code: def available_timezones():
      """Returns a set containing all available time zones.
  
      .. caution::
  
          This may attempt to open a large number of files, since the best way to
          determine if a given file on the time zone search path is to open it
          and check for the "magic string" at the beginning.
      """
      from importlib import resources
  
      valid_zones = set()
  
      # Start with loading from the tzdata package if it exists: this has a
      # pre-assembled list of zones that only requires opening one file.
      try:
          with resources.files("tzdata").joinpath("zones").open(
              "r"
          ) as f:
              for zone in f:
                  zone = zone.strip()
                  if zone:
                      valid_zones.add(zone)
      except (ImportError, FileNotFoundError):
          pass
  
      def valid_key(fpath):
          try:
              with open(fpath, "rb") as f:
                  return f.read(4) == b"TZif"
          except Exception:  # pragma: nocover
              return False
  
      for tz_root in TZPATH:
          if not os.path.exists(tz_root):
              continue
  
          for root, dirnames, files in os.walk(tz_root):
              if root == tz_root:
                  # right/ and posix/ are special directories and shouldn't be
                  # included in the output of available zones
                  if "right" in dirnames:
                      dirnames.remove("right")
                  if "posix" in dirnames:
                      dirnames.remove("posix")
  
              for file in files:
                  fpath = os.path.join(root, file)
  
                  key = os.path.relpath(fpath, start=tz_root)
                  if os.sep != "/":  # pragma: nocover
                      key = key.replace(os.sep, "/")
  
                  if not key or key in valid_zones:
                      continue
  
                  if valid_key(fpath):
                      valid_zones.add(key)
  
      if "posixrules" in valid_zones:
          # posixrules is a special symlink-only time zone where it exists, it
          # should not be included in the output
          valid_zones.remove("posixrules")
  
      return valid_zones
  
file_path: cpython/Lib/zoneinfo/_tzpath.py
imports: import os
  import sysconfig
label_desc: Use image node_id 6 for calling a global function with example usage: available_timezones() and returns: valid_zones, unknown, False