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.`

3 files changed, 73 insertions, 25 deletions

diff --git a/Library/Homebrew/dependency_collector.rb b/Library/Homebrew/dependency_collector.rb
index 91bcb85a3..50dffaab6 100644
--- a/Library/Homebrew/dependency_collector.rb
+++ b/Library/Homebrew/dependency_collector.rb
@@ -18,7 +18,7 @@ require 'set'
 class DependencyCollector
   # Define the languages that we can handle as external dependencies.
   LANGUAGE_MODULES = Set[
-    :chicken, :jruby, :lua, :node, :ocaml, :perl, :python, :python2, :python3, :rbx, :ruby
+    :chicken, :jruby, :lua, :node, :ocaml, :perl, :python, :rbx, :ruby
   ].freeze
 
   attr_reader :deps, :requirements
@@ -68,6 +68,9 @@ class DependencyCollector
     if tags.empty?
       Dependency.new(spec, tags)
     elsif (tag = tags.first) && LANGUAGE_MODULES.include?(tag)
+      # Next line only for legacy support of `depends_on 'module' => :python`
+      # It should be replaced by `depends_on :python => 'module'`
+      return PythonInstalled.new("2", spec, *tags) if tag == :python
       LanguageModuleDependency.new(tag, spec)
     else
       Dependency.new(spec, tags)
@@ -94,9 +97,8 @@ class DependencyCollector
     when :clt        then CLTDependency.new(tags)
     when :arch       then ArchRequirement.new(tags)
     when :hg         then MercurialDependency.new(tags)
-    when :python     then PythonInstalled.new(tags)
-    when :python2    then PythonInstalled.new("2", tags)
-    when :python3    then PythonInstalled.new("3", tags)
+    when :python, :python2 then PythonInstalled.new("2", *tags)
+    when :python3    then PythonInstalled.new("3", *tags)
     # Tiger's ld is too old to properly link some software
     when :ld64       then LD64Dependency.new if MacOS.version < :leopard
     else
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?
diff --git a/Library/Homebrew/requirements/python_dependency.rb b/Library/Homebrew/requirements/python_dependency.rb
index c3753d813..610bc1c6b 100644
--- a/Library/Homebrew/requirements/python_dependency.rb
+++ b/Library/Homebrew/requirements/python_dependency.rb
@@ -31,13 +31,12 @@ class PythonInstalled < Requirement
     end
   end
 
-  def initialize(*tags)
+  def initialize(version="2.6", *tags )
     # Extract the min_version if given. Default to python 2.X else
-    tags.flatten!
-    if /(\d+\.)*\d+/ === tags.first
-      @min_version = PythonVersion.new(tags.shift)
+    if /(\d+\.)*\d+/ === version.to_s
+      @min_version = PythonVersion.new(version)
     else
-      @min_version = PythonVersion.new("2.6")  # default
+      raise "Invalid version specification for Python: '#{version}'"
     end
 
     # often used idiom: e.g. sipdir = "share/sip" + python.if3then3
@@ -51,6 +50,18 @@ class PythonInstalled < Requirement
     # The name is used to generate the options like --without-python3
     @name = "python" + @if3then3
 
+    # Check if any python modules should be importable. We use a hash to store
+    # the corresponding name on PyPi "<import_name>" => "<name_on_PyPi>".
+    # Example: `depends_on :python => ['enchant' => 'pyenchant']
+    @imports = {}
+    tags.each do |tag|
+      if tag.kind_of? String
+        @imports[tag] = tag
+      elsif tag.kind_of? Hash
+        @imports.merge!(tag)
+      end
+    end
+
     # will be set later by the python_helper, because it needs the
     # formula prefix to set site_packages
     @site_packages = nil
@@ -78,10 +89,29 @@ class PythonInstalled < Requirement
     elsif @min_version.major == 2 && `python -c "import sys; print(sys.version_info[0])"`.strip == "3"
       @unsatisfied_because += "Your `python` points to a Python 3.x. This is not supported."
     else
-      true
+      @imports.keys.map do |module_name|
+        if not importable? module_name
+          @unsatisfied_because += "Unsatisfied dependency: #{module_name}\n"
+          @unsatisfied_because += "OS X System's " if from_osx?
+          @unsatisfied_because += "Brewed " if brewed?
+          @unsatisfied_because += "External " unless brewed? || from_osx?
+          @unsatisfied_because += "Python cannot `import #{module_name}`. Install with:\n  "
+          unless importable? 'pip'
+            @unsatisfied_because += "sudo easy_install pip\n  "
+          end
+          @unsatisfied_because += "pip-#{version.major}.#{version.minor} install #{@imports[module_name]}"
+          false
+        else
+          true
+        end
+      end.all?  # all given `module_name`s have to be `importable?`
     end
   end
 
+  def importable? module_name
+    quiet_system(binary, "-c", "import #{module_name}")
+  end
+
   # The full path to the python or python3 executable, depending on `version`.
   def binary
     @binary ||= begin
@@ -90,8 +120,15 @@ class PythonInstalled < Requirement
         # Note, we don't support homebrew/versions/pythonXX.rb, though.
         Formula.factory(@name).opt_prefix/"bin/python#{@min_version.major}"
       else
-        # This should find at least system python, because /usr/bin is in PATH:
-        which(@name)
+        begin
+          # Using the ORIGINAL_PATHS here because in superenv, the user
+          # installed external Python is not visible otherwise.
+          tmp_PATH = ENV['PATH']
+          ENV['PATH'] = ORIGINAL_PATHS.join(':')
+          which(@name)
+        ensure
+          ENV['PATH'] = tmp_PATH
+        end
       end
     end
   end
@@ -257,7 +294,7 @@ class PythonInstalled < Requirement
       import sys
 
       if sys.version_info[0] == #{version.major} and sys.version_info[1] == #{version.minor}:
-          if sys.executable.startswith('#{HOMEBREW_PREFIX}'):
+          if sys.executable.startswith('#{HOMEBREW_PREFIX}/opt/python'):
               # Fix 1)
               #   A setuptools.pth and/or easy-install.pth sitting either in
               #   /Library/Python/2.7/site-packages or in
@@ -285,7 +322,7 @@ class PythonInstalled < Requirement
                   from _sysconfigdata import build_time_vars
                   build_time_vars['LINKFORSHARED'] = '-u _PyMac_Error #{HOMEBREW_PREFIX}/opt/#{name}/Frameworks/Python.framework/Versions/#{version.major}.#{version.minor}/Python'
               except:
-                  pass  # remember: don't print here. Better to fail silent.
+                  pass  # remember: don't print here. Better to fail silently.
           # Fix 5)
           #   For all Pythons of the right major.minor version: Tell about homebrew's
           #   site-packages location. This is needed for Python to parse *.pth.
@@ -307,6 +344,10 @@ class PythonInstalled < Requirement
   end
 
   def hash
-    to_s.hash
+    # Requirements are a ComparableSet. So we define our identity by the
+    # selected python binary plus the @imports in order to support multiple:
+    #    depends_on :python => 'module1'
+    #    depends_on :python => 'module2'
+    (binary.to_s+@imports.to_s).hash
   end
 end