This is an experimental patch that implements lazy execution of top-level definitions in modules (functions, classes, imports, global constants).  See Tools/lazy_compile/README.txt for details.

Function definitions usually are cheap, as well as global constants definitions (unless they use complex comprehensions or call heavy functions for initialization). Creating a class is an order or more slower than creating a function. Creating enums and namedtuples is an order or two slower than creating a plain class.

Serhiy: There is a semi-common case where global constants can be quite expensive, specifically, initializing a global full of expensive to compute/serialize data so it will be shared post-fork when doing multiprocessing on a POSIX system. That said, that would likely be a case where lazy initialization would be a problem; you don't want each worker independently initializing the global lazily.

Also, for all practical purposes, aren't enums and namedtuples global constants too? Since they don't rely on any syntax based support at point of use, they're just a "function call" followed by assignment to a global name; you couldn't really separate the concept of global constants from enums/namedtuple definitions, right?