Chapter 2: Variables and Data Types in Python

1. Binary and character encoding

• Bit storage content is 0 and 1; bit is the smallest storage unit in the computer

• Data: Each symbol (English, number or symbol, etc.) will occupy a 1Bytes record, and each Chinese will occupy 2Byte

• Binary conversion: 1B(Byte)=8b(bit) ; 1KB=1024B

• [Online ASCII Table]( ASCII Table | Rookie Tutorial (runoob.com) )

• Character Encoding Development

2. Identifiers and reserved words in Python

2.1 Reserved words

• what are reserved words
  • There are some words that are assigned specific meanings by Python and these words should not
• View reserved words

import keyword
print(keyword.kwlist)

• what are the reserved words

['False', 'None', 'True', '__peg_parser__', 'and', 'as', 
 'assert', 'async', 'await', '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']

2.2 Identifiers

• what is an identifier
  • Variables, functions, classes, modules, and other objects are named after identifiers
• Identifier Naming Rules
  • Contains only letters, numbers, underscores_
  • cannot start with a number
  • cannot be a reserved word
  • Strictly case sensitive

3. Variables and data types in Python

3.1 Variables

• what is a variable

  • A variable is a labeled box in memory into which you put the data you need

    • name = "Python"
      

    • Among them: name - variable name; = - assignment operator ; Python - value

• components of variables

  • Identification: Identify the memory address where the object is stored, use the built-in function id(obj) to obtain
  • Type: Indicates the data type of the object, which can be obtained by using the built-in function type(obj)
  • Value: Indicates the specific data stored in the object, and the value can be printed out by using the function print(obj)

• Memory analysis graph

  

• code analysis

  • code writing

  name = '巧克力酸奶'
  print('标识', id(name))
  print('类型', type(name))
  print('值', name)
  

  • output result

  标识 2830801861584
  类型 <class 'str'>
  值 巧克力酸奶
  

3.2 Multiple assignment of variables

• After multiple assignments, the variable will point to the new space
• memory diagram

• code analysis

  • code writing

  name = "玛利亚"
  print(id(name))
  name = "楚留香"
  print(name)
  print(id(name))
  

  • Result analysis

  2151611071440
  楚留香
  2151611071728
  

3.3 Data Types

• common data types

type of data	character representation	example
integer type	int	98
floating point type	float	3.1415926
Boolean type	bool	TRUE，False
string type	str	"Life is too short, I use Python"

• Use the type() function to output the data type

3.3.1 Integer types

• English is integer, abbreviated as int , which can represent positive numbers, negative numbers and zero
• Different base representations of integers

Window platform, Win+Rafter input calc, can quickly dispatch the calculator

hexadecimal	base number	Every few days	Manifestations
decimal (default)	0,1,2,3,4,5,6,7,8,9	10	120
binary	0,1	2	0b101011
Octal	0,1,2,3,4,5,6,7	8	0o166
hexadecimal	0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F	16	0x76

• code demo

  • code writing

  # 整数类型，正数，负数，0
  n1 = 90
  n2 = -90
  n3 = 0
  print(n1, type(n1))
  print(n2, type(n2))
  print(n3, type(n3))
  # 正数可以表示为二进制，十进制，八进制，十六进制
  print("十进制", 118)
  print("二进制", 0b1010111)
  print("八进制", 0o176)
  print("十六进制", 0x78A2)
  

  • Result analysis

  90 <class 'int'>
  -90 <class 'int'>
  0 <class 'int'>
  十进制 118
  二进制 1010111
  二进制 87
  八进制 126
  十六进制 30882
  
  Process finished with exit code 0
  

3.3.2 Floating point type

• English is float , and the floating point number is composed of an integer part and a decimal part

• Floating point storage is non-deterministic

  • When using floating-point numbers for calculations, there may be cases where the decimal point is uncertain
  • Solution: import module decimal

• code demo

  • code writing

  a = 3.14159
  print(a, type(a))
  # 小数点位不确定
  n1 = 1.1
  n2 = 2.2
  n3 = 2.1
  print(n1 + n2)
  print(n1 + n3)
  # 解决方案：**导入模块decimal**
  from decimal import Decimal
  print(Decimal("1.1")+Decimal("2.2"))
  

  • Result analysis

  3.14159 <class 'float'>
  3.3000000000000003
  3.2
  3.3
  
  Process finished with exit code 0
  

3.3.3 Boolean type

• The English name boolean, abbreviated as bool , is used to represent true or false values

• True is true , False is false

• Boolean values ​​can be converted to positive numbers

  • True——>1
  • False——>0

• code demo

  • code writing

  f1 = True
  f2 = False
  print(f1, type(f1))
  print(f2, type(f2))
  
  # bool型可以转成整型运算
  print(f1 + 1)
  print(f2 + 1)
  

  • Result analysis

  True <class 'bool'>
  False <class 'bool'>
  2
  1
  
  Process finished with exit code 0
  

3.3.4 String type

• Strings are also known as immutable sequences of characters

• Can be defined using single quotes ' ', double quotes " ", triple quotes ''' ''' or""" """

• Strings defined by single quotes and double quotes must be on one line

• The character type defined by triple quotes can be distributed in multiple consecutive lines

• code demo

  • code writing

  str1 = '人生苦短，我用Python'
  str2 = "人生苦短，我用Python"
  str3 = """人生苦短，
  我用Python"""
  str4 = '''人生苦短，
  我用Python'''
  
  print(str1, type(str1))
  print(str2, type(str2))
  print(str3, type(str3))
  print(str4, type(str4))
  

  • Result analysis

  人生苦短，我用Python <class 'str'>
  人生苦短，我用Python <class 'str'>
  人生苦短，
  我用Python <class 'str'>
  人生苦短，
  我用Python <class 'str'>
  
  Process finished with exit code 0
  

3.4 Data type conversion

• Why data type conversion is needed
  • Needed when splicing data of different data types together
• Functions for data type conversion

Function name	effect	Precautions	example
str()	Convert other data types to strings	Can also be converted with quotes	str(123) 123’
int()	Convert other data types to integers	1. Text and decimal strings cannot be converted into integers 2. Floating point numbers are converted into integers: erasing and rounding	int(‘123’)
float()	Convert other data types to floating-point numbers	1. The text class cannot be converted into an integer 2. The integer is converted into a floating point number with a tail of .0	float(‘9.9’) float(9)

• Diagram of Data Transformation

• code demo

  • code writing

  name = '张三'
  age = 20
  
  print(type(name), type(age))
  # 说明name和age的数据类型不相同
  # print('我叫'+name+'今年'+age+'岁')
  # TypeError: can only concatenate str (not "int") to str
  print('我叫' + name + ',今年' + str(age) + '岁')
  
  print('------------str()：将其他类型转换成str类型--------------')
  a = 10
  b = 198.8
  c = False
  print(type(a), type(b), type(c))
  print(str(a), str(b), str(c), type(str(a)), type(str(b)), type(str(c)))
  
  print('------------int()：将其他类型转换成int类型--------------')
  s1 = '128'
  f1 = 98.1
  s2 = '76.77'
  ff = True
  s3 = 'Hell0'
  print(type(s1), type(f1), type(s2), type(ff), type(s3))
  print(int(s1), type(int(s1)))  # 将str转换成int类型，字符串为 数字串
  print(int(f1), type(int(f1)))  # 将float转换成int类型，截取整数部分，舍掉小数部分
  # print(int(s2), type(int(s2)))  # ValueError: invalid literal for int() with base 10: '76.77'mm，字符串为小数串
  print(int(ff), type(int(ff)))
  # print(int(s3), type(int(s3))) ValueError: invalid literal for int() with base 10: 'Hell0',字符串必须为整数数字串才能转换
  
  print('------------float()：将其他类型转换成float类型--------------')
  ss1 = '128.98'
  ss2 = '76'
  fff = True
  ss3 = 'Hell0'
  i = 98
  print(type(ss1), type(ss2), type(fff), type(ss3), type(i))
  print(float(ss1),type(float(ss1)))
  print(float(ss2),type(float(ss2)))
  print(float(fff),type(float(fff)))
  # print(float(ss3),type(float(ss3)))    # ValueError: could not convert string to float: 'Hell0'
  print(float(i),type(float(i)))
  

  • Result analysis

  <class 'str'> <class 'int'>
  我叫张三,今年20岁
  ------------str()：将其他类型转换成str类型--------------
  <class 'int'> <class 'float'> <class 'bool'>
  10 198.8 False <class 'str'> <class 'str'> <class 'str'>
  ------------int()：将其他类型转换成int类型--------------
  <class 'str'> <class 'float'> <class 'str'> <class 'bool'> <class 'str'>
  128 <class 'int'>
  98 <class 'int'>
  1 <class 'int'>
  ------------float()：将其他类型转换成float类型--------------
  <class 'str'> <class 'str'> <class 'bool'> <class 'str'> <class 'int'>
  128.98 <class 'float'>
  76.0 <class 'float'>
  1.0 <class 'float'>
  98.0 <class 'float'>
  
  Process finished with exit code 0
  

4. Comments in Python

• what is a note

  • Annotated text that explains the function of the code in the code can improve the readability of the code
  • The content of the comment will be ignored by the Python interpreter

• Common Annotation Types

  • Single-line comments --> start with **#** until the end of a newline
  • Multi-line comment --> There is no separate multi-line comment mark, and the code between a pair of triple quotes is called a multi-line comment
  • Chinese encoding statement comment --> add Chinese statement comment at the beginning of the file to specify the encoding format of the source code file
    • coding:gbk
    • coding:UTF-8

• code demo

  • code writing

  # 输出功能(单行注释）
  print('hello')
  '''嘿嘿，
  这里是多行注释
  啦啦啦啦啦'''