Python Object Attribute Variable Names Beginning With a Single Underscore

Cited from the book "Python How to Program"

Attribute names that begin with a single underscore have no special meaning in the syntax of the Python language itself. However, the single leading underscore is a convention among Python programmers who use the class. When a class author creates an attribute with a single leading underscore, the author does not want users of the class to access the attribute directly. If a program requires access to the attributes, the class author provides some other means for doing so. In this case, we provide access methods through which clients should manipulate the data.