Expert Python Programming - Third Edition

Constants

For constant global variables, an uppercase with an underscore is used. It informs the developer that the given variable represents a constant value.

There are no real constants in Python like those in C++, where const can be used. You can change the value of any variable. That's why Python uses a naming convention to mark a variable as a constant.

For example, the doctest module provides a list of option flags and directives (http://docs.python.org/lib/doctest-options.html) that are small sentences, clearly defining what each option is intended for, for example:

from doctest import IGNORE_EXCEPTION_DETAIL 
from doctest import REPORT_ONLY_FIRST_FAILURE 

These variable names seem rather long, but it is important to clearly describe ...