The reason a comma is put in between the book titles is because, while they are in quotation marks, they are still part of a list. When listing things, you put a comma in between each item. For example, if you were to say “He likes bananas, oranges, and carrots”, you would have to put commas between each item that “He” is said to like. How these commas are placed is a matter of debate for some people, since many are starting to avoid using the Oxford comma. If you do not use the Oxford comma, the sentence would read “He likes bananas, oranges and carrots.”
An underscore as the first character in an id is often used to indicate internal implementation that is not considered part of the API and should not be called by code outside that implementation. Python uses this for private member variables of classes, this is common in other languages such as C++ even though those provide keywords to indicate that members are private. It is extensively used to hide variables and functions used for implementations in header files . In fact the use of single underscore for this became so common that C compilers had to standardize on a double leading underscore (for instance __DATE__ ) for actual built-in variables to avoid conflicts with the ones in header files. Python uses double underscore to "mangle" a private id to make it much harder to refer to it, and " PHP reserves all function names starting with __ as magical."