[mayur-dhavan]

Fixes #715.

Problem

POSIX ${name:-default} applies the default when name is unset or empty. python-dotenv only applied it when the variable was unset, so an explicitly empty value leaked through:

dotenv.dotenv_values(stream=io.StringIO("b=\na=${b:-d}"))
# before: {'b': '', 'a': ''}
# after:  {'b': '', 'a': 'd'}

b unset already produced d, and ${b} without a default correctly stays empty - only the empty-with-default case was wrong.

Fix

In Variable.resolve (src/dotenv/variables.py), when a default was specified and the resolved value is the empty string, return the default. The no-default path (${name}) is unchanged, and a set non-empty value still wins.

Tests

Three new parametrized cases in test_dotenv_values_string_io: empty value from the environment, empty value from the file, and empty value without a default (unchanged behavior). Verified locally on Python 3.13: tests/test_main.py + tests/test_variables.py pass (165 passed). The pre-existing test_cli.py PermissionErrors are a sandbox restriction also present on unmodified master.

AI disclosure: this contribution was prepared with AI assistance (an automated coding agent acting on behalf of the author); the change and tests were validated locally against the project's test suite as described above.

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[mayur-dhavan]

Closing this as a duplicate: #716 (Oct 1) and #720 (Oct 4) both fix #715 with the same POSIX :- semantics (default applies when the variable is unset OR empty) and equivalent tests. I missed both when checking for existing work - a third PR on the same fix adds nothing. Sorry for the noise.