Add interpolate argument to avoid resolving proxied values.

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

@@ -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 +426,7 @@ 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'