Python module deps, ext. python fix and 10.6 fix

* Fixes Homebrew/homebrew#20572 by tweaking the logic that decides
  which python is used by the `python` object
  inside a formula. There was a bug when on 10.6
  there is no Python 2.7 but a :recommended
  Python was still treated as being available.

* Use the user's PATH when looking for an external
  Python. Until now only brewed or OS X system's
  python have been found by `depends_on :python`.

  But now we support any Python in PATH (e.g.
  pyenv's python).

* Further, instead of handling python modules
  and import tests in LanguageModuleDependency,
  these are now handled by:

        depends_on :python => 'numpy'  # for example

  The old style

        depends_on 'numpy' => :python

  is still supported and is only an alias
  for the newer style (only for :python, the
  other languages are not altered by this commit).

  The reasoning is that if a formula requires
  a python module, it basically also needs
  python itself - and further that specific
  version of python has to provide the module.
  So the `PythonInstalled` is the natural place
  to check for the availability of a python
  module.

  Using a python module and other tags like
  :optional or :recommended is done like so:

        depends_on :python => [:optional, 'numpy']

  Specifying another PyPi (Python Package index)
  name than the module import name is seldom used
  but supported, too:

         depends_on :python => ['enchant'=>'pyenchant']

  A last note: For clarity, you can define
  multiple depends_on statements with different
  modules to be importable.`

1 files changed, 15 insertions, 10 deletions

diff --git a/Library/Homebrew/python_helper.rb b/Library/Homebrew/python_helper.rb
index dcb698c9d..3db59b334 100644
--- a/Library/Homebrew/python_helper.rb
+++ b/Library/Homebrew/python_helper.rb
@@ -29,25 +29,30 @@ def python_helper(options={:allowed_major_versions => [2, 3]}, &block)
     if python_reqs.empty?
       raise "If you use python in the formula, you have to add `depends_on :python` (or :python3)!"
     end
-    # Now select those that are satisfied and matching the version.major
-    python_reqs = python_reqs.select do |p|
-      p.satisfied? &&
-      options[:allowed_major_versions].include?(p.version.major) &&
-      if p.optional? || p.recommended?
-        self.build.with?(p.name)
-      else
-        true
+    # Now select those that are satisfied and matching the version.major and
+    # check that no two python binaries are the same (which could be the case
+    # because more than one `depends_on :python => 'module_name' may be present).
+    filtered_python_reqs = []
+    while !python_reqs.empty?
+      py = python_reqs.shift
+      # this is ulgy but Ruby 1.8 has no `uniq! { }`
+      if !filtered_python_reqs.map{ |fpr| fpr.binary }.include?(py.binary) &&
+         py.satisfied? &&
+         options[:allowed_major_versions].include?(py.version.major) &&
+         self.build.with?(py.name) || !(py.optional? || py.recommended?)
+      then
+        filtered_python_reqs << py
       end
     end
 
     # Allow to use an else-branch like so: `if python do ... end; else ... end`
-    return false if python_reqs.empty?
+    return false if filtered_python_reqs.empty?
 
     # Sort by version, so the older 2.x will be used first and if no
     # block_given? then 2.x is preferred because it is returned.
     # Further note, having 3.x last allows us to run `2to3 --write .`
     # which modifies the sources in-place (for some packages that need this).
-    python_reqs.sort_by{ |py| py.version }.map do |py|
+    filtered_python_reqs.sort_by{ |py| py.version }.map do |py|
       # Now is the time to set the site_packages to the correct value
       py.site_packages = lib/py.xy/'site-packages'
       if !block_given?