python basics 3 - basic usage and naming of variables

4. Basic use of variables

4.1 Variable Definition

In Python, each variable must be assigned a value before it is used, and the variable will not be created until the variable is assigned
The equal sign (=) is used to assign values to variables
- = on the left is the variable name
- = on the right is the value stored in the variable

variable name = value

　　After the variable is defined, it can be used directly later

4.2 Types of variables

Create a variable in memory that will contain:

Name, stored data, type of stored data, address (label)

Defining variables in Python does not require specifying a type (as is required in many other high-level languages)
Data types can be divided into numeric and non- numeric
digital
- Integer (int)
- floating point (float)
- boolean (bool)
  - True True non-zero number - non-zero is true
  - false False 0
- complex
  - Mainly used for scientific calculations, such as: plane field problems, wave problems, inductance and capacitance problems, etc.
non-numeric
- string
- list
- tuple
- dictionary

Hint: In Python 2.x, integers are further classified according to the length of the stored value:

- int (integer)
- long (long integer)
- Use the type function to see the type of a variable

In [1]: type(name)

4.3.1 Computation between variables of different types

1) Numerical variables can be directly calculated

In Python, two numeric variables can perform arithmetic operations directly
If the variable is of type bool, when calculating
- True corresponds to the number 1
- False corresponds to 0

2) Use + to concatenate strings between string variables

In Python, you can use + concatenation between strings to generate new strings

In [1]: first_name = "三"
In [2]: last_name = "张"
In [3]: first_name + last_name
Out[3]: 'Three cards'

3) String variables can use * to repeatedly concatenate the same string with integers

In [1]: "-" * 50
Out[1]: '--------------------------------------------------'

4) No other calculations can be performed between numeric variables and strings

In [1]: first_name = "zhang"
In [2]: x = 10
In [3]: x + first_name
---------------------------------------------------------------------------
TypeError: unsupported operand type(s) for +: 'int' and 'str'
TypeError: `+` Unsupported operation types: `int` and `str`

4.3.2 Variable Input

The so-called input is to use code to obtain the information entered by the user through the keyboard
In Python, if you want to get the user's input information on the keyboard , you need to use the input function

function	illustrate
print(x)	output x to the console
type(x)	View the variable type of x

In Python you can use the input function to wait for user input from the keyboard
Anything entered by the user is considered a string

String variable = input("Prompt information:")

type conversion function

function	illustrate
int(x)	convert x to an integer
float(x)	convert x to a float

Define a floating-point variable and use the float function to convert it while receiving user input

price = float(input("Please enter the price:"))

2.5 Formatted output of variables

The unit price of apple is 9.00 yuan/catties, if you buy 5.00 catties, you need to pay 45.00 yuan

Use the print function to output formatted content
% is known as the formatting operator and is used to handle string formatting
- A string containing % is called a format string
- % is used with different characters , different types of data need to use different formatting characters

formatting characters	meaning
%s	string
%d	Signed decimal integer, %06d indicates the number of display digits of the output integer, and 0 is used to fill in the shortage.
%f	Floating point number, %.2f means that only two digits are displayed after the decimal point
%%	output %

print("Format string" % variable 1)

print("Format string" % (variable1, variable2...))

print("My name is %s, please take care of me!" % name)
print("My student number is %06d" % student_no)
print("The unit price of apples is %.02f yuan/catties, if you buy %.02f jins, you need to pay %.02f yuan" % (price, weight, money))
print("The data scale is %.02f%%" % (scale * 100))

Exercise - personal business card

need

In the console, the user is prompted to enter: name , company , position , phone number , email address
Output in the following format:

**************************************************
company name

Name (Position)

phone: phone
Email: Email
**************************************************

Code:

"""
In the console, prompt the user to input: name, company, position, phone number, email address
"""
name = input("Please enter your name: ")
company = input("Please enter the company: ")
title = input("Please enter the title: ")
phone = input("请输入电话：")
email = input("请输入邮箱：")

print("*" * 50)
print(company)
print()
print("%s (%s)" % (name, title))
print()
print("电话：%s" % phone)
print("邮箱：%s" % email)
print("*" * 50)

5. 变量的命名

5.1 标识符

标示符就是程序员定义的 变量名、函数名

标示符可以由字母、下划线 和数字组成
不能以数字开头
不能与关键字重名
关键字 就是在 Python 内部已经使用的标识符
关键字 具有特殊的功能和含义
开发者 不允许定义和关键字相同的名字的标示符

5.2 关键字

通过以下命令可以查看 Python 中的关键字

In [1]: import keyword
In [2]: print(keyword.kwlist)

import 关键字 可以导入一个 “工具包”
在 Python 中不同的工具包，提供有不同的工具

5.3 变量的命名规则

命名规则 可以被视为一种惯例，并无绝对与强制目的是为了 增加代码的识别和可读性

注意 Python 中的 标识符 是 区分大小写的

在定义变量时，= 的左右应该各保留一个空格
在 Python 中，可以按照以下方式命名
1. 每个单词都使用小写字母
2. 单词与单词之间使用 _下划线连接

　　　　　　　　例如：first_name、last_name、qq_number、qq_password

驼峰命名法

小驼峰式命名法

　　　　第一个单词以小写字母开始，后续单词的首字母大写。例如：firstName、lastName

大驼峰式命名法

　　　　每一个单词的首字母都采用大写字母。例如：FirstName、LastName、CamelCase