Python 2.x and 3.x support

New `depends_on :python` Dependency.
New `depends_on :python3` Dependency.

To avoid having multiple formulae with endings -py2 and -py3,
we will handle support for different pythons (2.x vs. 3.x)
in the same formula.
Further brewed vs. external python will be transparently supported.

The formula also gets a new object `python`, which is false if
no Python is available or the user has disabled it. Otherwise
it is defined and provides several support methods:

python.site_packages # the site-packages in the formula's Cellar
python.global_site_packages
python.binary # the full path to the python binary
python.prefix
python.version
python.version.major
python.version.minor
python.xy # => e.g. "python2.7"
python.incdir # includes of python
python.libdir # the python dylib library
python.pkg_config_path # used internally by brew
python.from_osx?
python.framework?
python.universal?
python.pypy?
python.standard_caveats # Text to set PYTHONPATH for python.from_osx?
python.if3then3 # => "" for 2.x and to "3" for 3.x.

Further, to avoid code duplication, `python` takes an optional
block that is run twice if the formula defines depends_on
:python AND :python3.

python do
  system python, 'setup.py', "--prefix=#{prefix}"
end

Read more in the Homebrew wiki.

1 files changed, 80 insertions, 0 deletions

diff --git a/Library/Homebrew/python_helper.rb b/Library/Homebrew/python_helper.rb
new file mode 100644
index 000000000..ef71b2a30
--- /dev/null
+++ b/Library/Homebrew/python_helper.rb
@@ -0,0 +1,80 @@
+
+# The python_helper is used in the Formula class when the user calls
+# `python`, `python2` or `python3`.
+
+# This method has a dual nature. For one, it takes a &block and sets up
+# the ENV such that a Python, as defined in the requirements, is the default.
+# If there are multiple `PythonInstalled` requirements, the block is evaluated
+# once for each Python. This makes it possible to easily support 2.x and
+# 3.x Python bindings without code duplication in formulae.
+# If you need to special case stuff, set :allowed_major_versions.
+# Second, inside the block, a formula author may call this method to access
+# certain convienience methods for the currently selected Python, e.g.
+# `python.site_packages`.
+def python_helper(options={:allowed_major_versions => [2, 3]}, &block)
+  if !block_given? and !@current_python.nil?
+    # We are already inside of a `python do ... end` block, so just return
+    # the current_python or false if the version.major is not allowed.
+    if options[:allowed_major_versions].include?(@current_python.version.major)
+      @current_python
+    else
+      false
+    end
+  else
+    # Look for PythonInstalled requirements for this formula
+    python_reqs = requirements.select{ |r| r.kind_of?(PythonInstalled) }
+    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
+      end
+    end
+
+    # Allow to use an else-branch like so: `if python do ... end; else ... end`
+    return false if 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|
+      # Now is the time to set the site_packages to the correct value
+      py.site_packages = lib/py.xy/'site-packages'
+      if block_given?
+        puts "brew: Python block (#{py.binary})..." if ARGV.verbose?
+        require 'superenv'
+        # Ensure env changes are only temporary by using `with_build_environment`
+        ENV.with_build_environment do
+          # In order to install into the Cellar, the dir must exist and be in the
+          # PYTHONPATH. This will be executed in the context of the formula
+          # so that lib points to the HOMEBREW_PREFIX/Cellar/<formula>/<version>/lib
+          puts "brew: Setting PYTHONPATH=#{py.site_packages}" if ARGV.verbose?
+          mkdir_p py.site_packages
+          ENV.append 'PYTHONPATH', py.site_packages, ':'
+          ENV['PYTHON'] = py.binary
+          ENV.prepend 'CMAKE_INCLUDE_PATH', py.incdir, ':'
+          ENV.prepend 'PKG_CONFIG_PATH', py.pkg_config_path, ':' if py.pkg_config_path
+          ENV.prepend 'PATH', py.binary.dirname, ':' unless py.from_osx?
+          # Track the state of the currently selected python for this block,
+          # so if this python_helper is called again _inside_ the block, we can
+          # just return the right python (see `else`-branch a few lines down):
+          @current_python = py
+          res = instance_eval(&block)
+          @current_python = nil
+          res
+        end
+      else
+        puts "brew: Using #{py.binary}" if ARGV.verbose?
+        # We return here with intention, because no block_given?
+        return py
+      end
+    end
+  end
+end