Python basic syntax notes and assignments

1. Identifier

• The first character must be a letter in the alphabet or an underscore _.
• The other parts of the identifier consist of letters, numbers and underscores.
• Identifiers are case sensitive.
• In Python 3, Chinese can be used as variable names, and non-ASCII identifiers are also allowed.

2. Python reserved words

[‘False’, ‘None’, ‘True’, ‘and’, ‘as’, ‘assert’, ‘break’, ‘class’, ‘continue’, ‘def’, ‘del’, ‘elif’, ‘else’, ‘except’, ‘finally’, ‘for’, ‘from’, ‘global’, ‘if’, ‘import’, ‘in’, ‘is’, ‘lambda’, ‘nonlocal’, ‘not’, ‘or’, ‘pass’, ‘raise’, ‘return’, ‘try’, ‘while’, ‘with’, ‘yield’]

3 escape character

• \t tab
• \n newline
• The character after \ represents the original meaning of the character
• r "" Everything in the quotation marks is the original meaning of the string
• f" "Format string constant.

4 placeholder

• %s string placeholder
• %f floating point placeholder
• %d integer placeholder

5. Basic data types

Standard data type

Number

Python3 supports int, float, bool, complex (plural). In Python 3, there is only one integer type int. The
built-in type() function can be used to query the type of object pointed to by a variable.

print(type(a), type(b), type(c), type(d))
print(type(a), type(b), type(c), type(d))

<class ‘int’> <class ‘float’> <class ‘bool’> <class ‘complex’>

In addition, isinstance can also be used to judge:

a = 111
isinstance(a, int)

True

The difference between isinstance and type is

• type() does not consider the subclass to be a superclass type.
• isinstance() will consider the subclass to be a superclass type.

5 + 4 # Addition
9
4.3-2 # Subtraction
2.3
3 * 7 # Multiplication
21
2/4 # Division, get a floating point number
0.5
2 // 4 # Division, get an integer
0
17% 3 # Take the remainder
2
2 ** 5 # Power
32

String (string)

str = 'Runoob'

print (str)          # 输出字符串
print (str[0:-1])    # 输出第一个到倒数第二个的所有字符
print (str[0])       # 输出字符串第一个字符
print (str[2:5])     # 输出从第三个开始到第五个的字符
print (str[2:])      # 输出从第三个开始的后的所有字符
print (str * 2)      # 输出字符串两次，也可以写成 print (2 * str)
print (str + "TEST") # 连接字符串

Output result

Runoob
Runoo
R
noo
noob
RunoobRunoob
RunoobTEST

String commonly used functions

• find() --> Check whether the string is included, and return the position of the string, if not, return -1

str="Hello World"
str.find("Hello") # 返回值：0
str.find("W") # 返回值：6, 这里需要注意下：空格也是一个字符。W前面有个空格，所以W位置是6
str.find("R") # 返回值：-1，并不包含在Hello World中，如果不包含返回-1

• index() --> Check whether the string contains the specified character, and return the starting index value, if not, an error will be reported

str.index("Hello") # 返回值：0
str.index("o") # 返回值：4
str.index("W") # 返回值：6
str.index("R") # 返回值：报错信息 ，因为R并不包含其中。 所以建议慎用，如果值不存在程序报错就完蛋了。

• len() --> Returns the length of the string, starting from 0

len(str) #返回值：10

• count() --> Collect the number of times the specified character appears in the string

str.count("o") 返回值：2, o字符在Hello World中存在两个。
 
# 也可以指定count()函数从某个位置开始查找。 语法为：count(" ",start,end)
str.count('o',5,10) 返回值：1, 原因：指定位置后会从索引5开始检索，以索引10结束。 5到10之间只存在一个'o'
str.count('o',4,len(str)) 返回值： 2，索引从4开始，到字符串结束。len(str)字符串长度

• replace() --> replace string

str.replace('hello','HELLO')  # 把小写的hello替换为大写的HELLO
str.replace('W','B')  # 把W替换为B

• split() string cutting

str.split('o') #以列表的形式返回["hell","w","rld"] ，hello world 里面的o被切割掉

• upper() --> convert all characters to uppercase

str.upper() #返回值为 HELLO WORLD

• title() --> convert the first letter to uppercase

str.title() #返回值：Hello World

• center() --> returns a new string with the original string centered and filled with spaces to the length and width

str.center(80) #返回值: ( Hello World ) 其字符串两头被空格填充

• join() --> Insert a specified character after the string to construct a new string

_str="_"
list=["I","Love","You"]
_str.join(list) # 返回值： I_Love_You 每个列表元素后面都插入一个下划线

• isspace() --> Detect whether the string contains only spaces, if it returns Trun, otherwise return False. In general, it is to judge non-empty verification

str=" "
strOne="早上好！"
str.isspace() # 返回trun
strOne.isspace #返回false

• isalnum() --> Check whether it contains only numbers or letters. Use: It can be used to judge the password. Generally, you cannot enter Chinese characters or spaces in the password.

strOne="a123"
strTwo="a 456"
strOne.isalnum() # 返回trun
strTwo.isalnum() # 返回false ，因为包含空格

• isdigit() --> Check if the character contains only digits, return Trun and False

str='123'
strone='a123'
str.isdigit() 返回trun 
strone.isdigit() 返回false

• isalpha() --> Check whether the string contains only letters

str="abcd"
strone="123abacd"
str.isalpha() # 返回 trun
strone.isalpha() # 返回false

operation

a,b = 6, 8 I want a=8 b=6. What should I do? Realize in 2 ways

method one

a,b = 6, 8
print(f"a = {a}\tb = {b}")
c = a
a = b
b = c
print(f"a = {a}\tb = {b}")

a = 6 b = 8
a = 8 b = 6

Method Two

a,b = 6, 8
print(f"a = {a}\tb = {b}")
a, b = b, a
print(f"a = {a}\tb = {b}")

a = 6 b = 8
a = 8 b = 6

---

Complete string reverse order and statistics

1. Design a program that requires only a string with a length of less than 31 to be entered, otherwise the user is prompted to re-enter
2. Print out the length of the string
3. Use slices to print out the string in reverse order

# 1
print("请输入一个字符串")
str1 = ''
s_input = True
while s_input:
    str1 = input("input:")
    if len(str1) >= 31:
        print("请重新输入一个字符串")
    else:
        s_input = False
print(f"str1: {str1}")
# 2
print(f"字符串长度: {len(str1)}")
# 3
str2 = str1[::-1]
print(f"str2: {str2}")

---

It is required to enter the user name and password from the keyboard to verify whether the format complies with the rules. If not, print out the reason for the non-compliance, and prompt to re-enter
• The length of the user name is 6-20, and the user name must start with a letter. The
length of the password is at least 6 digits , Cannot be a pure number, cannot have spaces

import re

user_name = ''
pwd = ''
s_input = True

while s_input:
    user_name = input("用户名:")
    reg = '^.{6,20}$'
    obj = re.match(reg, user_name)
    pwd = input("密码:")
    verify_user_name = user_name.isalpha() is False and obj is not None
    verify_user_pwd = pwd.isdigit() is False and len(pwd) > 6
    if verify_user_name and verify_user_pwd:
        print(f"用户名: {user_name}")
        print(f"用户名: {pwd}")
        s_input = False
    else:
        print(f"用户名或密码不对")