~~NOTOC~~
====== Python ======

<block right box round>
**Website**\\
[[http://www.python.org|www.python.org]]

**Standard library**\\
[[http://docs.python.org/2/library/|Python 2]]\\
[[http://docs.python.org/3/library/|Python 3]]
</block>

==== First Steps ===

  * **Use a [[programming:python:virtualenv|Python Virtual environment]]**  \\    If you are programming something more complex, use some python-packages not available in a simple python-environment, you should create a virtual-environment and store the used python-packages in a [[programming:python:requirements-file|requirements.txt]]


  * **Just one or two** python packages \\ for personal use in some automation-scripting can be installed in a **user environment**. \\ The default user environment is located in **''~/.local/lib''** but by setting some environment variables it can be moved somewhere else (You can add the following line to your .bashrc file - but you should not):

<code bash .bashrc>
...
export PYTHONUSERBASE=~/mypylibs
export PYTHONPATH=$PYTHONUSERBASE:$PYTHONPATH
...
</code>

  To install some packages you can now use the pip command:
<code bash>pip install --user SomePackage</code>

==== Little hints and hacks ===

=== Buffering ===
In case you want to continuously pipe the standard output of the terminal (STOUT) into a file then the output is buffered.  When outputting to a terminal, STDOUT will be line-buffered or not buffered at all, so you'll see output after each **''\n''** or for each character.

You can use the option **''-u''** to prevent python from buffering the output stream.
<code bash>python3 -u ${SCRIPT_FILE} > ${FILE_TO_WRITE} </code>

Or set an environment variable **''PYTHONUNBUFFERED=1''**, (this is what PyCharm uses)
<code bash>PYTHONUNBUFFERED=1 python3 ${SCRIPT_FILE} > ${FILE_TO_WRITE} </code>

When you always want to execute a python-script in unbuffered mode you can add it to the Shebang at the beginning of your script.
<code python>#!/usr/bin/env python3 -u</code>

== Performance ==
Completely unbuffered output can slow down your simulation (creates a lot system-calls). 

0-buffering, equal to PYTHONUNBUFFERD or **''-u''**:
<code bash>stdbuf -o0  python3 ${SCRIPT_FILE} > ${FILE_TO_WRITE} </code>

L-buffering; Linebuffer, a complete line is buffered (**best**)
<code bash>stdbuf -oL  python3 ${SCRIPT_FILE} > ${FILE_TO_WRITE} </code>

If it is not necessary to see all output written immediately, you can use user-defined buffers:
**''K''** Kilobyte, **''M''** Gigabyte or even **''G''** Gigabyte.







===== Tutorials =====
  * [[https://pythonmonk.com|pythonmonk]] (interactive tutorial)
  * [[https://wiki.python.org/moin/BeginnersGuide|Beginner's Guide to Python]] (Python 2)
  * [[http://learnpythonthehardway.org/book/|Learn Python the Hard Way]] (Python 2)
  * [[http://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_3|Non-Programmer's Tutorial for Python 3]] (Python 3)

===== Further Resources =====
  * [[http://legacy.python.org/dev/peps/pep-0008/|PEP 8 -- Style Guide for Python Code]]
  * [[http://www.pythontutor.com/visualize.html|Online Python Tutor -- Visualize program execution]]

  * Developing and testing Python Apps for [[https://developer.fedoraproject.org/tech/languages/python/multiple-pythons.html|multiple versions]] of Python.

{{tag>"python"}}