Use an enumeration when an attribute must accept a limited, named set of values. A book genre can be ADVENTURE, CLASSICS, DETECTIVE, FANTASY, HISTORIC, HORROR, ROMANCE, or SCIFI. Compared with strings, enumeration constants avoid case and spelling problems and make runtime comparisons efficient. section 2.3.2
Book genres in Python
Define Genre with Enum, then pass Genre.ADVENTURE or Genre.CLASSICS into Attributes. The catalogue stores the attributes and delegates matching to them, so genre changes remain encapsulated rather than leaking into Catalogue. section 2.3.2
Listing 2.4 (Program 2.2 Books-2): attributes.py
from enum import Enum
class Genre(Enum): #1
UNSPECIFIED = 0
ADVENTURE = 1
CLASSICS = 2
DETECTIVE = 3
FANTASY = 4
HISTORIC = 5
HORROR = 6
ROMANCE = 7
SCIFI = 8
def __str__(self): return self.name.lower()
class Attributes:
@staticmethod
def _equal_ignore_case(target_str, other_str): #2
if len(target_str) == 0:
return True
else:
return target_str.casefold() == other_str.casefold()
def __init__(self, title, last, first, year, genre):
self._title = title
self._last = last
self._first = first
self._year = year #3
self._genre = genre #3
@property
def title(self): return self._title
@property
def last(self): return self._last
@property
def first(self): return self._first
@property
def year(self): return self._year
@property
def genre(self): return self._genre
def is_match(self, target_attrs): #4
return (
Attributes._equal_ignore_case(target_attrs.title,
self._title)
and Attributes._equal_ignore_case(target_attrs.last,
self._last)
and Attributes._equal_ignore_case(target_attrs.first,
self._first)
and ( (target_attrs.year == 0)
or (target_attrs.year == self._year))
and ( (target_attrs.genre == Genre.UNSPECIFIED)
or (target_attrs.genre == self._genre))
)
def __str__(self):
return (f"{{TITLE: '{self._title}', LAST: '{self._last}', "
f"FIRST: '{self._first}', YEAR: {self._year}, " #5
f"GENRE: {self._genre}}}") #5
The listing shows named Genre constants, their numeric values, and a string representation used by the surrounding book-catalogue example. section 2.3.2
