# Basic usage
## Create an injectable
> **Note**: If the class needs dependencies, these will be resolved when the instance is retrieved.
If you wish to inject a singleton, use `singleton` decorator.
```python
from injection import singleton
@singleton
class ServiceA:
""" class implementation """
```
If you wish to inject a new instance each time, use `injectable` decorator.
```python
from injection import injectable
@injectable
class ServiceB:
""" class implementation """
```
If you have a constant (such as a global variable) and wish to register it as an injectable, use `set_constant`
function.
```python
from injection import set_constant
class ServiceC:
""" class implementation """
service_c = set_constant(ServiceC())
```
## Inject an instance
To inject one or several instances, use `inject` decorator.
_Don't forget to annotate type of parameter to inject._
```python
from injection import inject
@inject
def some_function(service_a: ServiceA):
""" function implementation """
```
If `inject` decorates a class, it will be applied to the `__init__` method.
_Especially useful for dataclasses:_
> **Note**: Doesn't work with Pydantic `BaseModel` because the signature of the `__init__` method doesn't contain the
> dependencies.
```python
from dataclasses import dataclass
from injection import inject
@inject
@dataclass
class SomeDataClass:
service_a: ServiceA = ...
```
## Get an instance
_Example with `get_instance` function:_
```python
from injection import get_instance
service_a = get_instance(ServiceA)
```
_Example with `get_lazy_instance` function:_
```python
from injection import get_lazy_instance
lazy_service_a = get_lazy_instance(ServiceA)
# ...
service_a = ~lazy_service_a
```
## Inheritance
In the case of inheritance, you can use the decorator parameter `on` to link the injection to one or several other
classes.
**Warning: if the child class is in another file, make sure that file is imported before injection.**
[_See `load_package` function._](utils.md#load_package)
_Example with one class:_
```python
class AbstractService(ABC):
...
@injectable(on=AbstractService)
class ConcreteService(AbstractService):
...
```
_Example with several classes:_
```python
class AbstractService(ABC):
...
class ConcreteService(AbstractService):
...
@injectable(on=(AbstractService, ConcreteService))
class ConcreteServiceOverload(ConcreteService):
...
```
If a class is registered in a package, and you want to override it, there is the `mode` parameter:
```python
@injectable
class InaccessibleService:
...
# ...
@injectable(on=InaccessibleService, mode="override")
class ServiceOverload(InaccessibleService):
...
```
## Recipes
A recipe is a function that tells the injector how to construct the instance to be injected. It is important to specify
the return type annotation when defining the recipe.
```python
from injection import injectable
@injectable
def service_d_recipe() -> ServiceD:
""" recipe implementation """
```
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