fix: apply ${VAR:-default} default to variables declared without a value

src/dotenv/variables.py

@@ -64,7 +64,9 @@ 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 ""
+        # A variable declared without a value (`FOO`) is unset, not empty, so
+        # a provided default applies to it just as it does to a missing name.
+        return result if result is not None else default
 
 
 def parse_variables(value: str) -> Iterator[Atom]:

tests/test_main.py

@@ -894,6 +894,9 @@ def test_dotenv_values_file(dotenv_path):
         # Undefined
         ({}, "a=${b}", True, {"a": ""}),
         ({}, "a=${b:-d}", True, {"a": "d"}),
+        # Declared without a value, but not empty
+        ({}, "b\na=${b:-d}", True, {"a": "d", "b": None}),
+        ({}, "b\na=${b}", True, {"a": "", "b": None}),
         # With quotes
         ({"b": "c"}, 'a="${b}"', True, {"a": "c"}),
         ({"b": "c"}, "a='${b}'", True, {"a": "c"}),

tests/test_variables.py

@@ -33,3 +33,16 @@ def test_parse_variables(value, expected):
     result = parse_variables(value)
 
     assert list(result) == expected
+
+
+@pytest.mark.parametrize(
+    "env,default,expected",
+    [
+        ({"a": "b"}, "c", "b"),
+        ({}, "c", "c"),
+        ({"a": ""}, "c", ""),
+        ({"a": None}, "c", "c"),
+    ],
+)
+def test_variable_resolve(env, default, expected):
+    assert Variable(name="a", default=default).resolve(env) == expected