I see there are some built in packages that I can import from any script like:
from datetime import date
today = date.today()
print today
How can I create a simple package and add it to the system library so that I can import it like datetime in the above example?
You're trying to make a module.
Start by installing the setuptools package; on either Windows or Linux you should be able to type pip install setuptools at a terminal to get that installed. You should now be able to write import setuptools at a python prompt without getting an error.
Once that's working, set up a directory structure containing a setup.py and a folder for your project's code to go in. The directory must contain a file called __init__.py, which allows you to import the directory as though it's a file.
some_folder/
| setup.py
| my_project/__init__.py
In setup.py, drop the following content:
# setup.py
from setuptools import setup
setup(name="My Awesome Project",
version="0.0",
packages=["my_project"])
In my_project/__init__.py, drop some stuff that you'd like to be able to import. Let's say...
# my_project/__init__.py
greeting = "Hello world!"
Now, in order to install the project at a system-wide level, run python setup.py install. Note that you'll need to run this as root if you're on Linux, since you're making changes to the system-wide python libraries.
After this, you should be able to run python from any directory you like and type:
>>> from my_project import greeting
>>> print greeting
Hello world!
>>>
Note that this is enough to tell you how to make a module, but there's one hell of a lot of stuff that setuptools can do for you. Take a look at https://pythonhosted.org/setuptools/setuptools.html for more info on building stuff, and https://docs.python.org/2/tutorial/modules.html for more info on how modules actually work. If you'd like to look at a package that (I hope) is reasonably simple, then I made my LazyLog module a couple of weeks ago on a train, and you're welcome to use it for reference.
The quick way, if you're just making something for your own use and not worrying about packaging, is to put the module (which could be as simple as a single file) in your system's site-packages directory. (On Debian-based systems, you probably want to use dist-packages instead).
To find out where your site-packages/dist-packages directory is, start Python and:
>>> from sys import path
>>> path
['', '/usr/lib/python3.4/site-packages/pip-7.1.2-py3.4.egg', '/usr/lib/python34.zip', '/usr/lib/python3.4', '/usr/lib/python3.4/plat-cygwin', '/usr/lib/python3.4/lib-dynload', '/usr/lib/python3.4/site-packages']
Note the last item in that example: /usr/lib/python3.4/site-packages. That's the sort of thing you're looking for. So in this example, if I save the following to /usr/lib/python3.4/site-packages/foo.py:
def bar():
print('Hello world!')
Then from anywhere on my system:
>>> from foo import bar
>>> bar()
Hello world!
If you really need a package, you can do it with boost, which allows interacts with C++. You may implement algorithms using C++ and compile it as a Python lib. However it's poorly documented. And as doc described, C API should be a basic option. Boost is built on C API any way.
Sample: I made it several years ago on a class. You can do: import tfidf.
If you simply want the module for personal use, just drop it in a folder and add that folder to the PYTHONPATH environment variable.
For example, create a folder in your home directory called ~/python-packages, then add the following line to your .bashrc (assuming you are using bash):
export PYTHONPATH=$HOME/python-packages`
Then, simply drop any modules/packages you want to make available in ~/python-packages.
Add you python script or package's path to sys.path or just move them to one of the location in sys.path. BUT i do not suggest to do this...
