Find default caldb_root once, based on whether directories exist

src/cal_tools/calcat_interface.py

 """Interfaces to calibration constant data."""
-import re
-import socket
 from datetime import date, datetime, time, timezone
 from functools import lru_cache
-from os import getenv
 from pathlib import Path
 from weakref import WeakKeyDictionary
 
@@ -367,6 +364,7 @@ class CalibrationData:
 
     calibrations = set()
     default_client = None
+    _default_caldb_root = ...
 
     def __init__(
         self,
@@ -409,9 +407,10 @@ class CalibrationData:
         self.event_at = event_at
         self.pdu_snapshot_at = event_at
         self.module_naming = module_naming
-        if caldb_root is not None:
-            caldb_root = Path(caldb_root)
-        self._caldb_root = caldb_root
+        if caldb_root is None:
+            self.caldb_root = self._get_default_caldb_root()
+        else:
+            self.caldb_root = Path(caldb_root)
 
         if client is None:
 
@@ -492,24 +491,19 @@ class CalibrationData:
         )
         return CalibrationData.default_client
 
-    @property
-    def caldb_root(self):
-        """Root directory for calibration constant data.
-
-        Returns:
-            (Path or None) Location of caldb store or
-                None if not available.
-        """
-
-        if self._caldb_root is None:
-            if getenv("SASE"):
-                # ONC
-                self._caldb_root = Path("/common/cal/caldb_store")
-            elif re.match(r"^max-(.+)\.desy\.de$", socket.getfqdn()):
-                # Maxwell
-                self._caldb_root = Path("/gpfs/exfel/d/cal/caldb_store")
-
-        return self._caldb_root
+    @staticmethod
+    def _get_default_caldb_root():
+        if CalibrationData._default_caldb_root is ...:
+            onc_path = Path("/common/cal/caldb_store")
+            maxwell_path = Path("/gpfs/exfel/d/cal/caldb_store")
+            if onc_path.is_dir():
+                CalibrationData._default_caldb_root = onc_path
+            elif maxwell_path.is_dir():
+                CalibrationData._default_caldb_root = maxwell_path
+            else:
+                CalibrationData._default_caldb_root = None
+
+        return CalibrationData._default_caldb_root
 
     @property
     def client(self):