Python basics review (full)

Python basics review

Second, brush up on the basics of python. Friends who have learned the basics of python, let's deepen their memory together! This will help you avoid bugs when writing code later.

Escapes

1. \n: newline
2. \t: horizontal tab character
3. \r: Overwrite (characters appearing later overwrite previous ones)
4. \b: Delete (delete the character before \b, equivalent to backspace on the keyboard)
5. : Make two backslashes represent a \ (==Output the above escape character and you can also add one more \ in front. For exampleprint(“\\n”)$\Rightarrow$\n)

Example

Cancel“”Transfer

print("你好\"小杰\"")#想要原样输出带引号的小杰就必须在每个引号前加\或者把外面的引号改成单引号

Unescaping other escapes

Add r in front of the string to cancel the escaping of the entire sentence

print(r"你好\n小杰\n今天有没有加油啊？")

The entire previous sentence is output as is$\to$Hello\nXiaojie\nHave you cheered today?

not/and/or logical operator precedence

not>and>or

Swap the values ​​of two variables

# 交换
# 法一：中间变量
a = "hello"
b = "world"
c = a
a = b 
b = c
print(a,b)
# 法二：python可直接这样操作
a,b = b,a
print(a,b)

Palindromes and slices

a = "山西运煤煤运西山"
b = a[::-1]#切片将a从后往前取 切片:[开始:结尾:步长]列表也可以采用这种方法！
if a==b:
    print(f"{
      
      a}是回文")

Common operations on strings

find

Function: Find element position

The first parameter: the string fragment to be found

Second parameter: the starting point to find

If there are multiple string fragments to be searched for, the subscript of the first one is returned.

If not found, return -1

a = "wdfuiewcuhwfwwdw"
print(a.find("w",2,7))

count

Count the number of times a string fragment appears in a string

Not found returns zero

The parameters are the same as find

a = "wdfuiewcuhwfwwdw"
print(a.count("w",2,7))

replace

Function: Replace the specified string fragment

Parameter 1: The string fragment to be replaced

Parameter 2: String fragment after replacement

Parameter three: the number of replacements, from front to back (replace all by default)

a = "wdfuiewcuhwfwwdw"
print(a.replace("w","c",2))

upper()&lower()

How to convert string to uppercase and lowercase

split&strip

split

How to split a string

a = "fwefc,12,123,dew,12w,wdc"
print(a.split(","))#有逗号的地方进行分割得到列表,后面的数字表示切几刀，默认全切

strip

Remove spaces from the beginning and end of the string (the spaces in the middle cannot be removed)

only()

Count string/list length

Common operations on lists

of the

a = [1,2,1]
del a #可以删除整个列表
del a[1]#也可以删除列表中的某个元素

append

Add elements to the list and stuff them all at once

insert

Function: Insert an element into the specified position

The first parameter: the insertion position

Second parameter: inserted content

li = [1,2,3,4]
li.insert(2,9)
print(li)

clear

Clear data from list

remove

• If there are duplicate elements, only the first one is removed
• Parameters are elements

pop

• Remove the last one by default
• Parameters are subscripts

index

• The first parameter is the element
• Second, the three parameters are the starting and ending positions
• Function: Get element subscript

reverse

Function: reverse sorting

a = ["py","c","go"]
a.reverse()
print(a)

extend

• Append data under the original list
• Note: The results of the extend function and list addition are the same, but the extend function will merge another list into the current list (it will not occupy new memory space), while the addition will return a new list (it will occupy new memory space). )

a = [1,2,3]
a.extend([4,5,6])
print(a)

sort

• Used to sort the list

• Sort according to ASCII code size rules/number size

• Data of the same type can be sorted

  a = [7,8,3,5]
  a.sort(reverse=True)#将a逆向排序
  print(a)
  

count

a = [1,2,3,4,7,2,2,2]
num = a.count(2)
print(num)

tuple

• only
• max/min #Find the maximum and minimum according to the ASCII code table
• ==When there is only one piece of data in the tuple, one must be added at the end,For example: tuplec = (10,)
• Data is immutable

gather

• When declared in braces, lists and dictionaries cannot be placed directly

• Can be used to deduplicate list/tuple/dictionary keys

  a = [1,1,2,2,3,3]
  a = list(set(a))
  print(a)#实现列表去重
  

  method

  add

  Function: add elements

  update

  Function: Merge collections

  a = {
        
        1,2,3,4,5}
  b = {
        
        "g","c","h"}
  a.add(8)
  a.update(b)
  print(a)
  

  remove

  Function: Delete elements in the collection. If there is direct deletion, no error will be reported.

  pop

  Function: Randomly delete elements in the set. If there are no elements in the set, an error will be reported.

discard

Function: Delete elements in the collection. If there is direct deletion, no operation will be performed.

intersection union

s1 = {
    
    1,2,3,5}
s2 = {
    
    12,2,3,1}
s3 = s1 & s2#取交集
s4 = s1 | s2#取并集
print(s3,s4)

dictionary

Add, delete, modify and check

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
dic["技能"] = "python"#增
del dic["名字"]#删
dic["名字"] = "zz"#改
print(dic["名字"])#查

get&keys

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
print(dic.get("名字"))#获取指定键的值
print(dic.keys())#获取所有键

items&values

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
print(dic.items())#获取所有键值对,对字典遍历的时候会用到这种方法
print(dic.values())#获取所有值

clear&copy

• clear: clear the dictionary

• copy: copy dictionary

I pop & drink

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
r = dic.pop("名字")#移除指定键，返回值为值
print(r,dic)
d = dic.popitem()#删除字典中最后一项，并以元组的形式返回该项所对应的键和值

setdefault

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
dic.setdefault("名字","python")#键无则增，有键则忽略
print(dic)

update

dic = {
    
    "名字":"织梦","年龄":18}#定义字典
dic2 = {
    
    "名字":"知梦","擅长":"python"}
dic.update(dic2)#原字典有相应键则改，无则增
print(dic)

judgment

in&not in

• Determine whether the segment character is in a string/tuple/list/dictionary
• When judging whether it is in the dictionary, you can only judge whether the corresponding key is in the dictionary.

dic = {
    
    "名字":"织梦","年龄":18}
print("名字"in dic)

is&is not

• Numbers/strings/tuples are all immutable data types. If they appear to be the same, they are exactly the same.
• Lists/dictionaries/sets are all mutable data types. If they appear to be the same, they are not actually the same object.

isinstance

• The return value is a bool value
• Syntax: isinstance(a,(int,str,float))

Extract elements from nested lists

a = [1,2,3,[4,5,6],[7,8,9]]
for i in a:
    if isinstance(i,list):
        for x in i:
            print(x)

Function related

variable parameter

• *argsThe types of and **kwargs are tuple and dictionary respectively

  def test(*args,**kwargs):
      print(args,kwargs)
  test(12,x=123)
  

Unpack

def test(*args):
	print(args)
test(*(1,2,3))#解包
a,b,c = (1,2,3)#元组的解包

Parameter order

def test(a,name="小杰",*args,**kwargs):
    pass