Missing Keys in Dictionaries: Semantics of C#, Java, PHP and Python

Today's popular languages implement Dictionaries: a structure that can efficiently look up values associated with a set of keys. On close examination, we find that different implementations provide programmers different options for missing keys: something that can make a big difference for the reliability and maintainability of applications. This article contrasts the implementation of Dictionaries in C#, PHP, Python and Java and provides implementations of Python-style semantics as a PHP function and C# extension method