An empty value ignores ${name:-default} #715

${name:-default} is the POSIX form that uses the default when name is unset or empty. An empty value is returned as itself.

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

a should be d. A missing b already becomes d. b=c still stays c. ${b} with no default stays empty.