code: def iter_sites(sites=None, site=None, renderer=None, setter=None, no_secure=False, verbose=None):
      
      if verbose is None:
          verbose = get_verbose()
  
      hostname = get_current_hostname()
  
      target_sites = env.available_sites_by_host.get(hostname, None)
  
      if sites is None:
          site = site or env.SITE or ALL
          if site == ALL:
              sites = list(six.iteritems(env.sites))
          else:
              sys.stderr.flush()
              sites = [(site, env.sites.get(site))]
  
      renderer = renderer #or render_remote_paths
      env_default = save_env()
      for _site, site_data in sorted(sites):
          if no_secure and _site.endswith('_secure'):
              continue
  
          # Only load site configurations that are allowed for this host.
          if target_sites is None:
              pass
          else:
              assert isinstance(target_sites, (tuple, list))
              if _site not in target_sites:
                  if verbose:
                      print('Skipping site %s because not in among target sites.' % _site)
                  continue
  
          env.update(env_default)
          env.update(env.sites.get(_site, {}))
          env.SITE = _site
          if callable(renderer):
              renderer()
          if setter:
              setter(_site)
          yield _site, site_data
  
      # Revert modified keys.
      env.update(env_default)
  
      # Remove keys that were added, not simply updated.
      added_keys = set(env).difference(env_default)
      for key in added_keys:
          # Don't remove internally maintained variables, because these are used to cache hostnames
          # used by iter_sites().
          if key.startswith('_'):
              continue
          del env[key]
docs: Iterates over sites, safely setting environment variables for each site.