list derivation(列表推导式)
generator derivation(生成器推导式)
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dict derivation (字典推导式)
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推导式(Derivation)
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l = [x*x for x in range(5)]
# [0, 1, 4, 9, 16] 直接返回列表而不是生成器
[i+1 if i>0 else 1 for i in l]
# [1, 2, 5, 10, 17]
[i+1 if i>0 else 2 for i in l]
# [2, 2, 5, 10, 17] # 说明 如果是取else的话,就不再去做更新的操作
[i for i in l if i >4 and i <=16] # [9,16]
vec = [[1,2,3], [4,5,6], [7,8,9]]
[num for elem in vec for num in elem]
# [1, 2, 3, 4, 5, 6, 7, 8, 9]
[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
# [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
```
改变字典的值
```python
l_d = [{'name':'femn','age':25},{'name':'femnyy','age':35}]
[i if i.get('age')> 25 else i for i in l_d]
# 虽然此时不能得到自己想要的改变 不能用i['name']='femnhh'去做改变
[i.update({'name':'femnhh'}) if i.get('age')> 25 else i for i in l_d]
# [{'age': 25, 'name': 'femn'}, None] # 相当于是匹配没有匹配成功则返回None
# 但此时l_d却已经是改变成想要的了
l_d
# [{'age': 25, 'name': 'femn'}, {'age': 35, 'name': 'femnhh'}]