Implement decorator to lazy initialise a property

lib/tfw/decorators/__init__.py

@@ -2,3 +2,4 @@
 # All Rights Reserved. See LICENSE file for details.
 
 from .rate_limiter import RateLimiter
+from .lazy_initialise import LazyInitialise

lib/tfw/decorators/lazy_initialise.py

@@ -0,0 +1,19 @@
+# Copyright (C) 2018 Avatao.com Innovative Learning Kft.
+# All Rights Reserved. See LICENSE file for details.
+
+
+class LazyInitialise:
+    """
+    Decorator that replaces a function with the value
+    it calculates on the first call.
+    """
+    def __init__(self, func):
+        self.func = func
+        self.__doc__ = func.__doc__
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            raise TypeError('Cannot get object property from class!')
+        value = self.func(instance)
+        setattr(instance, self.func.__name__, value)
+        return value