Using more complex structures – maps of lists

In Chapter 4, Built-in Data Structures – list, set, dict, we looked at the basic data structures available in Python. The recipes generally looked at the various structures in isolation.

We'll look at a common combination structure—the mapping from a key to a list. This is used to accumulate detailed information about an object identified by a given key. This recipe will transform a flat list of details into a structure that for one column contains values taken from other columns.

Getting ready

We'll work with an imaginary web log that's been transformed from the raw web format to a CSV (comma-separated value) format. This kind of transformation is often done with a regular expression that picks out the various syntactic groups. See the String parsing with regular expressions recipe in Chapter 1, Numbers, Strings, and Tuples, for information on how the parsing might work.

The raw data looks like the following:

[2016-04-24 11:05:01,462] INFO in module1...