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InvalidChildScopeError

Symptom

build_child_container(scope=...) raises it when the scope you pass is not deeper than the parent's:

modern_di.exceptions.container.InvalidChildScopeError: Scope of child container cannot be SESSION if parent scope is SESSION (child scope value must be strictly greater than parent scope value). Possible scopes are ['REQUEST', 'ACTION', 'STEP'].
See: https://modern-di.modern-python.org/troubleshooting/invalid-child-scope-error/

.parent_scope and .child_scope hold the two scopes, and .allowed_scopes holds the members listed after Possible scopes are. The Container(...) constructor never raises this error, because a root container can start at any scope.

Cause

A child's scope must have a strictly higher integer value than its parent's. A child at the same scope (Scope.SESSION under a SESSION parent) or a shallower one (Scope.APP under a SESSION parent) raises.

The check compares integer values, so a member of another IntEnum is accepted when its value is higher. The message and .allowed_scopes list only the deeper members of the parent's own enum, though. Under a STEP parent, or a parent at the last member of a custom enum, the message says Possible scopes are []. even though a higher-valued member of another enum would be accepted.

Fix

Pass a scope deeper than the parent's:

# Broken: SESSION is not deeper than SESSION
session_container.build_child_container(scope=Scope.SESSION)

# Works
request_container = session_container.build_child_container(scope=Scope.REQUEST)

You can also omit scope=. build_child_container() then takes the next deeper member of the parent's enum and never raises this error. When the enum has no deeper member it raises MaxScopeReachedError instead.

To go deeper than the parent's enum reaches, pass a member of your own IntEnum with a higher value:

import enum


class MyScope(enum.IntEnum):
    SUBSTEP = Scope.STEP + 1


step_container = Container(scope=Scope.STEP)
substep_container = step_container.build_child_container(scope=MyScope.SUBSTEP)

See also