Add public NULL_LABEL constant to Azure App Configuration Provider

[Copilot]

Users had no named constant for the null label ("\0") when configuring SettingSelector, requiring them to use an opaque string literal.

Changes

• _constants.py: Added NULL_LABEL = NULL_CHAR — a public alias for the existing internal NULL_CHAR constant representing "No Label"
• __init__.py: Exported NULL_LABEL in the public API (__all__)
• README.md: Updated example to use NULL_LABEL instead of "\0"
• CHANGELOG.md: Added feature entry
• tests/test_setting_selector.py: Added test_null_label_constant covering equality with NULL_CHAR and use in SettingSelector

Usage

from azure.appconfiguration.provider import load, SettingSelector, NULL_LABEL
from azure.identity import DefaultAzureCredential

selects = [
    SettingSelector(key_filter="*", label_filter=NULL_LABEL),
    SettingSelector(key_filter="*", label_filter="dev"),
]
config = load(endpoint=endpoint, credential=DefaultAzureCredential(), selects=selects)

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• pypi.org
  • Triggering command: /home/REDACTED/work/azure-sdk-for-python/azure-sdk-for-python/.venv/bin/python /home/REDACTED/work/azure-sdk-for-python/azure-sdk-for-python/.venv/bin/python /home/REDACTED/work/azure-sdk-for-python/azure-sdk-for-python/.venv/lib/python3.9/site-packages/pip/__pip-REDACTED__.py install --ignore-installed --no-user --prefix /tmp/pip-build-env-1scms1s3/overlay --no-warn-script-location --no-binary :none: --only-binary :none: -i REDACTED -- setuptools>=40.8.0 (dns block)

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Azure App Configuration Provider - Null Label</issue_title>
<issue_description>A constant for \0 should be added for the setting selector.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Azure App Configuration Provider - Null Label #43402

---

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[Copilot]

Pull request overview

This PR improves the Azure App Configuration Provider public API and docs by introducing a named constant for the “null label” ("\0") so users don’t need to rely on an opaque string literal when configuring SettingSelector.

Changes:

• Added public NULL_LABEL constant as an alias for the existing null-character label sentinel.
• Exported NULL_LABEL from the package public surface (__init__.py) and updated README example usage.
• Added changelog entry and a unit test validating NULL_LABEL behavior with SettingSelector.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/appconfiguration/azure-appconfiguration-provider/azure/appconfiguration/provider/_constants.py	Adds NULL_LABEL alias intended for public use.
sdk/appconfiguration/azure-appconfiguration-provider/azure/appconfiguration/provider/init.py	Exposes NULL_LABEL via public imports and __all__.
sdk/appconfiguration/azure-appconfiguration-provider/README.md	Replaces "\0" literal in examples with NULL_LABEL.
sdk/appconfiguration/azure-appconfiguration-provider/CHANGELOG.md	Documents the new public constant in the unreleased section.
sdk/appconfiguration/azure-appconfiguration-provider/tests/test_setting_selector.py	Adds coverage asserting NULL_LABEL is exported and works with SettingSelector.

[Copilot]

The triple-quoted string after NULL_LABEL = NULL_CHAR is a standalone module string literal (it does not attach as documentation to NULL_LABEL). This likely won’t show up in generated docs and is effectively dead code. Convert it to a normal comment (or a #: doc comment if you want Sphinx to pick it up), or move the text into an appropriate module/doc location.

Suggested change

	NULL_LABEL = NULL_CHAR
	"""Represents the null label (No Label) in Azure App Configuration. Use this constant
	with :class:`~azure.appconfiguration.provider.SettingSelector` to select configuration
	settings that have no label assigned."""
	#: Represents the null label (No Label) in Azure App Configuration.
	#: Use this constant with :class:`~azure.appconfiguration.provider.SettingSelector`
	#: to select configuration settings that have no label assigned.
	NULL_LABEL = NULL_CHAR

[Copilot]

The example uses a set literal for selects, but the provider API is typed/documented as List[SettingSelector] and the paragraph below relies on selector order for precedence. Sets are unordered, so this example can produce non-deterministic precedence. Use a list (e.g., [...]) to preserve order, and keep the explanation consistent with that.

Suggested change

	selects = {SettingSelector(key_filter="*", label_filter=NULL_LABEL), SettingSelector(key_filter="*", label_filter="dev")}
	selects = [
	SettingSelector(key_filter="*", label_filter=NULL_LABEL),
	SettingSelector(key_filter="*", label_filter="dev"),
	]