bpo-40536: Add zoneinfo.available_timezones (GH-20158)

This was not specified in the PEP, but it will likely be a frequently requested feature if it's not included.

This includes only the "canonical" zones, not a simple listing of every valid value of `key` that can be passed to `Zoneinfo`, because it seems likely that that's what people will want.

Lib/zoneinfo/_tzpath.py

@@ -102,6 +102,71 @@ def _validate_tzfile_path(path, _base=_TEST_PATH):
 del _TEST_PATH
 
 
+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.open_text("tzdata", "zones") 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
+
+
 class InvalidTZPathWarning(RuntimeWarning):
     """Warning raised if an invalid path is specified in PYTHONTZPATH."""