例 1：记录全班学生的成绩。
分析：定义一个 SimpleGradebook类，
学生名是字典self._grades的键，成绩是字典self._grades的值。
class SimpleGradebook():
def __init__(self):
self._grades = {}
def add_student(self, name):
self._grades[name] = []
def report_grade(self, name, score):
self._grades[name].append(score)
def average_grade(self, name):
grades = self._grades[name]
return self._grades, sum(grades) / len(grades)
book = SimpleGradebook()
book.add_student('qinlu')
book.report_grade('qinlu', 99)
print(book.average_grade('qinlu'))
({'qinlu': [99]}, 99.0)
字典可能因为功能过多导致结果多重嵌套。
例 2：扩充 SimpleGradebook类，按科目保存成绩。
分析：定义一个 BySubjectGradebook类，字典by_subject嵌套在字典self._grades内。
学生名是字典self._grades的键，科目、成绩是self._grades的值。
科目是字典by_subject的键，成绩是字典by_subject的值。
class BySubjectGradebook():
"""
report_grade(), average_grade()嵌套了两层的字典
"""
def __init__(self):
self._grades = {}
def add_student(self, name):
self._grades[name] = {}
def report_grade(self, name, subject, score):
by_subject = self._grades[name]
grade_list = by_subject.setdefault(subject, [])
grade_list.append(score)
def average_grade(self, name):
by_subject = self._grades[name]
total, count = 0, 0
for scores in by_subject.values():
total += sum(scores)
count += len(scores)
return self._grades, total / count
book = BySubjectGradebook()
book.add_student('qinlu')
book.report_grade('qinlu', 'Math', 99)
book.report_grade('qinlu', 'Math', 88)
book.report_grade('qinlu', 'Computer', 90)
book.report_grade('qinlu', 'Computer', 80)
print(book.average_grade('qinlu'))({'qinlu': {'Math': [99, 88], 'Computer': [90, 80]}}, 89.25)
例 3：需求变更，需记录每次成绩占总成绩的权重。（期中、期末考试所占的分量比随堂考大）
class WeightedGradebook():
def __init__(self):
self._grades = {}
def add_student(self, name):
self._grades[name] = {}
def report_grade(self, name, subject, score, weight):
by_subject = self._grades[name]
grade_list = by_subject.setdefault(subject, [])
grade_list.append(score, weight)
def average_grade(self, name):
by_subject = self._grades[name]
score_sum, score_count = 0, 0
for subject, scores in by_subject.items():
subject_avg, total_weight = 0, 0
for score, weight in scores:
#...
return score_sum / score_count该代码出现字典、元组的多层嵌套，应拆解为类。多层嵌套的代码，很难维护。
将下面的字典重构为类。
字典by_subject嵌套在字典self._students内。
{'qinlu': {'Math': [(99, 0.1), (88, 0.9)], 'Computer': [(90. 0.1), (80, 0.9)]}}分析：
① Gradebook()类，学生名是字典self._students的键；科目、成绩、权重是self._grades的值。
② Student()类，科目是字典self._subjects的键；成绩、权重是self._subjects的值。
③ Subject()类，成绩是列表self._grades的第一位；权重是列表self._grades的第二位。
从最底层开始重构，即考试成绩。这么简单的信息，没必要写成类。
namedtuple()命名元组。
'''
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'''
from collections import namedtuple
Grade = namedtuple('Grade', ('score', 'weight'))
# 科目类，该类包含考试成绩
class Subject():
def __init__(self):
self._grades = []
def report_grade(self, score, weight):
self._grades.append(Grade(score, weight))
def average_grade(self):
total, total_weight = 0, 0
# print(self._grades)
for grade in self._grades:
# print(grade)
total += grade.score * grade.weight
total_weight += grade.weight
return total / total_weight
# 学生类，该类包含学习课程
class Student():
def __init__(self):
self._subjects = {}
def subject(self, name):
if name not in self._subjects:
self._subjects[name] = Subject()
return self._subjects[name]
def average_grade(self):
total, count = 0, 0
for subject in self._subjects.values():
total += subject.average_grade()
count += 1
return total / count
# 成绩册类，包含所有学生考试成绩的容器类，该容器类以学生名字为键，可动态添加学生
class Gradebook():
def __init__(self):
self._students = {}
def student(self, name):
if name not in self._students:
self._students[name] = Student()
return self._students[name]
book = Gradebook()
qin = book.student('qinlu')
math = qin.subject('Math')
math.report_grade(99, 0.1)
math.report_grade(88, 0.9)
print(qin.average_grade())89.1
虽然代码量是原来的两倍，但更清晰，更易扩展，理解起来比原来容易。
