Add interpolate argument to avoid resolving proxied values.

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

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

@@ -188,10 +188,11 @@ class Env:
     }
     CLOUDSQL = 'cloudsql'
 
-    def __init__(self, **scheme):
+    def __init__(self, interpolate=True, **scheme):
         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):
@@ -395,7 +396,11 @@ def get_value(self, var, cast=None, default=NOTSET, parse_default=False):
         # Resolve any proxied values
         prefix = b'$' if isinstance(value, bytes) else '$'
         escape = rb'\$' if isinstance(value, bytes) else r'\$'
-        if hasattr(value, 'startswith') and value.startswith(prefix):
+        if (
+            self.interpolate
+            and hasattr(value, 'startswith')
+            and value.startswith(prefix)
+        ):
             value = value.lstrip(prefix)
             value = self.get_value(value, cast=cast, default=default)
 

tests/test_env.py

@@ -132,6 +132,10 @@ 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'