python basic note

[TOC]

python basic note

1. 基本运算

​ (1) + - * /

​ (2) 次方：x**y; 取模： x%y

2.类型

​ (1) type(x) : 查看变量类型

​ (2) float，int，bool（True，False），str

​ (3) list : [1,2,3] , [[1, "a"], [2, "b"], [3, "c"]]

• 数字索引: 正向索引，反向索引

  a = [1, 2, 3]
  print(a[0], a[-1])
  # output : 1 3
  

• list 切割

  a = [1,2,3,4,5,6]
  b = a[1:4]
  # [起点 ： 终点 ： 间隔]
  #  包含   不包含
  print(b)  # [2, 3, 4]
  print(a[:4])  # [1, 2, 3, 4]
  print(a[3:])  # [4, 5, 6]
  print(a[:])  # [1, 2, 3, 4, 5, 6]
  print(a[::2])  # [1, 3, 5]
  print(a[-4:-1])  # [3, 4, 5]
  

• list 改变、增加、删除 元素

  a = [1, 2, 3]
  b = [4, 5]
  c = a + b
  print(c)  # [1, 2, 3, 4, 5]
  print(c + [6])  # [1, 2, 3, 4, 5, 6]
  del (c[0])  # 删除 或 a.pop(0)
  print(c)  # [2, 3, 4, 5]
  c[0] = 0  # 改变
  print(c)  # [0, 3, 4, 5]
  
  a = [1, 2, 3]
  a.append(4)
  print(a)  # [1, 2, 3, 4]
  a.insert(0, 9)
  print(a)  # [9, 1, 2, 3, 4]
  

• list拷贝

  a = [1, 2, 3]
  aa = a
  aa[0] = 5
  print(a) # [5, 2, 3]
  print(id(a), id(aa))  # 71881712 71881712
  
  a = [1, 2, 3]
  aa = list(a)  # 或 a[:]
  aa[0] = 5
  print(a) # [1, 2, 3]
  print(id(a), id(aa))  # 74503352 73566288
  
  # 浅拷贝
  a = [[1, 2]]
  b = a[:] # 或 b = a 或 b = list(a)
  b[0][1] = 3
  print(a)  # [[1, 3]]
  
  # 深拷贝
  import copy
  a = [[1, 2]]
  b = copy.deepcopy(a)
  b[0][1] = 3
  print(a)  # [[1, 2]]
  

​ (4) tuple 元组

a = (1, 2, 3)
a = (1,)  # 单元素tuple， 多加个逗号

​ (5) dict

d = {
    1: "a",
    2: "b",
    3: "c"
}
for i in d: # 或 d.keys()
    print(d.get(i))  # 或 d[i]
    # a b c

作为 key 的元素必须不可变

​ (6) set

s = {1, 2, 3}  # 或 s = set([1, 2, 3])
print(1 in set)  # True

元素必须是不可变对象

3. 函数

# 可变参数
def avg(*args):
    return sum(args) / len(args)

print(avg(1, 2, 3))  # 2.0

# 返回多个返回值
def get(x, y):
    return x, y

x, y = get(1, 2)
print(get(1, 2))  # (1, 2)

Python解释器会把传入的一组参数组装成一个tuple传递给可变参数

4.列表生成式

a = [1, 2, 3, 4, 5, 6]
print([x*x for x in a if x % 3 == 2])  # [4, 25]
print(["hello %s" % x for x in a if x % 2 == 0])  # ['hello 2', 'hello 4', 'hello 6']
print([x * y for x in a[2:4] for y in a[3:5]])  # [12, 15, 16, 20]

5.包

import xx
import xx as yy
from xx import yy