Versions/#{xy}/lib/python#{xy}/site-packages"
+
     # Fix up the site-packages so that user-installed Python software survives
     # minor updates, such as going from 3.3.2 to 3.3.3:
 
@@ -267,7 +261,8 @@ class Python3 < Formula
       library_dirs << Formula["homebrew/dupes/tcl-tk"].opt_lib
     end
 
-    cfg = lib_cellar/"distutils/distutils.cfg"
+    cfg = prefix/"Frameworks/Python.framework/Versions/#{xy}/lib/python#{xy}/distutils/distutils.cfg"
+
     cfg.atomic_write <<-EOF.undent
       [install]
       prefix=#{HOMEBREW_PREFIX}
@@ -278,11 +273,9 @@ class Python3 < Formula
     EOF
   end
 
-  def xy
-    version.to_s.slice(/(3\.\d)/) || "3.6"
-  end
-
   def sitecustomize
+    xy = (prefix/"Frameworks/Python.framework/Versions").children.first.basename.to_s
+
     <<-EOF.undent
       # This file is created by Homebrew and is executed on each python startup.
       # Don't print from here, or else python command line scripts may fail!
@@ -313,7 +306,7 @@ class Python3 < Formula
           # the Cellar site-packages is a symlink to the HOMEBREW_PREFIX
           # site_packages; prefer the shorter paths
           long_prefix = re.compile(r'#{rack}/[0-9\._abrc]+/Frameworks/Python\.framework/Versions/#{xy}/lib/python#{xy}/site-packages')
-          sys.path = [long_prefix.sub('#{site_packages}', p) for p in sys.path]
+          sys.path = [long_prefix.sub('#{HOMEBREW_PREFIX/"lib/python#{xy}/site-packages"}', p) for p in sys.path]
 
           # Set the sys.executable to use the opt_prefix
           sys.executable = '#{opt_bin}/python#{xy}'
@@ -321,6 +314,7 @@ class Python3 < Formula
   end
 
   def caveats
+    xy = (prefix/"Frameworks/Python.framework/Versions").children.first.basename.to_s
     text = <<-EOS.undent
       Pip, setuptools, and wheel have been installed. To update them
         pip3 install --upgrade pip setuptools wheel
@@ -329,7 +323,7 @@ class Python3 < Formula
         pip3 install <package>
 
       They will install into the site-package directory
-        #{site_packages}
+        #{HOMEBREW_PREFIX/"lib/python#{xy}/site-packages"}
 
       See: https://github.com/Homebrew/brew/blob/master/docs/Homebrew-and-Python.md
     EOS
@@ -346,6 +340,7 @@ class Python3 < Formula
   end
 
   test do
+    xy = (prefix/"Frameworks/Python.framework/Versions").children.first.basename.to_s
     # Check if sqlite is ok, because we build with --enable-loadable-sqlite-extensions
     # and it can occur that building sqlite silently fails if OSX's sqlite is used.
     system "#{bin}/python#{xy}", "-c", "import sqlite3"