serum is a fresh take on Dependency Injection in Python 3.
serum is pure python and has no dependencies.
> pip install serum
from serum import inject, dependency, Context # Classes decorated with 'dependency' are injectable types. @dependency class Log: def info(self, message: str): raise NotImplementedError() class SimpleLog(Log): def info(self, message: str): print(message) class StubLog(SimpleLog): def info(self, message: str): pass @inject # Dependencies are injected using a class decorator... class NeedsLog: log: Log # ...and class level annotations... class NeedsSimpleLog: @inject # ...or using a function decorator def __init__(self, log: SimpleLog): self.log = log @inject class NeedsNamedDependency: named_dependency: str # class level annotations annotated with a type that is not # decorated with 'dependency' will be treated as a named # dependency # Contexts provide dependencies with Context(SimpleLog, named_dependency='this name is injected!'): assert isinstance(NeedsLog().log, SimpleLog) assert NeedsNamedDependency().named_dependency == 'this name is injected!' # Contexts will always provide the most specific # subtype of the requested type. This allows you to change which # dependencies are injected. with Context(StubLog): NeedsLog().log.info('Hello serum!') # doesn't output anything NeedsSimpleLog().log.info('Hello serum!') # doesn't output anything
inject is used to decorate functions and classes in which you want to inject
from serum import inject, dependency @dependency class MyDependency: pass @inject def f(dependency: MyDependency): assert isinstance(dependency, MyDependency) f()
Functions decorated with
inject can be called as normal functions.
not attempt to inject arguments given at call time.
@inject def f(dependency: MyDependency): print(dependency) f('Overridden dependency') # outputs: Overridden dependency
inject will instantiate classes decorated with
this way, your entire dependency graph can be specified using just
Instances of simple types and objects you want to instantiate yourself can be
injected using keyword arguments to
@inject def f(dependency: str): assert dependency == 'a named dependency' with Context(dependency='a named dependency'): f()
inject can also be used to decorate classes.
@inject class SomeClass: dependency: MyDependency
This is roughly equivalent to:
class SomeClass: @inject def __init__(self, dependency: MyDependency): self.__dependency = dependency @property def dependency(self) -> MyDependency: return self.__dependency
Dependencies that are specified as class level annotations can be overridden
using key-word arguments to
assert SomeClass(dependency='Overridden!').dependency == 'Overridden!'
Classes decorated with
dependency can be instantiated and injected
from serum import dependency, inject @dependency class Log: def info(self, message): print(message) @inject class NeedsLog: log: Log assert isinstance(NeedsLog().log, Log)
serum relies on being able to inject all dependencies for
dependency decorated classes
recursively. To achieve this,
serum assumes that the
dependency decorated classes can be called without any arguments.
This means that all arguments to
dependency decorated classes must be injected using
@dependency class SomeDependency: def method(self): pass @inject @dependency class ValidDependency: # OK! some_dependency: SomeDependency def __init__(self): ... @dependency class AlsoValidDependency: # Also OK! @inject def __init__(self, some_dependency: SomeDependency): ... @dependency class InvalidDependency: def __init__(self, a): ... @inject def f(dependency: InvalidDependency): ... f() # raises: # TypeError: __init__() missing 1 required positional argument: 'a' # The above exception was the direct cause of the following exception: # InjectionError Traceback (most recent call last) # ... # InjectionError: Could not instantiate dependency <class 'InvalidDependency'> # when injecting argument "dependency" in <function f at 0x10a074ea0>.
Note that circular dependencies preventing instantiation of
classes leads to an error.
@dependency class AbstractA: pass @dependency class AbstractB: pass class A(AbstractA): @inject def __init__(self, b: AbstractB): self.b = b class B(AbstractB): @inject def __init__(self, a: AbstractA): self.a = a @inject class Dependent: a: AbstractA with Context(A, B): Dependent().a # raises: CircularDependency: Circular dependency encountered while injecting <class 'AbstractA'> in <B object at 0x1061e3898>
Contexts provide implementations of dependencies. A
Context will always provide the most
specific subtype of the requested type (in Method Resolution Order).
@dependency class Super: pass class Sub(Super): pass @inject class NeedsSuper: instance: Super with Context(Sub): assert isinstance(NeedsSuper().instance, Sub)
It is an error to inject a type in an
Context that provides two or more equally specific subtypes of that type:
class AlsoSub(Super): pass with Context(Sub, AlsoSub): NeedsSuper() # raises: AmbiguousDependencies: Attempt to inject type <class 'Log'> with equally specific provided subtypes: <class 'MockLog'>, <class 'FileLog'>
Contexts can also be used as decorators:
context = Context(Sub) @context def f(): assert isinstance(NeedsSuper().instance, Sub)
You can provide named dependencies of any type using keyword arguments.
@inject class Database: connection_string: str connection_string = 'mysql+pymysql://root:firstname.lastname@example.org:3333/my_db' context = Context( connection_string=connection_string ) with context: assert Database().connection_string == connection_string
Contexts are local to each thread. This means that when using multi-threading
each thread runs in its own context
import threading @singleton class SomeSingleton: pass @inject def worker(instance: SomeSingleton): print(instance) with Context(): worker() # outputs: <SomeSingleton object at 0x101f75470> threading.Thread(target=worker).start() # outputs: <SomeSingleton object at 0x1035fb320>
To always inject the same instance of a dependency in the same
Context, annotate your type with
from serum import singleton @singleton class ExpensiveObject: pass @inject class NeedsExpensiveObject: expensive_instance: ExpensiveObject instance1 = NeedsExpensiveObject() instance2 = NeedsExpensiveObject() assert instance1.expensive_instance is instance2.expensive_instance
Singleton dependencies injected in different environments
will not refer to the same instance.
with Context(): instance1 = NeedsExpensiveObject() with Context(): assert instance1.expensive_instance is not NeedsExpensiveObject().expensive_instance
serum has support for injecting
MagicMocks from the builtin
unittest.mock library in unittests using the
function. Mocks are reset
when the environment context is closed.
from serum import mock @dependency class SomeDependency: def method(self): return 'some value' @inject class Dependent: dependency: SomeDependency context = Context() with context: mock_dependency = mock(SomeDependency) mock_dependency.method.return_value = 'some mocked value' instance = Dependent() assert instance.dependency is mock_dependency assert instance.dependency.method() == 'some mocked value' with context: instance = Dependent() assert instance.dependency is not mock_dependency assert isinstance(instance.dependency, SomeDependency)
mock uses its argument to spec the injected instance of
MagicMock. This means
that attempting to call methods that are not defined by the mocked
leads to an error
with context: mock_dependency = mock(SomeDependency) mock_dependency.no_method() # raises: AttributeError: Mock object has no attribute 'no method'
mock will only mock requests of the
exact type supplied as its argument, but not requests of
more or less specific types
from unittest.mock import MagicMock @dependency class Super: pass class Sub(Super): pass class SubSub(Sub): pass @inject class NeedsSuper: injected: Super @inject class NeedsSub: injected: Sub @inject class NeedsSubSub: injected: SubSub with Context(): mock(Sub) needs_super = NeedsSuper() needs_sub = NeedsSub() needs_subsub = NeedsSubSub() assert isinstance(needs_super.injected, Super) assert isinstance(needs_sub.injected, MagicMock) assert isinstance(needs_subsub.injected, SubSub)
match is small utility function for matching
with values of an environment variable.
# my_script.py from serum import match, dependency, Context, inject @dependency class BaseDependency: def method(self): raise NotImplementedError() class ProductionDependency(BaseDependency): def method(self): print('Production!') class TestDependency(BaseDependency): def method(self): print('Test!') @inject def f(dependency: BaseDependency): dependency.method() context = match( environment_variable='MY_SCRIPT_ENV', default=Context(ProductionDependency), PROD=Context(ProductionDependency), TEST=Context(TestDependency) ) with context: f()
> python my_script.py Production!
> MY_SCRIPT_ENV=PROD python my_script.py Production!
> MY_SCRIPT_ENV=TEST python my_script.py Test!
It can be slightly annoying to import some
Context and start it as a
context manager in the beginning of every IPython session.
Moreover, you quite often want to run an IPython REPL in a special context,
e.g to provide configuration that is normally supplied through command line
arguments in some other way.
To this end
serum can act as an IPython extension. To activate it,
add the following lines to your
c.InteractiveShellApp.extensions = ['serum']
Finally, create a file named
ipython_context.py in the root of your project. In it,
Context instance you would like automatically started to a global
# ipython_context.py from serum import Context context = Context()
IPython will now enter this context automatically in the beginning of every REPL session started in the root of your project.
If you've been researching Dependency Injection frameworks for python, you've no doubt come across this opinion:
You dont need Dependency Injection in python. You can just use duck typing and monkey patching!
The position behind this statement is often that you only need Dependency Injection in statically typed languages.
In truth, you don't really need Dependency Injection in any language,
statically typed or otherwise.
When building large applications that need to run in multiple environments however,
Dependency Injection can make your life a lot easier. In my experience,
excessive use of monkey patching for managing environments leads to a jumbled
mess of implicit initialisation steps and
if value is None type code.
In addition to being a framework, I've attempted to design
serum to encourage
designing classes that follow the Dependency Inversion Principle:
one should “depend upon abstractions, not concretions."
This is achieved by letting inheritance being the principle way of providing dependencies and allowing dependencies to be abstract.