fix(imagefield): skip post_init without dimensions (swev-id: django__django-16801)

[casey-brooks]

Summary

• skip wiring ImageField.update_dimension_fields when no width/height fields are configured
• add regression tests exercising post_init receiver counts for different ImageField setups

Observed problem

Every ImageField currently connects its post_init handler even when the model does not declare dimension fields. That extra receiver forces unnecessary signal dispatch overhead during model initialization.

Reproduction

A shell snippet that shows the excess receiver prior to this fix:

PYTHONPATH=/workspace/django/tests:/workspace/django python - <<'PY'
from django.conf import settings

settings.configure(
    INSTALLED_APPS=[
        "django.contrib.auth",
        "django.contrib.contenttypes",
        "model_fields",
    ],
    DATABASES={"default": {"ENGINE": "django.db.backends.sqlite3"}},
    SECRET_KEY="tests",
    USE_TZ=False,
)

import django

django.setup()

from django.db.models import signals
from model_fields import models

print("Person:", signals.post_init._live_receivers(models.Person)[0])
print("PersonWithHeight:", signals.post_init._live_receivers(models.PersonWithHeight)[0])
print("PersonTwoImages:", signals.post_init._live_receivers(models.PersonTwoImages)[0])
PY

Before this change the "Person" line contained a single update_dimension_fields receiver even though the model has no dimension fields. After the fix the same command prints an empty list for Person while models with dimension fields still report their listeners.

Testing

• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py model_fields.test_imagefield --parallel=1
• flake8 django/db/models/fields/files.py tests/model_fields/test_imagefield.py

Fixes #521

[casey-brooks]

Test & Lint Summary

• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py model_fields.test_imagefield --parallel=1 (49 passed, 0 failed, 0 skipped)
• flake8 django/db/models/fields/files.py tests/model_fields/test_imagefield.py (no issues)

[noa-lucent]

test

[noa-lucent]

Thanks for tightening up the ImageField signal registration. One blocker: the tail of TwoImageFieldTests.test_dimensions() was dropped in this patch, so we lost the assertions that cover the cached-dimension behaviour when you reset was_opened and when you reassign new images. Please put those expectations back so the regression coverage stays intact.

[casey-brooks]

Restored the cached-dimension assertions and reassignment coverage in TwoImageFieldTests.test_dimensions. Thanks for catching that!

[noa-lucent]

Thanks for restoring the dimension caching assertions — coverage looks good again. No further issues from my side.