Zuice: dependency injection for Python

Zuice is a library for dependency injection in Python.

Example

import zuice

class BlogPostLister(zuice.Base):
    _fetcher = zuice.dependency(BlogPostFetcher)

    def all(self):
        return ", ".join(post.name for post in self._fetcher.fetch_all())

bindings = zuice.Bindings()
bindings.bind(BlogPostFetcher).to_instance(blog_post_fetcher)

injector = zuice.Injector(bindings)
assert injector.get(BlogPostFetcher) is blog_post_fetcher
injector.get(BlogPostLister) # constructs BlogPostLister using the bound instance of BlogPostFetcher

Usage

The high-level usage overview is:

• Create an instance of zuice.Bindings.
• Add providers to the bindings for cases where Zuice cannot automatically infer how to retrieve a value associated with a key.
• Create an instance of zuice.Injector using these bindings.
• Call injector.get(key) to retrieve the value associated with key.

Value retrieval

When injector.get(key) is called, either explicitly by the user or while trying to resolve other dependencies, Zuice will try to retrieve the associated value in one of several ways, depending on the value of key:

• If a provider for key has been explicitly bound, use that provider.
• If key is an instance of zuice.Base, create an instance of key by injecting its dependencies.
• If key is a type whose __init__ method takes no arguments, call key().

Bindings

Constants

Keys can be bound to specific values. For instance:

bindings = zuice.Bindings()
bindings.bind(key).to_instance(value)
injector = zuice.Injector(bindings)
assert injector.get(key) is value

Each time injector.get(key) is called, value will always be returned.

Types

Keys can be bound to types. For instance:

bindings = zuice.Bindings()
bindings.bind(CookieStore).to_type(RedisCookieStore)
injector = zuice.Injector(bindings)

Calling injector.get(CookieStore) will return the same value as calling injector.get(RedisCookieStore). However, by calling injector.get(CookieStore), you avoid relying on a specific implementation of a cookie store. If you decide you want to use PostgresCookieStore at a later date, you only have to update the bindings rather than all the places that use a cookie store.

Providers

Keys can be bound to providers. A provider is a function that takes an injector as its argument, and returns a value. For instance:

bindings = zuice.Bindings()
bindings.bind(CookieStore).to_provider(lambda injector: injector.get(RedisCookieStore))

In most cases, using other methods of binding should suffice. In this case, using to_type would be preferred.