> The standard library provides the LazyLoader class to solve some of these inefficiency problems. It permits imports at the module level to work mostly like inline imports do.

The use of these sorts of Python import internals is highly non-obvious. The Stack Overflow Q&A I found about it (https://stackoverflow.com/questions/42703908/) doesn't result in an especially nice-looking UX.

So here's a proof of concept in existing Python for getting all imports to be lazy automatically, with no special syntax for the caller:

  import sys
  import threading # needed for python 3.13, at least at the REPL, because reasons
  from importlib.util import LazyLoader # this has to be eagerly imported!
  class LazyPathFinder(sys.meta_path[-1]): # <class '_frozen_importlib_external.PathFinder'>
      @classmethod
      def find_spec(cls, fullname, path=None, target=None):
          base = super().find_spec(fullname, path, target)
          base.loader = LazyLoader(base.loader)
          return base
  sys.meta_path[-1] = LazyPathFinder

We've replaced the "meta path finder" (which implements the logic "when the module isn't in sys.modules, look on sys.path for source code and/or bytecode, including bytecode in __pycache__ subfolders, and create a 'spec' for it") with our own wrapper. The "loader" attached to the resulting spec is replaced with an importlib.util.LazyLoader instance, which wraps the base PathFinder's provided loader. When an import statement actually imports the module, the name will actually get bound to a <class 'importlib.util._LazyModule'> instance, rather than an ordinary module. Attempting to access any attribute of this instance will trigger the normal module loading procedure — which even replaces the global name.

Now we can do:

  import this # nothing shows up
  print(type(this)) # <class 'importlib.util._LazyModule'>
  rot13 = this.s # the module is loaded, printing the Zen
  print(type(this)) # <class 'module'>

That said, I don't know what the PEP means by "mostly" here.