Add interpolate argument to avoid resolving proxied values.

The argument is already documented but not implemented yet.
Fixes joke2k#415

CHANGELOG.rst

@@ -17,6 +17,8 @@ Added
   `#468 <https://github.com/joke2k/django-environ/pull/468>`_.
 - Added capability to handle comments after #, after quoted values, like ``KEY= 'part1 # part2' # comment``
   `#475 <https://github.com/joke2k/django-environ/pull/475>`_.
+- Added support for ``interpolate`` parameter
+  `#419 <https://github.com/joke2k/django-environ/pull/419>`_.
 
 Changed
 +++++++

docs/tips.rst

@@ -226,7 +226,7 @@ Proxy value
 ===========
 
 Values that being with a ``$`` may be interpolated. Pass ``interpolate=True`` to
-``environ.Env()`` to enable this feature:
+``environ.Env()`` to enable this feature (``True`` by default):
 
 .. code-block:: python
 

environ/environ.py

@@ -197,11 +197,12 @@ class Env:
     VAR = re.compile(r'(?<!\\)\$\{?(?P<name>[A-Z_][0-9A-Z_]*)}?',
                      re.IGNORECASE)
 
-    def __init__(self, **scheme):
+    def __init__(self, interpolate=True, **scheme):
         self._local = threading.local()
         self.smart_cast = True
         self.escape_proxy = False
         self.prefix = ""
+        self.interpolate = interpolate
         self.scheme = scheme
 
     def __call__(self, var, cast=None, default=NOTSET, parse_default=False):
@@ -425,7 +426,8 @@ def _get_value(self, var_name, cast=None, default=NOTSET,
             value = default
 
         # Expand variables
-        if isinstance(value, (bytes, str)) and var_name not in NOT_EXPANDED:
+        if self.interpolate and isinstance(value, (bytes, str)) \
+            and var_name not in NOT_EXPANDED:
             def repl(match_):
                 return self.get_value(
                     match_.group('name'), cast=cast, default=default,

tests/test_env.py

@@ -134,6 +134,10 @@ def test_bool_true(self, value, variable):
     def test_proxied_value(self):
         assert self.env('PROXIED_VAR') == 'bar'
 
+    def test_not_interpolated_proxied_value(self):
+        env = Env(interpolate=False)
+        assert env('PROXIED_VAR') == '$STR_VAR'
+
     def test_escaped_dollar_sign(self):
         self.env.escape_proxy = True
         assert self.env('ESCAPED_VAR') == '$baz'