面向非程序员的 Python 2.6 教程/字典
本章节将介绍字典。如果您打开一本字典，您应该注意到每个条目都包含两个部分，一个单词和该单词的定义。单词是找到单词含义的关键，而单词的含义被认为是该键的值。在 Python 中，字典有键和值。键用于查找值。以下是一个使用字典的示例
def print_menu():
print '1. Print Dictionary'
print '2. Add definition'
print '3. Remove word'
print '4. Lookup word'
print '5. Quit'
print
words = {}
menu_choice = 0
print_menu()
while menu_choice != 5:
menu_choice = input("Type in a number (1-5): ")
if menu_choice == 1:
print "Definitions:"
for x in words.keys():
print x, ": ", words[x]
print
elif menu_choice == 2:
print "Add definition"
name = raw_input("Word: ")
means = raw_input("Definition: ")
words[name] = means
elif menu_choice == 3:
print "Remove word"
name = raw_input("Word: ")
if name in words:
del words[name]
print name, " was removed."
else:
print name, " was not found."
elif menu_choice == 4:
print "Lookup Word"
name = raw_input("Word: ")
if name in words:
print "The definition of ", name, " is: ", words[name]
else:
print "No definition for ", name, " was found."
elif menu_choice != 5:
print_menu()
这是我的输出
1. Print Dictionary 2. Add definition 3. Remove word 4. Lookup word 5. Quit Type in a number (1-5): 2 Add definition Word: Python Definition: A snake, a programming language, and a British comedy. Type in a number (1-5): 2 Add definition Word: Dictionary Definition: A book where words are defined. Type in a number (1-5): 1 Definitions: Python: A snake, a programming language, and a British comedy. Dictionary: A book where words are defined. Type in a number (1-5): 4 Lookup Word Word: Python The definition of Python is: A snake, a programming language, and a British comedy. Type in a number (1-5): 3 Remove Word Word: Dictionary Dictionary was removed. Type in a number (1-5): 1 Definitions: Python: A snake, a programming language, and a British comedy. Type in a number (1-5): 5
该程序类似于之前章节中列表的名称列表（请注意，列表使用索引，而字典不使用索引）。以下是程序的工作原理
- 首先定义函数
print_menu。print_menu只是打印一个菜单，该菜单稍后在程序中使用两次。 - 接下来是奇怪的代码行
words = {}。该行只是告诉 Pythonwords是一个字典。 - 接下来的几行只是让菜单正常工作。
for x in words.keys():
print x, ": ", words[x]
- 这会遍历字典并打印所有信息。函数
words.keys()返回一个列表，该列表随后被for循环使用。keys()返回的列表没有特定的顺序，因此如果您想要按字母顺序排列，则必须对其进行排序。类似于列表，语句words[x]用于访问字典的特定成员。当然，在这种情况下，x是一个字符串。 - 接下来，代码行
words[name] = means将一个单词和定义添加到字典中。如果name已经存在于字典中，means将替换之前存在的内容。
if name in words:
del words[name]
- 查看 name 是否在 words 中，如果在的话就将其移除。表达式
name in words如果name是words中的键，则返回 True，否则返回 False。代码行del words[name]删除键name以及与该键关联的值。
if name in words:
print "The definition of ", name, " is: ", words[name]
- 检查 words 是否包含某个键，如果包含的话，就打印与之关联的定义。
- 最后，如果菜单选择无效，则会重新打印菜单以供您查看。
回顾：字典有键和值。键可以是字符串或数字。键指向值。值可以是任何类型的变量（包括列表，甚至字典（这些字典或列表当然可以包含字典或列表本身（吓人吗？:-) )）。以下是用列表在字典中使用的一个示例
max_points = [25, 25, 50, 25, 100]
assignments = ['hw ch 1', 'hw ch 2', 'quiz ', 'hw ch 3', 'test']
students = {'#Max': max_points}
def print_menu():
print "1. Add student"
print "2. Remove student"
print "3. Print grades"
print "4. Record grade"
print "5. Print Menu"
print "6. Exit"
def print_all_grades():
print '\t',
for i in range(len(assignments)):
print assignments[i], '\t',
print
keys = students.keys()
keys.sort()
for x in keys:
print x, '\t',
grades = students[x]
print_grades(grades)
def print_grades(grades):
for i in range(len(grades)):
print grades[i], '\t', '\t',
print
print_menu()
menu_choice = 0
while menu_choice != 6:
print
menu_choice = input("Menu Choice (1-6): ")
if menu_choice == 1:
name = raw_input("Student to add: ")
students[name] = [0] * len(max_points)
elif menu_choice == 2:
name = raw_input("Student to remove: ")
if name in students:
del students[name]
else:
print "Student:", name, "not found"
elif menu_choice == 3:
print_all_grades()
elif menu_choice == 4:
print "Record Grade"
name = raw_input("Student: ")
if name in students:
grades = students[name]
print "Type in the number of the grade to record"
print "Type a 0 (zero) to exit"
for i in range(len(assignments)):
print i + 1, assignments[i], '\t',
print
print_grades(grades)
which = 1234
while which != -1:
which = input("Change which Grade: ")
which = which - 1
if 0 <= which < len(grades):
grade = input("Grade: ")
grades[which] = grade
elif which != -1:
print "Invalid Grade Number"
else:
print "Student not found"
elif menu_choice != 6:
print_menu()
这是样本输出
1. Add student
2. Remove student
3. Print grades
4. Record grade
5. Print Menu
6. Exit
Menu Choice (1-6): 3
hw ch 1 hw ch 2 quiz hw ch 3 test
#Max 25 25 50 25 100
Menu Choice (1-6): 5
1. Add student
2. Remove student
3. Print grades
4. Record grade
5. Print Menu
6. Exit
Menu Choice (1-6): 1
Student to add: Bill
Menu Choice (1-6): 4
Record Grade
Student: Bill
Type in the number of the grade to record
Type a 0 (zero) to exit
1 hw ch 1 2 hw ch 2 3 quiz 4 hw ch 3 5 test
0 0 0 0 0
Change which Grade: 1
Grade: 25
Change which Grade: 2
Grade: 24
Change which Grade: 3
Grade: 45
Change which Grade: 4
Grade: 23
Change which Grade: 5
Grade: 95
Change which Grade: 0
Menu Choice (1-6): 3
hw ch 1 hw ch 2 quiz hw ch 3 test
#Max 25 25 50 25 100
Bill 25 24 45 23 95
Menu Choice (1-6): 6
以下是程序的工作原理。基本上，变量 students 是一个字典，键是学生的姓名，值是他们的成绩。前两行只是创建两个列表。下一行 students = {'#Max': max_points} 创建一个新的字典，键为 {#Max}，值为 [25, 25, 50, 25, 100]，因为这是 max_points 在赋值时的情况（我使用键 #Max，因为 # 排列在任何字母字符之前）。接下来定义 print_menu。接下来，print_all_grades 函数在以下几行中定义
def print_all_grades():
print '\t',
for i in range(len(assignments)):
print assignments[i], '\t',
print
keys = students.keys()
keys.sort()
for x in keys:
print x, '\t',
grades = students[x]
print_grades(grades)
注意，首先使用 keys 函数从 students 字典中获取键，代码行 keys = students.keys()。keys 是一个列表，因此所有列表函数都可以用于它。接下来，键在代码行 keys.sort() 中进行排序，因为它是列表。for 用于遍历所有键。成绩作为列表存储在字典中，因此赋值 grades = students[x] 会使 grades 等于存储在键 x 处的列表。函数 print_grades 只是打印一个列表，并在接下来的几行中定义。
程序的后续代码行实现了菜单的各种选项。代码行 students[name] = [0] * len(max_points) 将学生添加到其姓名的键中。符号 [0] * len(max_points) 只是创建了一个与 max_points 列表长度相同的 0 列表。
移除学生条目只是删除学生，类似于电话簿示例。记录成绩选项稍微复杂一些。成绩在代码行 grades = students[name] 中检索，获取学生 name 成绩的引用。然后，在代码行 grades[which] = grade 中记录成绩。您可能注意到，grades 从未放回 students 字典中（如没有 students[name] = grades）。缺少该语句的原因是，grades 实际上是 students[name] 的另一个名称，因此更改 grades 会更改 student[name]。
字典提供了一种简单的方法来将键与值关联起来。这可以用来轻松跟踪附加到各个键的数据。