“写函数”也就是自己写一个算法，用来实现一些功能。比如最简单的给电脑两个参数，让他计算这两个参数的关系。

```a = 5
b = 10
a, b = b, a
print(a) # 10
print(b) # 5
# 这个看起来不是函数，但是这种用法在函数中常用```

```def is_even(num):
return num % 2 == 0
is_even(10)  # True```

```def split_lines(s):
return s.split('\n')
split_lines('50\n python\n snippets')
# ['50', ' python', ' snippets']```

```import sys
print(sys.getsizeof(5)) # 28
print(sys.getsizeof("Python")) # 55```

```language = "sumer-minus"   # SUMER是我公众号，minus是我一个网名
reversed_language = language[::-1]
print(reversed_language)  # sunim-remus```

```def repeat(string, n):
return (string * n)
a=repeat('oop!',10)
print(a)
>>>oop!oop!oop!oop!oop!oop!oop!oop!oop!oop!```

```def palindrome(string):
return string == string[::-1]
print(palindrome('python')) # False```

```strings = ['老虎', '老鼠', '蟑螂']
print('kill'.join(strings))
# 老虎kill老鼠kill蟑螂```

```def union(a,b):
return list(set(a + b))
union([1, 2, 3, 4, 5], [6, 2, 8, 1, 4])
# 答案：[1,2,3,4,5,6,8]```

```def unique_elements(numbers):
return list(set(numbers))
unique_elements([1, 2, 3, 2, 4])
# 答案：[1, 2, 3, 4]```

```def average(*args):
return sum(args, 0.0) / len(args)
average(5, 8, 2)
# 5.0```

```def unique(list):
if len(list)==len(set(list)):
print("所有数据都是唯一的")
else:
print("并不唯一")
unique([1,2,3,4,5])
# 所有数据都是唯一的```

```def most_frequent(list):
return max(set(list), key = list.count)
numbers = [1, 2, 3, 2, 4, 3, 1, 3]
most_frequent(numbers)
# 3```

π在math 库中是有定义的,就叫做math.pi

```import math
return (deg * math.pi) / 180.0
# 1.5707963267948966```

```import time
start_time = time.time()
a,b = 5,10  #随便一个过程
c = a+b     #随便一个过程
end_time = time.time()
time_taken = (end_time- start_time)*(10**6)
print("运行这个耗费微秒数为:", time_taken)
# 运行这个耗费微秒数为: 2.1457672119140625```

```from functools import reduce
import math
def gcd(numbers):
return reduce(math.gcd, numbers)
gcd([24,108,90])
# 6```

```x = lambda a, b, c : a + b + c
print(x(5, 10, 20))
# 35
# 或者也可以这样
def x(a, b, c):
return a + b + c
print(x(5, 10, 30))
# 35```

```def multiply(n):
return n*n
list1 =[1, 2, 3]
result = list(map(multiply, list1))
print(result)
# [1, 4, 9]```

```arr = [1, 2, 3, 4, 5]
def Even(x):
return list(filter(lambda x: x % 2 == 0, arr))
Even(arr)
# [2, 4]```

```numbers = [1, 2, 3]
def squares(nums):
return [number**2 for number in nums]
squares(numbers)
# [1, 4, 9]```

```def rotate(arr, d):
return arr[d:] + arr[:d]
if __name__ == '__main__':
arr = [1, 2, 3, 4, 5]
arr = rotate(arr, 2)
print (arr)
# 结果：[3, 4, 5, 1, 2]```

```def add(a, b):
return a + b
def subtract(a, b):
return a - b
a, b = 5, 10
print((subtract if a > b else add)(a, b))  # 15
print((subtract if a < b else add)(a, b))  # -5```