Detailed explanation of how to quickly transform python

1. python installation pyenv

Installing Python under windows

(1) Open the web browser and visit the official Python site http://www.python.org

(2) Click Download, or click download on the homepage

(3) Download the required version, the latest version is 3.5.2

(4) After the download is completed, double-click and execute the installation

Installing Python and pyenv under CentOS

pyenv: pyenv is a python version manager. Since different programmers may use different pythons for development, they still need to communicate with each other on the same machine. No impact. The main function of pyenv is to set different directories to use different versions, and it can easily install python.

1. Install dependency packages

yum groupinstall "Development Tools" "Server Plataform Development" -y

2. Install pyenv

(1) Passed git clone pyenv program

$ git clone https://github.com/yyuu/pyenv.git ~/.pyenv

(2) Set the environment variable of pyenv

$ echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.bash_profile
$ echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.bash_profile

(3) Add pyenv init to the environment variable

$ echo 'eval "$(pyenv init -)"' >> ~/.bash_profile

(4 ) Reread your shell

$ exec $SHELL
$ source ~/.bash_profile

3. Use and install pyenv python

List all available versions

pyenv install -l

Install the specified version

$ pyenv install 3.5.2
Downloading Python-3.5.2.tar.xz...
-> https://www.python.org/ftp/python/3.5.2/Python-3.5.2.tar.xz

Switch local python version

$ pyenv local 2.7 .12

Switch global python version

pyenv global 3.5.2

4. Uninstall pyenv

CentOS uninstall

rm -rf `pyenv root`

Mac OS X

brew uninstall pyenv

2. Interactive interpreter

When starting python, the following prompt will appear:

$ python
Python 3.5.2 (default, Aug 17 2016, 18:31:29)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-17)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

You can enter interactively with python again, such as

>>> print('Hello World')
When you press Enter, you will get the following output
Hello World

Of course, in many computer languages, it is customary to It ends with a semicolon, but Python is different. One line is one line, regardless of the content.
What is described above is an interactive interpreter. You can get output by typing some characters to achieve your own purposes.

3. What is an algorithm?

Before starting to change, first explain what computer programming is. Simply put, it tells the computer what to do. The computer can do many things, but it will not do it by itself. Programmers tell it the specific details like feeding a child, and use a language that the computer can understand - algorithm. “Algorithm” is just another fancy way of saying “steps” or “recipe”—a detailed description of how to do something. For example:
How to make cold skin:
(1) Cut the cold skin into thin strips
(2) Put the cold skin into a container
(3) Add mahjong, vinegar, garlic juice, chili and other seasonings
(4) Stir for about 1 minute
In fact, the above steps are algorithms. They tell the vinegar program how to do it, how long to do it, what to do, and what to do.

4. Numbers and expressions

Number operations

The interactive python interpreter can be used as a very powerful calculator

>>> 3666665444566 + 1123134546
3667788579112
>>> 588822456 * 33334678
19628206969929168

Of course there are some special ones. The following two examples are in different versions

Python 2.7.5 (default, Jun 24 2015, 00:41:19)
[GCC 4.8.3 20140911 (Red Hat 4.8.3-9)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> ; 1/2
0
Python 3.5.2 (default, Aug 17 2016, 18:31:29)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-17)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/2
0.5

Divisible

>>> 20 / 3
6.666666666666667
>>> 20 // 3
6

remainder

>>> ; 122 % 3
2
>>> 2.75 % 0.5
0.25

power

>>> (-3) ** 2
9
>>> (-3) ** 3
-27
>>> 3 ** 3
27

Long integer

The display method of long integers is different after python3.0. Before 3.0, there will be an L

Python 2.7.5 (default, Jun 24 2015, 00:41:19)
[GCC 4.8.3 20140911 (Red Hat 4.8.3-9)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>> ;> 3000000000000000000000000000
300000000000000000000000000L
Python 3.5.2 (default, Aug 17 2016, 18:31:29)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-17)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 00000000000000000000000000
243000000000000000000000000000000000000000000000000000000000000000000

5. Variables

Variable is another concept that needs to be familiar with. A variable basically represents (or refers to) a value name

>>> x = 3

>>> * 2
6

Note: Variable names can include letters, numbers and underscores. Variables cannot start with a number.

6. Obtain user input

When writing a program, we may not know the values ​​of some variables. It may be through the user's input, but the interpreter knows its value, so we may need to obtain the user's input. To obtain the user's input, use the input() function (the raw_input function is replaced by the input function after 3.0)

>>> input('Please input your name? ')

Please input your name? Antony

'Antony'

We can also store the obtained value in A variable

>>> name = input('Please input your name? ')

Please input your name? superman

>>> name

'superman '

Calculation

>>> x = input('Please input a number x: ')

Please input a number x: 34

>> > y = input('Please input a number y: ')

Please input a number y: 56
>>> x
'34'
>>> y
'56'
>>> int(x) * int(y)
1904

7. Function

The function is similar to a small program and can be used to implement specific functions. Python has many functions, print() is one function. Defining functions will be introduced later. For now, we will introduce how to use Python's built-in functions.

pow()

pow represents power, which can be used to calculate exponentiation

>>> 2 ** 3

8

>> ;> pow(2,3)

8
>>> pow(3,2)
9

abs()

abs means absolute Value

>>> abs(-10)

10

round()

The round function will round floating point numbers

>>> 1 / 2
0.5
>>> round(1 / 2)
0
>>> round(5 / 2)
2

floor()

floor is rounded down. For example, a person’s age is 32.9 years old, which is 32 years and nine months. At this time, it cannot be said that he is already three years old. Three years old, use the floor function at this time

>>> import math
>>> math.floor(32.9)
32

The floor function is After python3.0, the value is no longer a decimal, and the certificate is obtained directly. The following is the output before 3.0

>>> import math
>>> math.floor(32.9)
32.0

int()

int means taking an integer

>>> int(256.125)
256

input ()

Input means obtaining user input, which has been introduced above. Without further ado, here is an example

>>> input('Do you want me? ')
Do you want me? No!!!
'No!!!'

exit()

Interactive interpreter used to exit python

$ python
Python 3.5.2 (default, Aug 17 2016, 18:31:29)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-17)] on linux
Type "help" , "copyright", "credits" or "license" for more information.
>>> exit()
$

8. Module

The module is for To extend the functionality of python, you need to use the special command import to import modules. The floor function mentioned earlier is in the module named math.

>>> import math
>>> math.floor(333.20)
333

Sometimes you may only need to use a certain module A function, and you don’t want to write the name of the module every time, you can use another form.

>>> from math import sqrt
>>> sqrt(9)
3.0

9. Save in execution script

To store the command permanently for later use, you can write it to a file.
The beginning of the file needs to be marked, because there is a standard implementation method in Unix: add # before the first line of the script! . After that, add the absolute path to the program that interprets the script.

For example:

$ cat first.py
#!/usr/bin/env python3.5
name = input('Please input your name: ')
print('Hello, ' + name)
$ chmod +x first.py
$ ./first.py
Please input your name: Antony
Hello, Antony

Summary

Algorithm: An algorithm is a detailed description of how to complete a task. In fact, when writing a program, you must use a language that the computer can understand (such as Python) to describe the algorithm. This type of machine-friendly description is called a program, which mainly contains expressions and statements.

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