[containerapp] Fix compose create not expanding ${VAR:-default} for unset environment variables

[Copilot]

az containerapp compose create was passing literal strings like ${VAR_C:-bar} as environment variable values instead of expanding them to their defaults (bar) when the variable was unset in the local shell.

Changes

azext_containerapp/_compose_utils.py

• Added _expand_env_var_substitution(value): explicit Docker Compose variable substitution expansion covering all three syntax forms:
  • ${VAR:-default} — use default when unset or empty
  • ${VAR-default} — use default only when unset
  • ${VAR} — substitute value (empty string when unset)
• Added resolve_environment_from_service(service): overrides the core azure-cli function, applying explicit expansion as a second pass after pycomposefile processing

azext_containerapp/custom.py

• Redirected resolve_environment_from_service import from azure.cli.command_modules.containerapp._compose_utils to the local module, so the extension's expansion logic is always applied

Tests

• test_containerapp_compose_unit.py (new): 17 unit tests for _expand_env_var_substitution and resolve_environment_from_service covering set/unset/empty var combinations, the primary bug scenario, and non-string passthrough
• test_containerapp_compose_environment.py: added scenario test test_containerapp_compose_create_with_unset_env_defaults for E2E validation

Example

# docker-compose.yml
services:
  app:
    environment:
      VAR_C: ${VAR_C:-bar}

# VAR_C not set in shell
az containerapp compose create --compose-file-path docker-compose.yml ...
# Before: "value": "${VAR_C:-bar}"
# After:  "value": "bar"

Related command

az containerapp compose create

General Guidelines

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• My extension version conforms to the Extension version schema

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

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[azure-client-tools-bot-prd]

️✔️Azure CLI Extensions Breaking Change Test

️✔️Non Breaking Changes

[azure-client-tools-bot-prd]

Hi Copilot,
Please write the description of changes which can be perceived by customers into HISTORY.rst.
If you want to release a new extension version, please update the version in setup.py as well.

[Yong Zhang (yonzhan)]

containerapp