Python学习第二天

一、Python 2 or 3?

In summary : Python 2.x is legacy, Python 3.x is the present and future of the language

Python 3.0 was released in 2008. The final 2.x version 2.7 release came out in mid-2010, with a statement of

extended support for this end-of-life release. The 2.x branch will see no new major releases after that. 3.x is

under active development and has already seen over five years of stable releases, including version 3.3 in 2012,

3.4 in 2014, and 3.5 in 2015. This means that all recent standard library improvements, for example, are only

available by default in Python 3.x.

Guido van Rossum (the original creator of the Python language) decided to clean up Python 2.x properly, with less regard for backwards compatibility than is the case for new releases in the 2.x range. The most drastic improvement is the better U服务器托管网nicode support (with all text strings being Unicode by default) as well as saner bytes/Unicode separation.

Besides, several aspects of the core language (such as print and exec being statements, integers using floor division) have been adjusted to be easier for newcomers to learn and to be more consistent with the rest of the language, and old cruft has been removed (for example, all classes are now new-style, “range()” returns a memory efficient iterable, not a list as in 2.x).

py2与3的详细区别

PRINT IS A FUNCTION

The statement has been replaced with a print() function, with keyword arguments to replace most of the special syntax of the old statement (PEP 3105). Examples:

Old:print"The answer is",2*2New:print("The answer is",2*2) Old:print服务器托管网x,# Trailing comma suppresses newline New: print(x, end=" ") # Appends a space instead of a newline Old:print# Prints a newline New:print()# You must call the function! Old:print>>sys.stderr,"fatal error"New:print("fatal error",file=sys.stderr) Old:print(x, y)# prints repr((x, y)) New:print((x, y))# Not the same as print(x, y)!

You can also customize the separator between items, e.g.:

print("There are ,2**32,"> possibilities!", sep="")

ALL IS UNICODE NOW

从此不再为讨厌的字符编码而烦恼

还可以这样玩： (A,*REST,B)=RANGE(5)

>>> a,*rest,b=range(5) >>> a,rest,b (0, [1,2,3],4) /strong>

某些库改名了

　　还有谁不支持Python3?

One popular module that don’t yet support Python 3 is Twisted (for networking and other applications). Most

actively maintained libraries have people working on 3.x support. For some libraries, it’s more of a priority than

others: Twisted, for example, is mostly focused on production servers, where supporting older versions of

Python is important, let alone supporting a new version that includes major changes to the language. (Twisted is

a prime example of a major package where porting to 3.x is far from trivial

二、Python安装

windows

1、下载安装包 https://www.python.org/downloads/ 2、安装 默认安装路径：C:python27 3、配置环境变量 【右键计算机】--》【属性】--》【高级系统设置】--》【高级】--》【环境变量】--》【在第二个内容框中找到 变量名为Path 的一行，双击】--> 【Python安装目录追加到变值值中，用 ； 分割】 如：原来的值;C:python27，切记前面有分号

linux、Mac

无需安装，原装Python环境 ps：如果自带2.6，请更新至2.7

三、Hello World程序

在linux 下创建一个文件叫hello.py,并输入

1	print("Hello World!")

然后执行命令:python hello.py ,输出

localhost:~ jieli$ vim hello.py localhost:~ jieli$ python hello.py Hello World!

指定解释器

上一步中执行 python hello.py时，明确的指出 hello.py 脚本由 python 解释器来执行。

如果想要类似于执行shell脚本一样执行python脚本，例：./hello.py，那么就需要在 hello.py 文件的头部指定解释器，如下：

1 2 3

#!/usr/bin/env python print"hello,world"

如此一来，执行：./hello.py即可。

ps：执行前需给予 hello.py 执行权限，chmod 755 hello.py

在交互器中执行　

除了把程序写在文件里，还可以直接调用python自带的交互器运行代码，　

localhost:~ jieli$ python Python2.7.10(default,Oct232015,18:05:06) [GCC4.2.1Compatible Apple LLVM7.0.0(clang-700.0.59.5)] on darwin Type"help","copyright","credits"or"license"formore information. >>>print("Hello World!") Hello World!

四、变量字符编码　　

Variablesare used to store information to be referenced and manipulated in a computer program. They also provide a way of labeling data with a descriptive name, so our programs can be understood more clearly by the reader and ourselves. It is helpful to think of variables as containers that hold information. Their sole purpose is to label and store data in memory. This data can then be used throughout your program.

声明变量

#_*_coding:utf-8_*_ name="Alex Li"

上述代码声明了一个变量，变量名为： name，变量name的值为：”Alex Li”　

变量定义的规则：

• 变量名只能是 字母、数字或下划线的任意组合
• 变量名的第一个字符不能是数字
• 以下关键字不能声明为变量名
  [‘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’]

变量的赋值

name="Alex Li" name2=name print(name,name2) name="Jack" print("What is the value of name2 now?")

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1. 前言

  大家好，我是herosunly。985院校硕士毕业，现担任算法研究员一职，热衷于机器学习算法研究与应用。曾获得阿里云天池比赛第一名，CCF比赛第二名，科大讯飞比赛第三名。拥有多项发明专利。对机器学习和深度学习拥有自己独到的见解。曾经辅导过若干个非计算机专业…