code: def _create(self):
          
          if self.readonly:
              raise VirtualenvReadonlyException()
          args = ['virtualenv']
          if self.system_site_packages:
              args.append('--system-site-packages')
          if self.python is None:
              args.append(self.name)
          else:
              args.extend(['-p', self.python, self.name])
          proc = subprocess.Popen(args, cwd=self.root, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
          output, error = proc.communicate()
          returncode = proc.returncode
          if returncode:
              raise VirtualenvCreationException((returncode, output, self.name))
          self._write_to_log(output, truncate=True)
          self._write_to_error(error, truncate=True)
docs: Executes `virtualenv` to create a new environment.