Python 3 Object-Oriented Programming - Third Edition

Generator expressions

Sometimes we want to process a new sequence without pulling a new list, set, or dictionary into system memory. If we're just looping over items one at a time, and don't actually care about having a complete container (such as a list or dictionary) created, creating that container is a waste of memory. When processing one item at a time, we only need the current object available in memory at any one moment. But when we create a container, all the objects have to be stored in that container before we start processing them.

For example, consider a program that processes log files. A very simple log might contain information in this format:

Jan 26, 2015 11:25:25 DEBUG This is a debugging message. Jan 26, 2015 11:25:36 INFO ...