Fix empty expanded value for duplicate key

Example problematic file:

```bash
hello=hi
greetings=${hello}
goodbye=bye
greetings=${goodbye}
```

It would result in `greetings` being associated with the empty string
instead of `"bye"`.

The problem came from the fact that bindings were converted to a dict,
and so deduplicated by key, before being interpolated.  The dict would
be `{"hello": "hi", "greetings": "${goodbye}", "goodbye": "bye"}` in the
earlier example, which shows why interpolation wouldn't work: `goodbye`
would not be defined when `greetings` was interpolated.

This commit fixes that by passing all values in order, even if there are
duplicated keys.

CHANGELOG.md

@@ -7,7 +7,9 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
-*No unreleased change at this time.*
+### Fixed
+
+- Fix potentially empty expanded value for duplicate key (#260 by [@bbc]).
 
 ## [0.14.0] - 2020-07-03
 

src/dotenv/main.py

@@ -18,7 +18,7 @@
 
 if IS_TYPE_CHECKING:
     from typing import (
-        Dict, Iterator, Match, Optional, Pattern, Union, Text, IO, Tuple
+        Dict, Iterable, Iterator, Match, Optional, Pattern, Union, Text, IO, Tuple
     )
     if sys.version_info >= (3, 6):
         _PathLike = os.PathLike
@@ -83,9 +83,13 @@ def dict(self):
         if self._dict:
             return self._dict
 
-        values = OrderedDict(self.parse())
-        self._dict = resolve_nested_variables(values) if self.interpolate else values
-        return self._dict
+        if self.interpolate:
+            values = resolve_nested_variables(self.parse())
+        else:
+            values = OrderedDict(self.parse())
+
+        self._dict = values
+        return values
 
     def parse(self):
         # type: () -> Iterator[Tuple[Text, Optional[Text]]]
@@ -211,7 +215,7 @@ def unset_key(dotenv_path, key_to_unset, quote_mode="always"):
 
 
 def resolve_nested_variables(values):
-    # type: (Dict[Text, Optional[Text]]) -> Dict[Text, Optional[Text]]
+    # type: (Iterable[Tuple[Text, Optional[Text]]]) -> Dict[Text, Optional[Text]]
     def _replacement(name, default):
         # type: (Text, Optional[Text]) -> Text
         default = default if default is not None else ""
@@ -229,7 +233,7 @@ def _re_sub_callback(match):
 
     new_values = {}
 
-    for k, v in values.items():
+    for (k, v) in values:
         new_values[k] = __posix_variable.sub(_re_sub_callback, v) if v is not None else None
 
     return new_values

tests/test_main.py

@@ -337,6 +337,8 @@ def test_dotenv_values_file(dotenv_file):
 
         # Re-defined and used in file
         ({"b": "c"}, "b=d\na=${b}", True, {"a": "d", "b": "d"}),
+        ({}, "a=b\na=c\nd=${a}", True, {"a": "c", "d": "c"}),
+        ({}, "a=b\nc=${a}\nd=e\nc=${d}", True, {"a": "b", "c": "e", "d": "e"}),
     ],
 )
 def test_dotenv_values_stream(env, string, interpolate, expected):