# How to Write, Package and Distribute a Library in Python

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Python is a great programming language, but packaging is one of its weakest points. It is a well-known fact in the community. Installing, importing, using and creating packages has improved a lot over the years, but it's still not on par with newer languages like Go and Rust that learned a lot from the struggles of Python and other mature languages.

In this tutorial, you'll learn everything you need to know about writing, packaging and distributing your own packages.

## How to Write a Python Library

A Python library is a coherent collection of Python modules that is organized as a Python package. In general, that means that all modules live under the same directory and that this directory is on the Python search path.

Let's quickly write a little Python 3 package and illustrate all these concepts.

### The Pathology Package

Python 3 has an excellent Path object, which is a huge improvement over Python 2's awkward os.path module. But it's missing one crucial capability—finding the path of the current script. This is very important when you want to locate access files relative to the current script.

In many cases, the script can be installed in any location, so you can't use absolute paths, and the working directory can be set to any value, so you can't use a relative path. If you want to access a file in a sub-directory or parent directory, you must be able to figure out the current script directory.

Here is how you do it in Python:

To access a file called 'file.txt' in a 'data' sub-directory of the current script's directory, you can use the following code: print(open(str(script_dir/'data/file.txt').read())

With the pathology package, you have a built-in script_dir method, and you use it like this:

Yep, it's a mouthful. The pathology package is very simple. It derives its own Path class from pathlib's Path and adds a static script_dir() that always returns the path of the calling script.

Here is the implementation:

Due to the cross-platform implementation of pathlib.Path, you can derive directly from it and must derive from a specific sub-class (PosixPath or WindowsPath). The script dir resolution uses the inspect module to find the caller and then its filename attribute.

### Testing the Pathology Package

Whenever you write something that is more than a throwaway script, you should test it. The pathology module is no exception. Here are the tests using the standard unit test framework:

### The Python Path

Python packages must be installed somewhere on the Python search path to be imported by Python modules. The Python search path is a list of directories and is always available in sys.path. Here is my current sys.path:

Note that the first empty line of the output represents the current directory, so you can import modules from the current working directory, whatever it is. You can directly add or remove directories to/from sys.path.

You can also define a PYTHONPATH environment variable, and there a few other ways to control it. The standard site-packages is included by default, and this is where packages you install using via pip go.

## How to Package a Python Library

Now that we have our code and tests, let's package it all into a proper library. Python provides an easy way via the setup module. You create a file called setup.py in your package's root directory. Then, to create a source distribution, you run: python setup.py sdist

To create a binary distribution called a wheel, you run: python setup.py bdist_wheel

Here is the setup.py file of the pathology package:

It includes a lot of metadata in addition to the 'packages' item that uses the find_packages() function imported from setuptools to find sub-packages.

Let's build a source distribution:

The warning is because I used a non-standard README.md file. It's safe to ignore. The result is a tar-gzipped file under the dist directory:

And here is a binary distribution:

The pathology package contains only pure Python modules, so a universal package can be built. If your package includes C extensions, you'll have to build a separate wheel for each platform:

For a deeper dive into the topic of packaging Python libraries, check out How to Write Your Own Python Packages.

## How to Distribute a Python Package

Python has a central package repository called PyPI (Python Packages Index). When you install a Python package using pip, it will download the package from PyPI (unless you specify a different repository). To distribute our pathology package, we need to upload it to PyPI and provide some extra metadata PyPI requires. The steps are:

• Create an account on PyPI (just once).

### Create an Account

You can create an account on the PyPI website. Then create a .pypirc file in your home directory:

For testing purposes, you can add a "pypitest" index server to your .pypirc file:

If this is the first release of your package, you need to register it with PyPI. Use the register command of setup.py. It will ask you for your password. Note that I point it to the test repository here:

Now that the package is registered, we can upload it. I recommend using twine, which is more secure. Install it as usual using pip install twine. Then upload your package using twine and provide your password (redacted below):

For a deeper dive into the topic of distributing your packages, check out How to Share Your Python Packages.

## Conclusion

In this tutorial, we went through the fully fledged process of writing a Python library, packaging it, and distributing it through PyPI. At this point, you should have all the tools to write and share your libraries with the world.

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