# Package HowTo

This is a brief introduction to python packages. How to make them, how
to install them, how to upload them, how to maintain them, how to work
with them. For details consult https://packaging.python.org and other
sites.


## Why packages?

When working on your project, you typically end up with some scripts,
functions and classes that are of more general interest. As a first
step, you make some modules, i.e. separate python files, where you
collect this code. These modules can be easily imported from other
scripts in the very same directory.

For example, consider a module `addition.py` and a script `analyze.py`
both in the same directory:
```txt
├── addition.py
└── analyze.py
```

In `addition.py` we define a function `add_two()`:
```
def add_two(x):
    return x + 2
```

We can use this function in `analyze.py` by importing it from
`addition.py`:
```
from addition import add_two

x = 5
y = add_two(x)
```

This works, however, only if scripts and modules are in the same
directory (or if modules are in sub-directories). To make modules
available in other places of your file system or even for other
people, you need to turn the modules into packages.


## Minimal package

First we make a project directory for our new package. Usually the
name of the project directory is the same as the one of the
package. Here, however, we call the proejct directory `packagehowto`
and the name of the package `numerix`.

A package is a directory, and the name of the package is the name of
this directory, here `numerix`. Inside the package directory we put
all the modules, for now just `addition.py`.

What makes this directory a package is the presence of a file named
`__init__.py`. This file is executed when the package is imported. For
now we leave it empty.

The package directory resides in the `src/` directory of the project:
```txt
packagehowto/
├── pyproject.toml
└── src/
    └── numerix
        ├── __init__.py
        └── addition.py
```

A package also needs a `pyproject.toml` file. This file contains some
metadata about the package and informations about how to build a
package. As the bare minimum the content of the `pyproject.toml` file
specifies `setuptools` to be used for building the project:

```txt
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"
```

With this `pyproject.toml` file, and `addition.py` and `__init__.py`
in the `src/numerix` directory, you can install the `numerix` package
on your machine and import it from wherever you want.

To install the project, go to the project root, here `packagehowto/` and run
from your shell
```sh
pip3 install .
```
This installs the package of the current project folder `.` somewhere
in your home directory. From anywhere in your home file system you now
can import this package. The import line of our `analyze.py` file
needs to look like this:
```
from numerix.addition import add_two
```
From the addition module of the numerix package the function add_two
is imported.

You would need to reinstall the package whenever you change your
package, for exmple when you add a new function or when you just fix
some package code. This is tedious, and that is why there is a `-e`
option for `pip install`. So install your package with
```sh
pip3 install -e .
```
Then all future changes on your package are immediately available
without the need to reinstall the package again.


## The `__init__.py` file

This file is your package. In fact, you could write a package that
only has an `__init__.py` file in the package directory. All code you
want to make available in you package could be place in the
`__init__.py` file. For example, if you define a `add_four()` function
in `__init__.py` like this:
```
def add_four(x):
    return x + 4
```
then it can be imported directly from the package:
```
from numerix import add_four
```

Alternatively, you can define all your functions in module files, like
we did with the function `add_two()` in the `addition.py` module. In
the `__init__.py` file you can import this function and this way make
it available directly from the package. With this line in `__init__.py`
```
from .addition import add_two
```
(note the `.` in front of `addition`!) you can import `add_two()`
without specifying the module:
```
from numerix import add_two
```


## Multiple modules within a package

You can have as many modules as you need in your package. Let's add a
module `numbers.py` to the `numerix` package
```txt
    └── numerix
        ├── __init__.py
        ├── addition.py
        └── numbers.py
```
with the following content:
```
from .addition import add_two

def three():
    return add_two(1)
```
This makes a function `three()` available that can be imported via
```
from numerix.numbers import three
```

Note, that the `numbers.py` module imports a function from the
`addition.py` module via a relative import with the `.` in front of
`addition`.


## Distribute your package

Full pyproject.toml file...

README.md

LICENSE

pypi.org

```sh
python3 -m build
```

```sh
python3 -m twine upload dist/*
```

token


## Package version


## Unit tests

pytest

coverage
```sh
pytest -v --cov-report html --cov numerix tests/
```

## Documentation