where is this documented?

[Skaperen]

the following code:

foo = {}.update(os.environ)

has a behavior that i have been unable to find documentation that describes (assuming import os has been done). does anyone know if such documentation exists and where that documentation can be found? i have PDF documentation for python version 3.5.2 but a reference in an HTML document of 3.4 or later should be good enough.