# Flat overrides and live creation probes

Prerequisite: [addressing](../beginner/addresses.md) and
[lifetimes](../beginner/lifetimes.md). 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](../advanced/overrides.md) 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.

```{literalinclude} ../downloads/02_intermediate/08_spell_override_construction.py
:language: python
:pyobject: main
:linenos:
```


## Runnable examples

- [Spell override construction](../examples/intermediate/08-spell-override-construction.md) — Intermediate 08
- [Live creation probe](../examples/intermediate/18-live-creation-probe.md) — Intermediate 18

[All intermediate examples](../examples/intermediate/index.md) · [Level contents](index.md) · [Full contents](../contents.md)

[Canonical page source](https://github.com/Synaptic724/melder/blob/1211e838099a342c9dde48f064b1e772b6060e13/docs/intermediate/overrides.md)