From d29e0e9f789bbd056abf9dcefcdbe8f16451be9a Mon Sep 17 00:00:00 2001
From: Mayur Dhavan <118877752+mayur-dhavan@users.noreply.github.com>
Date: Mon, 26 Oct 2026 00:07:09 +0530
Subject: [PATCH 1/2] Apply ${name:-default} when the variable is set but empty

---
 src/dotenv/variables.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/dotenv/variables.py b/src/dotenv/variables.py
index 667f2f26..5554b4dc 100644
--- a/src/dotenv/variables.py
+++ b/src/dotenv/variables.py
@@ -64,7 +64,12 @@ 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 ""
+        # POSIX ${name:-default} also applies when the variable is set but empty.
+        if result == "" and self.default is not None:
+            return self.default
+        return result
 
 
 def parse_variables(value: str) -> Iterator[Atom]:

From 31aecc08b552918365b0f282e953fa4f27896f88 Mon Sep 17 00:00:00 2001
From: Mayur Dhavan <118877752+mayur-dhavan@users.noreply.github.com>
Date: Mon, 26 Oct 2026 00:07:44 +0530
Subject: [PATCH 2/2] Test ${name:-default} with a set-but-empty variable

---
 tests/test_main.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/test_main.py b/tests/test_main.py
index 930ab171..38592997 100644
--- a/tests/test_main.py
+++ b/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"}),
+        # Defined but empty: ${name:-default} applies (POSIX semantics)
+        ({"b": ""}, "a=${b:-d}", True, {"a": "d"}),
+        ({}, "b=\na=${b:-d}", True, {"a": "d", "b": ""}),
+        ({"b": ""}, "a=${b}", True, {"a": ""}),
         # With quotes
         ({"b": "c"}, 'a="${b}"', True, {"a": "c"}),
         ({"b": "c"}, "a='${b}'", True, {"a": "c"}),