brew/Library/Homebrew/python_helper.rb

# 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?
        return py
      else
        puts "brew: Python block (#{py.binary})..." if ARGV.verbose? && ARGV.debug?
        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: Appending to 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?
          ENV.prepend 'LDFLAGS', "-F#{py.framework}" if py.brewed?
          # 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
      end
    end
  end
end