python-dependency-injector/README.md

# Objects

Dependency management tool for Python projects.

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## Introduction

Python ecosystem consists of a big amount of various classes, functions and 
objects that could be used for applications development. Each of them has its 
own role.

Modern Python applications are mostly the composition of well-known open 
source systems, frameworks, libraries and some turnkey functionality.

When application goes bigger, its amount of objects and their dependencies 
also increased extremely fast and became hard to maintain.

`Objects` is designed to be developer's friendly tool for managing objects 
and their dependencies in formal, pretty way. Main idea of `Objects` is to 
keep dependencies under control.

## Entities

Current section describes main `Objects` entities and their interaction.

### Providers

Providers are strategies of accessing objects.

All providers are callable. They describe how particular objects will be 
provided. For example:

```python
"""`NewInstance` and `Singleton` providers example."""

from objects.providers import NewInstance
from objects.providers import Singleton


# NewInstance provider will create new instance of specified class
# on every call.
new_object = NewInstance(object)

object_1 = new_object()
object_2 = new_object()

assert object_1 is not object_2

# Singleton provider will create new instance of specified class on first call,
# and return same instance on every next call.
single_object = Singleton(object)

single_object_1 = single_object()
single_object_2 = single_object()

assert single_object_1 is single_object_2
```

### Injections

Injections are additional instructions, that are used for determining 
dependencies of objects.

Objects can take dependencies in various forms. Some objects take init 
arguments, other are using attributes or methods to be initialized. Injection, 
in terms of `Objects`, is an instruction how to provide dependency for the 
particular object.

Every Python object could be an injection's value. Special case is a `Objects` 
provider as an injection's value. In such case, injection value is a result of 
injectable provider call (every time injection is done).

Injections are used by providers.

```python
"""`KwArg` and `Attribute` injections example."""

import sqlite3

from objects.providers import Singleton
from objects.providers import NewInstance

from objects.injections import KwArg
from objects.injections import Attribute


class ObjectA(object):

    """ObjectA has dependency on database."""

    def __init__(self, database):
        """Initializer.

        Database dependency need to be injected via init arg."""
        self.database = database

    def get_one(self):
        """Select one from database and return it."""
        return self.database.execute('SELECT 1').fetchone()[0]


# Database and `ObjectA` providers.
database = Singleton(sqlite3.Connection,
                     KwArg('database', ':memory:'),
                     KwArg('timeout', 30),
                     KwArg('detect_types', True),
                     KwArg('isolation_level', 'EXCLUSIVE'),
                     Attribute('row_factory', sqlite3.Row))

object_a = NewInstance(ObjectA,
                       KwArg('database', database))

# Creating several `ObjectA` instances.
object_a_1 = object_a()
object_a_2 = object_a()

# Making some asserts.
assert object_a_1 is not object_a_2
assert object_a_1.database is object_a_2.database
assert object_a_1.get_one() == object_a_2.get_one() == 1
```

Also injections could be used by any callable with `@inject` decorator:

```python
"""`@inject` decorator example."""

from objects.providers import NewInstance

from objects.injections import KwArg
from objects.injections import inject


object_a = NewInstance(object)
object_b = NewInstance(object)


@inject(KwArg('a', object_a))
@inject(KwArg('b', object_b))
def example_callback(a, b):
    """This function has dependencies on object a and b.

    Dependencies are injected using `@inject` decorator.
    """
    assert a is not b
    assert isinstance(a, object)
    assert isinstance(b, object)


example_callback()
```

### Catalogs

Catalogs are named set of providers.