Ask for forgiveness, not for permission (#175)

References: 

- https://docs.python.org/3/glossary.html#term-eafp
- https://blogs.msdn.microsoft.com/pythonengineering/2016/06/29/idiomatic-python-eafp-versus-lbyl/
- https://docs.quantifiedcode.com/python-anti-patterns/readability/asking_for_permission_instead_of_forgiveness_when_working_with_files.html
- https://jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/

Author: Synthetica9

function_caching.rst

@@ -52,9 +52,9 @@ is a generic cache:
         memo = {}
         @wraps(function)
         def wrapper(*args):
-            if args in memo:
+            try:
                 return memo[args]
-            else:
+            except KeyError:
                 rv = function(*args)
                 memo[args] = rv
                 return rv