python 学习之：字典默认值的用法

默认值

从字典中读取时使用默认值

dic = {"a":1,"b":2, "c":3, "d":4}

key = "e"
value = 0
if key in dic:
    value = dic[key]
else:
    value = 0

1
2
3
4
5
6
7
8
9

value = dic.get("e",0)
# 同时也可以将列表作为默认值
value2 = dic.get("e",[1,2,3])
1
2
3

value
1

0
1

value2
1

[1, 2, 3]
1

创建字典时使用默认值

words = ["apple","banana","airline","bank","panda"]
by_letters = dict()

1
2
3

for word in words:
    first_letter = word[0]
    if first_letter in by_letters:
        by_letters[first_letter].append(word)
    else:
        by_letters[first_letter] = [word]
1
2
3
4
5
6

by_letters
1

{'a': ['apple', 'airline'], 'b': ['banana', 'bank'], 'p': ['panda']}
1

setdefault 方法

by_letters = {}
for word in words:
    by_letters.setdefault(word[0],[]).append(word)
1
2
3

by_letters
1

{'a': ['apple', 'airline'], 'b': ['banana', 'bank'], 'p': ['panda']}
1

collection 中的 defaultdict

from collections import defaultdict
by_letters = defaultdict(list)
for word in words:
    by_letters[word[0]].append(word)
1
2
3
4

by_letters
1

defaultdict(list,
            {'a': ['apple', 'airline'],
             'b': ['banana', 'bank'],
             'p': ['panda']})
1
2
3
4