Dictionaries

Abstract

In addition to integers, floats, strings, and lists, Python has a powerful built-in Python data type called a dictionary. Dictionaries are useful for storing tables of information with a unique identifier for each record. Dictionaries provide a mapping from a set of keys to a set of values. In other words, given a key, a dictionary can look up the value associated with it. These have many applications in GIS, including reading GIS attribute tables or text data files and modifying them within a script. Also, they are often used to store pairs of items that go together. For example, soil science uses standard classifications for soil, abbreviated with terms such as ‘Ap’ and ‘Cg’; However, more explicit names such as ‘Plinthic Acrisol’ (for ‘Ap’) and ‘Gleyic Chernozem’ (for ‘Cg’) are needed for some analysis. A dictionary can be used to store these terms so that the abbreviations are associated with the full names. This chapter shows the dictionary syntax for creating associations like this and then it shows how to access, update, and modify dictionaries.