Fix ${name:-default} ignoring default for empty values

src/dotenv/variables.py

@@ -64,7 +64,13 @@ def __hash__(self) -> int:
     def resolve(self, env: Mapping[str, Optional[str]]) -> str:
         default = self.default if self.default is not None else ""
         result = env.get(self.name, default)
-        return result if result is not None else ""
+        if result is None:
+            return ""
+        if result == "" and self.default is not None:
+            # POSIX: ${name:-default} falls back to the default when name
+            # is unset or empty (issue #715)
+            return default
+        return result
 
 
 def parse_variables(value: str) -> Iterator[Atom]:

tests/test_main.py

@@ -894,6 +894,10 @@ def test_dotenv_values_file(dotenv_path):
         # Undefined
         ({}, "a=${b}", True, {"a": ""}),
         ({}, "a=${b:-d}", True, {"a": "d"}),
+        # Empty value with default: ${name:-default} treats empty as unset (#715)
+        ({}, "b=\na=${b:-d}", True, {"a": "d", "b": ""}),
+        ({"b": ""}, "a=${b:-d}", True, {"a": "d"}),
+        ({}, "b=\na=${b}", True, {"a": "", "b": ""}),
         # With quotes
         ({"b": "c"}, 'a="${b}"', True, {"a": "c"}),
         ({"b": "c"}, "a='${b}'", True, {"a": "c"}),