PythonInstalled: Allow formulae to set/append PYTHONPATH

Improve robustness of `PYTHONPATH` by first unsetting it (during
`satisfy`) so that the `PythonInstalled` can get the `python.version`
and so forth and then, after that, setting the `PYTHONPATH` to our
`global_site_packages`.
In the `python_helper` we append to the `PYTHONPATH` so if that var has
been set in a formula, it is respected.
Brew audit does no longer complain about setting the
`ENV['PYTHONPATH']`.

Library/Homebrew/cmd/audit.rb

@@ -568,13 +568,7 @@ class FormulaAuditor
       end
     end
 
-    if f.requirements.any?{ |r| r.kind_of?(PythonInstalled) }
+    unless f.requirements.any?{ |r| r.kind_of?(PythonInstalled) }
-      # Don't check this for all formulae, because some are allowed to set the
-      # PYTHONPATH. E.g. python.rb itself needs to set it.
-      if text =~ /ENV\.append.*PYTHONPATH/ || text =~ /ENV\[['"]PYTHONPATH['"]\]\s*=[^=]/
-        problem "Don't set the PYTHONPATH, instead declare `depends_on :python`"
-      end
-    else
       # So if there is no PythonInstalled requirement, we can check if the
       # formula still uses python and should add a `depends_on :python`
       unless f.name.to_s =~ /(pypy[0-9]*)|(python[0-9]*)/

Library/Homebrew/requirements/python_dependency.rb

@@ -80,6 +80,9 @@ class PythonInstalled < Requirement
   # We look for a brewed python or an external Python and store the loc of
   # that binary for later usage. (See Formula#python)
   satisfy :build_env => false do
+    begin
+      # Unset the PYTHONPATH during these tests, but later ensure it is restored.
+      pythonpath = ENV['PYTHONPATH']
       ENV['PYTHONPATH'] = nil
       @unsatisfied_because = ''
       if binary.nil? || !binary.executable?
@@ -113,6 +116,9 @@ class PythonInstalled < Requirement
           end
         end
       end
+    ensure
+      ENV['PYTHONPATH'] = pythonpath
+    end
   end
 
   def importable? module_name
@@ -196,7 +202,7 @@ class PythonInstalled < Requirement
     end
   end
 
-  # Is the brewed Python installed
+  # Is the brewed Python installed?
   def brewed?
     @brewed ||= begin
       require 'formula'
@@ -245,6 +251,18 @@ class PythonInstalled < Requirement
     # Most methods fail if we don't have a binary.
     return if binary.nil?
 
+    ENV['PYTHONHOME'] = nil  # to avoid fuck-ups.
+    ENV['PYTHONPATH'] = nil  # first unset, because global_site_packages may fail
+    ENV['PYTHONPATH'] = if brewed? then nil; else global_site_packages.to_s; end
+
+    # For non-system python's we add the opt_prefix/bin of python to the path.
+    ENV.prepend_path 'PATH', binary.dirname unless from_osx?
+    ENV.append_path 'CMAKE_INCLUDE_PATH', incdir
+    ENV.append_path 'PKG_CONFIG_PATH', pkg_config_path if pkg_config_path
+    # We don't set the -F#{framework} here, because if Python 2.x and 3.x are
+    # used, `Python.framework` is ambiguous. However, in the `python do` block
+    # we can set LDFLAGS+="-F#{framework}" because only one is temporarily set.
+
     # Write our sitecustomize.py
     file = global_site_packages/"sitecustomize.py"
     ohai "Writing #{file}" if ARGV.verbose? && ARGV.debug?
@@ -256,17 +274,6 @@ class PythonInstalled < Requirement
 
     file.write(sitecustomize)
 
-    # For non-system python's we add the opt_prefix/bin of python to the path.
-    ENV.prepend_path 'PATH', binary.dirname unless from_osx?
-
-    ENV['PYTHONHOME'] = nil  # to avoid fuck-ups.
-    ENV['PYTHONPATH'] = if brewed? then nil; else global_site_packages.to_s; end
-    ENV.append_path 'CMAKE_INCLUDE_PATH', incdir
-    ENV.append_path 'PKG_CONFIG_PATH', pkg_config_path if pkg_config_path
-    # We don't set the -F#{framework} here, because if Python 2.x and 3.x are
-    # used, `Python.framework` is ambiguous. However, in the `python do` block
-    # we can set LDFLAGS+="-F#{framework}" because only one is temporarily set.
-
     # Udpate distutils.cfg (later we can remove this, but people still have
     # their old brewed pythons and we have to update it here)
     # Todo: If Jack's formula revisions arrive, we can get rid of this here!