Determine if the member should be set to the default string with just the or operator

If you look at source of openpyxl, ```or` Make a note of some brilliant idioms that use the `` operator on strings.

In the self.title = title or self._default_title part of the code below, if the argument title of __init__ is None or an empty string, the value of the class variable _default_title " Sheet " is set to self.title as the default string.

child.py

class _WorkbookChild(object):

    __title = ""
    _id = None
    _path = "{0}"
    __parent = None
    _default_title = "Sheet"

    def __init__(self, parent=None, title=None):
        self.__parent = parent
        self.title = title or self._default_title
        self.HeaderFooter = HeaderFooter()

Addendum) There was a detailed explanation in this article. http://qiita.com/keisuke-nakata/items/e0598b2c13807f102469