一、字典、元組的多重巢狀
例 1：記錄全班學生的成績。
分析：定義一個 SimpleGradebook類，
學生名是字典self._grades的鍵，成績是字典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)
字典可能因為功能過多導致結果多重巢狀。
例 2：擴充 SimpleGradebook類，按科目儲存成績。
分析：定義一個 BySubjectGradebook類，字典by_subject巢狀在字典self._grades內。
學生名是字典self._grades的鍵，科目、成績是self._grades的值。
科目是字典by_subject的鍵，成績是字典by_subject的值。
class BySubjectGradebook():
"""
report_grade(), average_grade()巢狀了兩層的字典
"""
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：需求變更，需記錄每次成績佔總成績的權重。（期中、期末考試所佔的分量比隨堂考大）
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
book = WeightedGradebook()
book.add_student('qinlu')
book.report_grade('qinlu', 'Math', 99, 0.1)
book.report_grade('qinlu', 'Math', 88, 0.6)
book.report_grade('qinlu', 'Computer', 90, 0.1)
book.report_grade('qinlu', 'Computer', 80, 0.6)
print(book.average_grade('qinlu'))
該程式碼出現字典、元組的多層巢狀，應拆解為類。多層巢狀的程式碼，很難維護。
二、巢狀結構重構為類
將下面的字典重構為類。
字典by_subject巢狀在字典self._students內。
{'qinlu': {'Math': [(99, 0.1), (88, 0.9)], 'Computer': [(90. 0.1), (80, 0.9)]}}
分析：
-
Gradebook()類，學生名是字典
self._students的鍵；科目、成績、權重是self._grades的值。 -
Student()類，科目是字典
self._subjects的鍵；成績、權重是self._subjects的值。 -
Subject()類，成績是列表
self._grades的第一位；權重是列表self._grades的第二位。
從最底層開始重構，即考試成績。這麼簡單的資訊，沒必要寫成類。
namedtuple()命名元組。
from collections import namedtuple
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Grade = namedtuple('Grade', ('score', 'weight'))
# 科目類，該類包含考試成績
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
# 學生類，該類包含學習課程
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
# 成績冊類，包含所有學生考試成績的容器類，該容器類以學生名字為鍵，可動態新增學生
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
雖然程式碼量是原來的兩倍，但更清晰，更易擴充套件，理解起來比原來容易。