Flat overrides and live creation probes

Prerequisite: addressing and lifetimes. A flat override supplies constructor inputs for the object being resolved. A factory can instead capture configuration at its registration site. The saved mailer lesson demonstrates both with concrete values.

Choose where configuration belongs

The factory returns a mailer configured for its fixed environment. The separately registered many class accepts host and port through meld(override={...}). Its assertions compare both resulting configurations.

An override is not a new binding or a global configuration edit. Pair the choice with the intended instance lifetime; repeated reuse and fresh construction are different operations. Follow the complete script's lifetime choices first.

Inspect before constructing

has_live_creation(...) asks whether an addressed object already exists. The live-probe lesson checks False before its first meld and True afterward, then uses describe_live_creation_status(...) for detail. A diagnostic path can use these reads without using a meld merely to discover whether something is alive. Given a spell_id as spell=, both probes answer for exactly that version, also one a notch has parked or a copy borrowed from a linked conduit, while meld builds only the selected version.

Continue to deep overrides when the target is inside a dependency graph rather than a parameter of the object you are resolving.

For a registered resolvable=False definition, supply its consumer's value through the same override API. The supplied object remains your reference; this does not transfer lifecycle ownership. Missing ordinary required arguments use Python's normal constructor error, including in cached execution. Existing whole-child overrides still eagerly construct registered children before replacement; they do not prune the child's constructor inputs. Plan for that existing behavior when replacing a branch.

The workflow in code

These are the core steps from the saved example. Use its complete linked script for all class definitions and setup.

def main() -> None:
    book = md.Spellbook()
    book.bind(spell=production_mailer, existence="unique",
              spellframe="mail", binding_name="mailer")
    book.bind(spell=SmtpMailer, existence="many",
              spellframe="mail", binding_name="raw-mailer")
    conduit = book.conjure()

    mailer = conduit.meld(spellframe="mail", binding_name="mailer")
    assert (mailer.host, mailer.port) == ("smtp.example.com", 2525)
    print("factory-configured:", mailer.host, mailer.port)

    overridden = conduit.meld(
        spellframe="mail", binding_name="raw-mailer",
        override={"host": "smtp.test.local", "port": 1025},
    )
    assert (overridden.host, overridden.port) == ("smtp.test.local", 1025)
    print("meld-site override:", overridden.host, overridden.port)

Runnable examples

Spell override construction — Intermediate 08
Live creation probe — Intermediate 18

All intermediate examples · Level contents · Full contents

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