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211 lines
9.0 KiB
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Translating-----geekpi
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Creating your First App on Linux with Python and Flask
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================================================================================
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Whether playing on Linux or working on Linux there is high chance you have come across a program written in python. Back in college I wish they thought us Python instead of Java like they do today, it’s fun to learn and useful in building practical applications like the yum package manager.
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In this tutorial I will take you through how I built a simple application which displays useful information like [memory usage per process][1], CPU percentage etc using python and a micro framework called flask.
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### Prerequisites ###
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Python Basics, Lists, Classes, Functions, Modules.
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HTML/CSS (basic)
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You don’t need to be an advanced python programmer to follow this tutorial, But before you go further I recommend you read https://wiki.python.org/moin/BeginnersGuide/NonProgrammers
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### Installing Python 3 on Linux ###
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On most Linux distributions python is installed by default. This is how you can find out the python version on your system.
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[root@linux-vps ~]# python -V
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Python 2.7.5
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We will be using python version 3.x to build our app. As per [Python.org][2] all improvements are now only available in this version which is not backward compatible with python 2.
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**Caution**: Before your proceed I strongly recommend you try this tutorial out on a Virtual machine, since python is a core component of many Linux Distributions any accidents may cause permanent damage to your system.
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This step is for RedHat based variants like CentOS (6&7), Debian based variants like Ubuntu,Mint and Rasbian can skip this step as you should have python version 3 installed by default. If not use apt-get instead of yum to install the relevant packages below.
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[leo@linux-vps] yum groupinstall 'Development Tools'
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[leo@linux-vps] yum install -y zlib-dev openssl-devel sqlite-devel bzip2-devel
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[leo@linux-vps] wget https://www.python.org/ftp/python/3.4.2/Python-3.4.2.tgz
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[leo@linux-vps] tar -xvzf Python-3.4.2.tgz
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[leo@linux-vps] cd Python-3.4.2
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[leo@linux-vps] ./configure
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[leo@linux-vps] make
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# make altinstall is recommended as make install can overwrite the current python binary,
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[leo@linux-vps] make altinstall
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After a successful, installation you should be able to access the python 3.4 shell with the command below.
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[leo@linux-vps]# python3.4
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Python 3.4.2 (default, Dec 12 2014, 08:01:15)
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[GCC 4.8.2 20140120 (Red Hat 4.8.2-16)] on linux
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Type "help", "copyright", "credits" or "license" for more information.
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>>> exit ()
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### Installing packages in python with PIP ###
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Python comes with it’s own package manager, similar to yum and apt-get. You will need to use it to download, install and uninstall packages.
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[leo@linux-vps] pip3.4 install "packagename"
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[leo@linux-vps] pip3.4 list
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[leo@linux-vps] pip3.4 uninstall "packagename"
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### Python Virtual Environment ###
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In Python a virtual environment is a directory where your projects dependencies are installed. This is a good way to segregate projects with different dependencies. It also allows you to install packages without the need for sudo access.
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[leo@linux-vps] mkdir python3.4-flask
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[leo@linux-vps] cd python3.4-flask
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[leo@linux-vps python3.4-flask] pyvenv-3.4 venv
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To create the virtual environment you will need to use the “pyvenv-3.4” command. This will create a directory called “lib” inside the venv folder where the dependencies for this project will be installed. It will also create a bin folder which will contain pip and python executables for this virtual environment.
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### Activating the Virtual Environment for our Linux system information project ###
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[leo@linux-vps python3.4-flask] source venv/bin/activate
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[leo@linux-vps python3.4-flask] which pip3.4
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~/python3.4-flask/venv/bin/pip3.4
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[leo@linux-vps python3.4-flask] which python3.4
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~/python3.4-flask/venv/bin/python3.4
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### Installing flask with PIP ###
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Lets go ahead and install out first module the flask framework which will take care of the routing and template rendering of our app.
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[leo@linux-vps python3.4-flask]pip3.4 install flask
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### Creating your first app in flask. ###
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Step 1:Create directories where your app will reside.
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[leo@linux-vps python3.4-flask] mkdir app
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[leo@linux-vps python3.4-flask]mkdir app/static
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[leo@linux-vps python3.4-flask]mkdir app/templates
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Inside the python3.4-flask folder create a folder called app which will contain two sub-folders “static” and “templates”. Our python script will reside inside the app folder, files like css/js inside the static folder and templates folder will contain our html templates.
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Step 2:Create an initialization file inside the app folder.
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[leo@linux-vps python3.4-flask] vim app/_init_.py
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from flask import Flask
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app = Flask(__name__)
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from app import index
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This file will create a new instance of Flask and load our python program stored in a file called index.py which we will create next.
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[leo@linux-vps python3.4-flask]vim app/index.py
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from app import app
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@app.route('/')
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def index():
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import subprocess
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cmd = subprocess.Popen(['ps_mem'],stdout=subprocess.PIPE,stderr=subprocess.PIPE)
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out,error = cmd.communicate()
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memory = out.splitlines()
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return
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Routing in flask is handled by the route decorator. It is used to bind a URL to a function.
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@app.route('/')
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@app.route('/index')
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In order to run a shell command in python you can use the Popen class from Subprocess module.
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subprocess.Popen(['ps_mem'],stdout=subprocess.PIPE,stderr=subprocess.PIPE)
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This class will take a list as an argument, the first item of the list will default to being executable while the next item will be considered the option. Here is another example
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subprocess.Popen(['ls', ‘-l’],stdout=subprocess.PIPE,stderr=subprocess.PIPE)
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stdout and stderr will store the output and error of this command respectively. You can then access this output via the communicate method of the Popen class.
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out,error = cmd.communicate()
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To display the output in a better way via the html template, I have used the splitlines () method,
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memory = out.splitlines()
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More information on python subprocess module is available in the docs at the end of this tutorial.
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Step 3: Create an html template where we can display the output of our command.
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In order to do this we need to use the Jinja2 template engine in flask which will do the template rendering for us.
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Your final index.py file should look as follows
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from flask import render_template
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from app import app
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def index():
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import subprocess
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cmd = subprocess.Popen(['ps_mem'],stdout=subprocess.PIPE,stderr=subprocess.PIPE)
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out,error = cmd.communicate()
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memory = out.splitlines()
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return render_template('index.html', memory=memory)
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Now create an index.html template inside the templates folder, flask will search for templates in this folder.
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[leo@linux-vps python3.4-flask]vim app/templates/index.html
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Memory usage per process
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{% for line in memory %}
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{{ line.decode('utf-8') }}
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{% endfor %}
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The Jinja2 template engine allows you to use the “{{ … }}” delimiter to print results and {% … %} for loops and value assignment. I used the “decode()” method for formatting purposes.
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Step 4: Running the app.
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[leo@linux-vps python3.4-flask]vim run.py
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from app import app
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app.debug = True
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app.run(host='174.140.165.231', port=80)
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The above code will run the app in debug mode. If you leave out the IP address and port it will default to localhost:5000.
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[leo@linux-vps python3.4-flask] chmod +x run.py
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[leo@linux-vps python3.4-flask] python3.4 run.py
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I have added more code to the app so that it gives you cpu, I/O and load avg as well.
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You can access the code to this app [here][3].
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This is a brief introduction to flask and I recommend you reading the tutorials and docs below for indepth information.
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http://flask.pocoo.org/docs/0.10/quickstart/#
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https://docs.python.org/3.4/library/subprocess.html#popen-constructor
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http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world
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--------------------------------------------------------------------------------
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via: http://techarena51.com/index.php/how-to-install-python-3-and-flask-on-linux/
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作者：[Leo G][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://techarena51.com/
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[1]:http://techarena51.com/index.php/linux-memory-usage/
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[2]:https://wiki.python.org/moin/Python2orPython3
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[3]:https://github.com/Leo-g/python-flask-cmd
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