Use power of LazyInitialise to murder TFW/TAOENV global state

lib/envvars/__init__.py

@@ -4,9 +4,20 @@
 from collections import namedtuple
 from os import environ
 
+from tfw.decorators import LazyInitialise
 
-def prefixed_envvars_to_namedtuple(prefix: str, tuple_name: str):
-    envvars = {envvar.replace(prefix, '', 1): environ.get(envvar)
-               for envvar in environ.keys()
-               if envvar.startswith(prefix)}
-    return namedtuple(tuple_name, envvars)(**envvars)
+
+class LazyEnvironment:
+    def __init__(self, prefix, tuple_name):
+        self._prefix = prefix
+        self._tuple_name = tuple_name
+
+    @LazyInitialise
+    def environment(self):
+        return self.prefixed_envvars_to_namedtuple()
+
+    def prefixed_envvars_to_namedtuple(self):
+        envvars = {envvar.replace(self._prefix, '', 1): environ.get(envvar)
+                   for envvar in environ.keys()
+                   if envvar.startswith(self._prefix)}
+        return namedtuple(self._tuple_name, envvars)(**envvars)