defaultdict 在python中的基本用法

from collections import defaultdict
# 使用 int 作为默认工厂函数，意味着当访问不存在的键时，会创建一个默认值为 0 的新条目。
# int() 会返回 0
word_counts = defaultdict(int)
# 访问一个不存在的键 'apple'，它会自动创建并赋值为 int() 的结果 (0)
print(word_counts['apple'])  # 输出: 0
# 现在 'apple' 键存在了，我们可以像普通字典一样操作它
word_counts['apple'] += 1
print(word_counts['apple'])  # 输出: 1
word_counts['banana'] += 1
word_counts['apple'] += 1
print(word_counts)  # 输出: defaultdict(<class 'int'>, {'apple': 2, 'banana': 1})

counts = {}
my_list = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
for item in my_list:
    if item in counts:
        counts[item] += 1
    else:
        counts[item] = 1
print(counts)

from collections import defaultdict
counts = defaultdict(int)
my_list = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
for item in my_list:
    counts[item] += 1  # 如果 'item' 不存在，会自动创建为 0，然后加 1
print(counts)

words_by_initial = {}
words = ['apple', 'banana', 'apricot', 'cat', 'dog']
for word in words:
    initial = word[0]
    if initial in words_by_initial:
        words_by_initial[initial].append(word)
    else:
        words_by_initial[initial] = [word]
print(words_by_initial)

from collections import defaultdict
words_by_initial = defaultdict(list)
words = ['apple', 'banana', 'apricot', 'cat', 'dog']
for word in words:
    initial = word[0]
    words_by_initial[initial].append(word) # 如果 'initial' 不存在，会自动创建为空列表 []
print(words_by_initial)

from collections import defaultdict
sales_data = [
    {'product': 'A', 'amount': 100},
    {'product': 'B', 'amount': 50},
    {'product': 'A', 'amount': 75},
    {'product': 'C', 'amount': 200},
    {'product': 'B', 'amount': 120},
]
total_sales = defaultdict(float) # 或者 int，根据你的数据类型
for item in sales_data:
    product = item['product']
    amount = item['amount']
    total_sales[product] += amount
print(total_sales)
# 输出: defaultdict(<class 'float'>, {'A': 175.0, 'B': 170.0, 'C': 200.0})

d = defaultdict(int)
print(d['a']) # 0

d = defaultdict(list)
d['a'].append(1)
print(d) # defaultdict(<class 'list'>, {'a': [1]})

d = defaultdict(set)
d['a'].add(1)
d['a'].add(2)
print(d) # defaultdict(<class 'set'>, {'a': {1, 2}})

d = defaultdict(str)
print(d['a']) # ''

def create_default_value():
    return "Not Found"
d = defaultdict(create_default_value)
print(d['unknown_key']) # Not Found

d = defaultdict(lambda: {'count': 0, 'items': []})
d['product1']['count'] += 1
d['product1']['items'].append('item_x')
print(d['product1']) # {'count': 1, 'items': ['item_x']}

counts = {}
my_list = ['apple', 'banana', 'apple']
for item in my_list:
    counts[item] = counts.setdefault(item, 0) + 1
print(counts) # {'apple': 2, 'banana': 1}

words_by_initial = {}
words = ['apple', 'banana', 'apricot']
for word in words:
    initial = word[0]
    words_by_initial.setdefault(initial, []).append(word)
print(words_by_initial) # {'a': ['apple', 'apricot'], 'b': ['banana']}