Run Python from Ruby, stream stdout to logs and print stderr as the exception message

python_cmd.rb

require 'pty'

class PythonCmd
  class ExecutionError < StandardError; end

  attr_reader :file

  def initialize(file, args = [])
    @file = file
    @args = args
  end

  def run!
    stderr_reader, stderr_writer = IO.pipe

    PTY.spawn('python3', @file, *@args, err: stderr_writer.fileno) do |stdout, _stdin, pid|
      begin
        stdout.sync
        stdout.each { |line| Nephew::Application.logger.info line }
      rescue Errno::EIO => e
        Nephew::Application.logger.debug "Errno::EIO error, but this probably means
          that the process has finished giving output: #{e.message}".squish
      ensure
        ::Process.wait(pid)
        stderr_writer.close
      end

      if $CHILD_STATUS.exitstatus.nonzero?
        msg = <<-EOL.squish
          Non-zero exit code: #{$CHILD_STATUS.exitstatus}: '#{stderr_reader.read}'
          (file #{@file}, args #{@args})
        EOL

        raise ExecutionError, msg
      end
    end
  rescue PTY::ChildExited => e
    raise ExecutionError, e.message
  ensure
    stderr_reader.close
  end
end