Expert Python Programming

The leading and trailing underscores are used to mark the privacy and special elements.

CamelCase, where words are capitalized and grouped
mixedCase, which is like CamelCase, but starts with a lower case character
UPPERCASE, and UPPER_CASE_WITH_UNDERSCORES
lowercase and lower_case_with_underscores
_leading and trailing_ underscores, and sometimes __doubled__

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