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Computer and Python 2.1 basis

2.1.1 CPU Memory Hard Disk Operating System

CPU
- Computing Center computer, the brain of the computer
RAM
- Local data temporarily stored data, provisional application loaded
- Common memory size: 4G, 8G, 16G
- Read speed
hard disk
- Local long-term storage of data
- 1TB=1024G 1GB=1024MB 1MB=1024KB 1KB=1024B
- Read speed is slow
operating system
- a softweare
- Connect your computer hardware and all software

2.1.2 Python history

Python founder Guido van Rossum (Guido van Rossum), Python advocating beautiful, clear and simple

Python currently has two versions: Python2.x and Python3.x

Roughly the difference between the two versions of the source code
- Python2.x: Because of the early C, Java, and other languages of cattle involved in writing to supplement, resulting in duplication of code and more redundant, non-standard
- Python3.x: code standards, clear and simple

2.1.3 Programming Language Category

Compiled
- The code is compiled into binary all at once, and then execute
- Compiler: each row is to compile the source code into machine language, and save it as a binary file
- Advantages and disadvantages
  - Advantages: high efficiency, due compiled into a binary file, so you can run from a separate locale.
  - Disadvantages: low efficiency of development can not be cross-platform
- Representatives language: C, C ++, Go
Interpreted
- Progressive interpreted as a binary, line by line execution
- Interpreter: only in the implementation of the program, one by one statement interpreted into machine language to perform computer
- Advantages and disadvantages
  - Advantages: development of high efficiency, cross-platform, flexible modification
  - Disadvantages: low efficiency
- Representatives language: Python, Ruby, PHP

2.1.4 dynamic and static languages (extension)

Dynamic Languages
- Do data type checking during operation language
- Representatives language: Python, Ruby
Static language
- During the compilation of data type checking, writing in the program is to declare all variables
- Representatives language: C, C ++, java

2.1.5 strongly typed and weakly typed language definition language (extension)

Strongly typed language:
- Always strongly typed language, variables must be coerced into another data type to the specified type.
Weakly typed language definitions:
- Data types can be neglected language, variables can be given different types of data values

2.1.6 Python advantages and disadvantages

advantage
- Elegant, clear and simple
- Development of high efficiency - a very strong third-party libraries
- High-level language - regardless of the underlying details
- Portability - can be ported to almost run on all platforms
- Scalability - may be used by using a part program written in C or C ++
- Embeddable - can be embedded in the Python C or C ++ program
Shortcoming
- Slow
- Code can not be encrypted
- Thread can not take advantage of multi-CPU

2.1.6 Python species

Different Python interpreter

CPython
- Python official version, using the C language, C language recognition may be converted into bytecode
JPython
- Identification can be converted into Java bytecode
ironPython
- It can be converted into a language recognizable .net bytecode
pypy
- Dynamic compilation

2.2 Python installation

Python's official website

In the official website to download the installation package corresponding to the system
installation steps

Note: Sometimes after installation there will be "Disable path length limit" button, if there is, click on it, Path length disables the system automatically limits
an examination

Open win + r, enter cmd carriage return, open the Windows command interface

Appear above the interface is the installation was successful.

2.3 a first python code

Default python 3.x version, python 2.x version special instructions.

Writing in notepad ++ inprint ("hello world")
Save the file as t1.py

win + r, enter cmd, enter the command interface file to run just

Successful operation

2.4 Variable

Why variables?
- When the expression is too complex to be assigned to a variable is not easy to write
What is a variable?
- Variable value stored in the memory

How to define variables?

name = "小明"
#这种形式就是在定义一个名称叫name的变量

name ： 变量名
= ： 赋值
"小明" ： 值

Explanation

Variables can only point data can not point to a variable. Variables in memory is uniquely named.

2.4.1 Naming

Variable naming rules:

Variables must be a combination of letters, numbers, or underscore
The first letter variable names may not be numbers
python keywords can not be used as a variable
- ['and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'exec', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'not', 'or', 'pass', 'print', 'raise', 'return', 'try', 'while', 'with', 'yield']
不建议使用中文或拼音
变量名称要有意义
变量名称最好不要过长
变量名称区分大小写
变量名称推荐写法：
- 驼峰体：OldBoy
- 下划线：old_boy

2.5 常量

常量：常量就是把变量名称大写，一直保持不变的量。

NAME = "小明"
#类似以上的就是常量，赋值方法和变量类似；但是python没有真正意义上的常量。

2.6 注释

为什么需要注释？
- 进行解释说明，便于理解
如何注释？
- 单行注释：#
- 多行注释：''' 内容 ''' """ 内容 """
```
# 单行注释

'''
多行注释
'''

"""
多行注释
"""
```
在哪使用？
- 在难以理解的代码后进行注释
- 在函数，类等进行注释

2.7 基础数据类型

2.7.1 数据类型

number —— 数字
string —— 字符串
list —— 列表
tuple —— 元组
set —— 集合
dictionary —— 字典

2.7.2 int 整型

整型即为整数类型

范围
- 32位机器：-231～231-1，即-2147483648～2147483647
- 64位机器：-263～263-1，即-9223372036854775808～9223372036854775807

数值运算：

+：加法
-：减法
*：乘法
/：除法，得浮点数
//：除法，得整数
%：取余
**：乘方

2.7.3 str 字符串

什么是字符串？
- 在python中用单引号''或双引号""或三个单引号括'''或三个双引号"""起来的都是字符串
- 单引号和双引号没有区别，可以配合使用。
- 在内容有换行时，可以使用三个单引号

字符串的操作

字符串的拼接

str1 = "hello"
str2 = "world"
str = str1 + str2
print (str)

字符串的复制

str1 = "hello"
str = str1 * 2
print (str)

2.7.4 bool 布尔类型

bool 布尔值：True(真) False(假)

2.7.5 type() 判断数据类型

利用type()进行数据类型的判断

date1 = 123
date2 = "123"
date3 = True

print (type(date1),type(data2),type(data3))

2.7.6 数据类型转换

实例：

data1 = 123
data2 = str(data1)
print (type(data2))

函数	描述
str(x)	将x转换为字符串
int(x)	将x转换为整数（必须为数字的前提）

2.8 用户交互 input

利用input可以进行用户之间的交互，交互获得的内容是字符串

usename = input('输入用户名：')
print (usename,type(usename))

2.9 流程控制语句 if

基本结构：

if 条件:
    结果
else:
    结果

实例：

if 1 > 2:
    print ("ok")
else:
    print ("not ok")

多项选择结构：

if 条件:
    结果
elif 条件:
    结果
elif 条件:
    结果
else:
    结果

实例：

num = int(input("输入数字:"))
if num == 3:
    print ("1")
elif num == 4:
    print ("2")
elif num == 7:
    print ("3")
else:
    print ("5")

嵌套判断

if 条件:
    if 条件:
        结果
    else:
        结果
else:
    结果

实例：

if 3 > 2:
    if 5 < 6:
        print ("ok")
    else:
        print ("not ok")
else:
    print ("False")