Python 避免字典和元组的多重嵌套
一、字典、元组的多重嵌套
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
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'))
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。
二、嵌套结构重构为类
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。