# nv-utils-registry
This is a generic mapping container that implements some registration logic.

All the logic lies behind 


## Installation
To install this package, run:

    pip install nv_utils_registry

As part of `nv.utils` namespace package, the `registry` module will be available 
as one of the library `collections`:

    import nv.utils.collections.registry as registry


## Usage

The registry is a simple container 
that can be used to store and retrieve objects by a unique identifier class as key.

As a regular Python dictionary,
only hashable objects can be used as registry keys.

If the key class implements an `iter_defaults` method
that returns an iterable of default keys in a hierarchical order
(i.e. from the most specific to the most general),
the registry will return the value associated with the first default key that matches an existing key.

The registry accepts a default value that will be returned if no key 
(including any alternative keys provided by `iter_defaults`) is found.
However, if no default is provided,
the registry will raise a `KeyError` in the absence of a valid key.


### Dictionary with a default

Let's start with the simplest and silly version of it: 
one that does nothing but implementing a dictionary with strings as keys and
a default value:

    from nv.utils.collections.registry import BaseContentRegistry

    class SillyRegistry(BaseRegistry):
        registry_key_constructor = str

If you use this registry, you can store and retrieve objects by strings:

    registry = SillyRegistry(default='oops')

    # You may use it as a regular dictionary
    registry['foo'] = 'bar'
    registry['foo']
    # 'bar'

    registry['whatever']
    # 'oops'

    # Or use its specialized methods
    registry.get_content('foo')
    # 'bar'

    registry.get_content('whatever', default='use this instead')
    # 'use this instead'

    registry.register_content('whatever', 'you got it!')
    registry.get_content('whatever')
    # 'you got it!'

So far, nothing really exciting.


### Using the default hierarchy

Let's move to a more useful version of this collection.
Let's assume that you want to store and retrieve classes by a hierarchy of keys.

    from dataclasses import dataclass

    # Let's set frozen=true and order=True to have a hasahble dataclass
    @dataclass(frozen=True, order=True)
    class AnimalKey(BaseContentRegistry):
        category: str
        subcategory: str

        def iter_defaults(self):
            # This will teach how to find alternative default keys for this key,
            # in this case, by adding a '*' progressively
            cls = self.__class__
            yield cls(self.category, "*")
            yield cls("*", "*")

Notice that we have implemented `iter_defaults` method, which will yield
a sequence of keys that will teach the dictionary how to look for a sequence of defaults
from the more specific to the more generic.

    # Let's create our custom registry
    class AnimalRegistry(BaseRegistry):
        registry_key_constructor = AnimalKey

    registry = AnimalRegistry()

    class GenericAnimal:
        pass

    # Now let's register some animal classes using a convenient decorator
    @registry.register('mammal', 'canine')
    class Canine:
        pass

    @registry.register('mammal', 'feline')
    class Feline:
        pass

    # You can use the register_content method as well
    class Ape:
        pass
    
    registry.register_content('mammal', 'ape', Ape)

    # Now let's add a default for the category 'mammal'
    @registry.register('mammal', '*')
    class GenericMammal:
        pass

    registry.register_content('reptile', 'turtle', Turtle)

    # Now let's add a default for all categories
    @registry.register('*', '*')
    class GenericAnimal:
        pass

    # Now let's retrieve the classes
    cat_cls = registry.get_content('mammal', 'feline')         # Feline
    dog_cls = registry.get_content('mammal', 'canine')         # Canine
    hamster_cls = registry.get_content('mammal', 'rodent')     # GenericMammal
    sea_turtle_cls = registry.get_content(category='reptile', subcategory='turtle')     # Turtle
    chameleon_cls = registry.get_content('reptile', 'lizard')  # GenericAnimal
    
    # The decorator, get_content and register_content will recreate the key object
    # by building it from its args or kwargs,
    # but if you have the key object you can use the dictionary directly

    canine_key = AnimalKey('mammal', 'canine')
    wolf_cls = registry[canine_key]

    fox_cls = registry.get_content_by_key(canine_key, default=None)


### Customizing the behavior of our registry

The `BaseRegistry` class provides a way to customize its behavior while dealing with
duplicate registrations (or changes to any of the existing values).

    class AnimalRegistry(BaseRegistry, change_behavior=BaseRegistry.ChangeBehavior.RAISE):
        registry_key_constructor = AnimalKey

    registry = AnimalRegistry()

    # Let's register some animal classes using a convenient decorator
    @registry.register('mammal', 'canine')
    class Canine:
        pass

    # This shall raise a TypeError
    @registry.register('mammal', 'canine')
    class AnotherCanine:
        pass

    # This shall also raise a TypeError
    del registry[AnimalKey('mammal', 'canine')]

    # However, this will not raise an error, as we are registering
    # the same content under the same key again.
    registry.register_content(Canine, 'mammal', 'canine')

This behavior can be customized by setting the `change_behavior` attribute to `WARN`
if you want to be warned when you try to register under a key that is already registered,
or to `IGNORE` if you want to ignore duplicate registrations (default).

The comparison is made via `==` operator of the key objects. 
If you want to customize the comparison, you can override the `__eq__` method.


### Using other mappings under the hood

If you need an alternative mapping to store the content, no problem. 
You can provide your custom mapping at the `__init__` method,
and that mapping will serve as the underlying storage.

Please notice that if you change the underlying mapping directly,
the registry will not be aware of the changes!

    from collections import OrderedDict

    custom_mapping = OrderedDict()

    class AnimalRegistry(BaseRegistry):
        registry_key_constructor = AnimalKey

    # Registry instance will use the custom mapping
    registry = AnimalRegistry(custom_mapping)


### Bugs, suggestions, findings, etc.

Please submit any bug reports, suggestions, patches, etc. to
this github repo.
