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Constructor dependency injection๏
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.
import melder as md
class Database:
"""A small stand-in for a database; this example performs no I/O."""
def query(self) -> str:
return "42 rows"
class ReportService:
def __init__(self, database: Database) -> None: # โ injected
self.database = database
def run(self) -> str:
return "report over " + self.database.query()
book = md.Spellbook()
book.bind(spell=Database, existence="unique")
book.bind(spell=ReportService, existence="unique")
with book.conjure() as conduit:
report: ReportService = conduit.meld("ReportService")
assert isinstance(report.database, Database)
print(report.run())
This complete example prints report over 42 rows: melding the consumer first
constructs its registered dependency and injects it into the constructor.
Runnable examples๏
Constructor di by annotation โ Intermediate 15
Typed melding โ Beginner 16