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Python学习day14-函数进阶（2）
三元表达式
三元表达式就是我们在leetcode评论里见得最多的大佬常用的那些一行代码，对，就是那些之前怎么看也看不懂的，觉得特别厉害的大佬们写的，直接return加一堆参数就出结果那种。
首先，三元表达式只支持双分支结构，不支持多分支结构，这是一个注意点，另外就是，三元表达式只是看起来特别厉害，而实际上它既没有节省时间也没有节省空间，只是看起来比较厉害而已。
例如：
cat = '1'
dog = '2'
if dog == '2':
print('dog')
else:
print('cat')
# 上面这段代码用三元表达式表示就是
print('dog')if dog == '2' else print('cat')
# 优点是省略了一些符号和换行
# 缺点是，如果代码很复杂，那么除了写的人基本没人能看懂，代码可移植性和扩展性非常差
# 所以其实平时码代码不是特别推荐这种写法，这是一种方式，显得很帅的方式，但没什么用
列表推导式
列表推导式即是用三元表达式的形式去生成一个列表，其优点是在生成的时候可以直接对列表的每一项进行算数操作。
print(F"[i for i in range(10)]:{[i for i in range(10)]}")
# 上述式子执行结果是
# [i for i in range(10)]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# 对每项操作可以如此：
print(F"[i**3 for i in range(10)]:{[i**3 for i in range(10)]}")
# 结果是：
# [i**3 for i in range(10)]:[0, 1, 8, 27, 64, 125, 216, 343, 512, 729]
字典生成式
和列表类似，我们用三元表达式的方式来生成字典，同样的可以在生成的时候就直接对字典的key或value的值进行操作。
xxxxxxxxxx
print(i:i**2 for i in range(10))
# 结果为：
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
这里字典有有一个比较有用的方法，叫做zip()，通过zip压缩，然后通过解压缩可以生成一个字典。
xxxxxxxxxx
keys = ['name','age','gender']
values = ['nick',19,male]
res = zip(key,values)
print(F"zip(key,values):{zip(keys,value)}")
info_dict = {k:v for k,v in res}
print(f"info_dict:{info_dict}")
# 输出结果为
# zip(key,values):<zip object at 0x0000023854370D48>
# info_dict:{'name': 'nick', 'age': 19, 'gender': 'male'}
生成器
生成器的标志就是yield关键字，但凡定义函数的时候里面有这个关键字，该函数就是个生成器函数，所以其实生成器的本质还是一个迭代器，也就是一个函数。
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def func():
print(1)
yield
print(2)
yield
g = func()
print(g)
实际上迭代器的用处不是很多，所以升车鞥器是提供了一个非常方便的自定义迭代器的途径。
yield的意义在于，只要函数中出现yield关键字，在调用函数就不会继续执行函数体代码，而是返回一个值，就是yield后面返回的值，然后才会继续执行函数。
所以yield和return是不是有些相似？确实，return是返回值然后结束当前函数，而yield是返回一个值然后继续执行该函数，大概就只是这些区别了。
生成器表达式
列表推导式相当于直接给你一堆散装鸡蛋，而生成器表达式则相当于给你一只老母鸡。
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lt = [i for i in range(100)]
print(lt)
# 一次性取一百个值，相当于一堆鸡蛋
g = (i for i in range(100))
print(g.__next__)
print(g.__next__)
print(g.__next__)
print(g.__next__)
# 一次只能取一个值，相当于母鸡下蛋，一次只能一个(或许有时候是双蛋？不过这个无所谓了)
匿名函数
从名字就可以看出，匿名函数，也就是没有名字的函数，他没有绑定的名字，使用一次就被收回，加括号就可以运行。关键字lambda
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# lambda的用法
# 实际上lambda x:x*x就相当于以下式子
def f(x):
return x*x
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lambda x,y: x + y
print(lambda x,y: x + y)
res = (lambda x, y: x + y)(1, 2)
print(res)
# 输出结果为：
# <function <lambda> at 0x00000284CEB70268>
# 3
匿名函数通常与一些内置函数联用
例如max() sorted() filter() 列表中的sort()
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salary_dict = {
'nick':2000,
'jason':40000,
'tank':2999,
'sean':10000
}
如果我们想从上面的字典中取出来薪资最高的人，第一想到的可能是max方法，可是max对字典使用的话是按照key排序的，不是按照value，可以按照如下方式来实现：
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def func(k):
return salary_dict[k]
print(f'{max(salary_dict,key = func)}')
# 或者以下这种，更好理解
print(f'{max(salary_dict,key = lambda name:salary_dict[name])}')