--- a/configmix/config.py	Mon Mar 07 09:11:22 2022 +0100
+++ b/configmix/config.py	Fri Mar 11 01:53:08 2022 +0100
@@ -451,14 +451,23 @@
         #
         # PY2.7 compat: must be set before calling the superclass' __init__
         #
-        self.__lookup_cache = {}
-        self.__interpolation_cache = {}
+        self.enable_cache()
         super(Configuration, self).__init__(*args, **kwds)
 
     def clear_cache(self):
         """Clear the internal lookup cache and the interpolation cache"""
-        self.__lookup_cache.clear()
-        self.__interpolation_cache.clear()
+        if self.__lookup_cache is not None:
+            self.__lookup_cache.clear()
+        if self.__interpolation_cache is not None:
+            self.__interpolation_cache.clear()
+
+    def disable_cache(self):
+        self.__lookup_cache = None
+        self.__interpolation_cache = None
+
+    def enable_cache(self):
+        self.__lookup_cache = {}
+        self.__interpolation_cache = {}
 
     def __getitem__(self, key):
         """Mapping and list interface that forwards to :meth:`~.getvarl_s`
@@ -834,28 +843,33 @@
         """
         if not path:
             return self
-        v = self.__lookup_cache.get(path, _MARKER)
-        if v is not _MARKER:
-            if v is _MISSING:
-                raise KeyError(
-                    "Configuration variable %r not found"
-                    " (negative internal cache value)" % (path,))
-            else:
-                return v
+        use_cache = self.__lookup_cache is not None
+        if use_cache:
+            v = self.__lookup_cache.get(path, _MARKER)
+            if v is not _MARKER:
+                if v is _MISSING:
+                    raise KeyError(
+                        "Configuration variable %r not found"
+                        " (negative internal cache value)" % (path,))
+                else:
+                    return v
         eiref = self.expand_if_reference
         try:
             v = eiref(super(Configuration, self).__getitem__(path[0]))
             for p in path[1:]:
                 v = eiref(v[p])
         except TypeError:
-            self.__lookup_cache[path] = _MISSING
+            if use_cache:
+                self.__lookup_cache[path] = _MISSING
             raise KeyError(
                 "Configuration variable %r not found"
                 "(missing intermediate keys?)" % (path,))
         except KeyError:
-            self.__lookup_cache[path] = _MISSING
+            if use_cache:
+                self.__lookup_cache[path] = _MISSING
             raise
-        self.__lookup_cache[path] = v
+        if use_cache:
+            self.__lookup_cache[path] = v
         return v
 
     def _lookupref(self, key):
@@ -936,21 +950,23 @@
         start = s.find(_STARTTOK, 0)
         if start < 0:
             return s
-        res = self.__interpolation_cache.get(s, _MARKER)
-        if res is not _MARKER:
-            if res is _MISSING:
-                warnings.warn("Cannot interpolate variables in string "
-                              "%r (cached)" % (s, ),
-                              UserWarning,
-                              stacklevel=1)
-                raise KeyError("Cannot interpolate variables in string "
-                               "%r (cached)" % (s, ))
-            else:
-                return res
+        use_cache = self.__interpolation_cache is not None
+        if use_cache:
+            res = self.__interpolation_cache.get(s, _MARKER)
+            if res is not _MARKER:
+                if res is _MISSING:
+                    warnings.warn("Cannot interpolate variables in string "
+                                  "%r (cached)" % (s, ),
+                                  UserWarning,
+                                  stacklevel=1)
+                    raise KeyError("Cannot interpolate variables in string "
+                                   "%r (cached)" % (s, ))
+                else:
+                    return res
         res = []
         res_append = res.append
         rest = 0
-        use_cache = True
+        cacheable = True
         while start != -1:
             res_append(s[rest:start])
             end = s.find(_ENDTOK, start)
@@ -968,14 +984,15 @@
                 elif EMPTY_FILTER in filters:
                     varvalue = _EMPTY_STR
                 else:
-                    self.__interpolation_cache[s] = _MISSING
+                    if use_cache and cacheable:
+                        self.__interpolation_cache[s] = _MISSING
                     warnings.warn("Cannot interpolate variable %r in string "
                                   "%r" % (varname, s, ),
                                   UserWarning,
                                   stacklevel=1)
                     raise
             if not cacheable:
-                use_cache = False
+                cacheable = False
             varvalue = self._apply_filters(filters, varvalue)
             rest = end + 2
             #
@@ -983,7 +1000,7 @@
             # the whole `s` is just one expansion
             #
             if (start == 0) and (rest == len_s):
-                if use_cache:
+                if use_cache and cacheable:
                     self.__interpolation_cache[s] = varvalue
                 return varvalue
             if varvalue is None:
@@ -994,7 +1011,7 @@
             start = s.find(_STARTTOK, rest)
         res_append(s[rest:])
         res = _EMPTY_STR.join(res)
-        if use_cache:
+        if use_cache and cacheable:
             self.__interpolation_cache[s] = res
         return res