concerns 1.0

# Concerns

[Python](https://www.python.org/) implementation of the
[Ruby](https://www.ruby-lang.org/)’s
[ActiveSupport::Concern](https://api.rubyonrails.org/v5.1/classes/ActiveSupport/Concern.html)
behavior.

## Use examples

Note: the class context is the fourth argument of the
[metaclass](https://www.python.org/doc/essays/metaclasses/) `__init__` method,
formally “namespace”:

```python
def __init__(cls, name: str, bases: tuple, namespace: dict) -> None:
...
```

It is a dictionary representation of the class body.

### Using a class as mix-in

If the aspect is a class, its context will be merged to the target context on
its factoring.

```python
from typing import NamedTuple
from concerns import concern


class SerialMixin:

def serialize(self) -> str:
return json.dumps(self._asdict(), sort_keys=True)

@classmethod
def deserialize(cls, data: str):
return cls(**json.loads(data))


class PersonBase(NamedTuple):
name: str
surname: str


class Person(PersonBase):
concern(SerialMixin)
```

### Using a mapping as default values map

If the aspect is a mapping (a dictionary, by example), it itself will be merged
to the target context on its factoring.

```python
from concerns import concern

class Animal:
# Too simple use, avoid doing it
concern({voice='grrr'})

def __init__(self, *, voice: str = None):
if voice:
self.voice = voice

def say(self) -> None:
print(self.voice)
```

### Function to change behavior

If the aspect is a callable, the target class context will be passed as
parameter to the aspect for processing before its factoring.

```python
from concerns import concern
from collections.abc import MutableMapping
from datetime import datetime

def make_timed(context: MutableMapping) -> None:
"""
Reusable aspect function to add creating time to objects
"""

def set_created_attr(attr: str) -> None:
context[attr] = None
def new(cls, name: str, bases: tuple, _dict: dict):
self = super().__new__(cls, name, bases, _dict)
setattr(self, attr) = datetime.now()
return self
context['__new__'] = new
context['set_created_attr'] = set_created_attr


class Register:
concern(make_timed)

... # more class attributes

set_created_attr('created_at')
```