Constructor, explicit, and collection DI๏
Beginner โ Intermediate. Annotations, explicit targets, collections.
Annotate a constructor parameter. Melder builds the graph.
What an annotation selects depends on what it is. A class selects the spells of that type โ a class
bound bare, or an existing object of that class โ and nothing else: a category that happens to share the
class's name does not turn its members into providers. A Protocol selects the spells bound under it as
their contract. A TYPE_CHECKING-only import, which leaves a string at runtime, resolves by name to a
type or a contract, never to a string category. list[...] gathers the group the annotation's kind names:
every spell of the class, every implementer of the Protocol, or โ for a string โ every member of the
category or type of that name.
class ReportService:
def __init__(self, database: Database) -> None: # โ injected
self.database = database
book.bind(spell=Database, existence="unique")
book.bind(spell=ReportService, existence="unique")
conduit = book.conjure()
report = conduit.meld("ReportService")
When an annotation isn't specific enough โ two implementations of one shape, or
a named binding โ declare the target with SpellMap as the parameter default:
class Consumer:
def __init__(self, store=md.SpellMap(PrimaryStore)) -> None:
self.store = store
SpellMap takes the full address: a concrete spell, a spellframe, a
binding_name, or a combination. It obeys an exactly-one law โ a target
that matches zero spells or several fails at conjure(), not at runtime.
When a plain annotation has several providers (two classes that share a name,
two bindings of one class), conjure() refuses with an AMBIGUOUS_PROVIDER
report that lists every candidate's address and the fix โ a SpellMap
default, a meld override, or one provider โ never a guess.
Ask for a list and get every implementation โ the plugin pattern in one annotation:
class Dispatcher:
def __init__(self, handlers: list[Handler]) -> None: # โ all of them
self.handlers = handlers
book.bind(spell=EmailHandler, existence="unique",
spellframe=Handler, binding_name="email")
book.bind(spell=SmsHandler, existence="unique",
spellframe=Handler, binding_name="sms")
Runnable examples๏
Constructor di by annotation โ Intermediate 15
Spellmap defaults โ Intermediate 16
Collection di โ Intermediate 17
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