From 2bc4c5215bf78d2122ad83197ef18b64aa7a6119 Mon Sep 17 00:00:00 2001
From: grabner <pjg.github@ubergrabner.net>
Date: Sat, 20 May 2017 16:25:49 -0400
Subject: added support for recursive evaluation of "ENV" expansions

---
 iniherit/interpolation.py | 21 ++++++++++++++++++++-
 iniherit/test.py          | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/iniherit/interpolation.py b/iniherit/interpolation.py
index 96b213d..d368f44 100644
--- a/iniherit/interpolation.py
+++ b/iniherit/interpolation.py
@@ -60,7 +60,26 @@ def interpolate(parser, base_interpolate, section, option, rawval, vars):
     raise CP.InterpolationDepthError(option, section, rawval)
   if '%(SUPER)s' in value:
     raise InterpolationMissingSuperError(option, section, rawval, 'SUPER')
-  return base_interpolate(parser, section, option, value, vars)
+  if base_interpolate is None:
+    return value
+  vars = dict(vars)
+  while True:
+    # ok, this is... uh... "tricky"... basically, we don't want to
+    # pre-emptively expand SUPER & ENV expressions because then we may
+    # trip invalid expressions that aren't actually used. thus, we
+    # only expand keys that are actually requested, and we detect by
+    # catching InterpolationMissingOptionError's...
+    try:
+      return base_interpolate(parser, section, option, value, vars)
+    except CP.InterpolationMissingOptionError as err:
+      for key, val in list(vars.items()):
+        if err.reference.lower() in val.lower():
+          newval = interpolate(parser, None, section, key, val, vars)
+          if newval != val:
+            vars[key] = newval
+            break
+      else:
+        raise
 
 #------------------------------------------------------------------------------
 def interpolate_super(parser, src, dst, section, option, value):
diff --git a/iniherit/test.py b/iniherit/test.py
index 6b2c3ad..d4b93b9 100644
--- a/iniherit/test.py
+++ b/iniherit/test.py
@@ -453,6 +453,41 @@ class TestIniherit(unittest.TestCase):
       )
     self.assertMultiLineEqual(str(cm.exception), err)
 
+  #----------------------------------------------------------------------------
+  def test_cascading_env_interpolate(self):
+    # test that if a key contains an interpolation of another key
+    # can in turn interpolate an "%(ENV:...)s" style expansion.
+    files = {k: textwrap.dedent(v) for k, v in {
+      'config.ini' : '''
+        [DEFAULT]
+        kw1 = %(kw2)s
+        kw2 = %(ENV:UNDEFINED:-defval)s
+      ''',
+    }.items()}
+    parser = ConfigParser(loader=ByteLoader(files))
+    parser.read('config.ini')
+    self.assertEqual(parser.get('DEFAULT', 'kw2'), 'defval')
+    self.assertEqual(parser.get('DEFAULT', 'kw1'), 'defval')
+
+  #----------------------------------------------------------------------------
+  def test_subclass_override(self):
+    # test that subclasses that override `ConfigParser._interpolate`,
+    # but that still directly call it, works...
+    class SomeOtherConfigParser(ConfigParser):
+      def _interpolate(self, section, option, rawval, vars):
+        return ConfigParser._interpolate(self, section, option, rawval, vars)
+    files = {k: textwrap.dedent(v) for k, v in {
+      'config.ini' : '''
+        [DEFAULT]
+        kw1 = %(kw2)s
+        kw2 = %(ENV:SOMEVAL:-defval)s
+      ''',
+    }.items()}
+    parser = SomeOtherConfigParser(loader=ByteLoader(files))
+    parser.read('config.ini')
+    self.assertEqual(parser.get('DEFAULT', 'kw2'), 'defval')
+    self.assertEqual(parser.get('DEFAULT', 'kw1'), 'defval')
+
 
 #------------------------------------------------------------------------------
 # end of $Id$
-- 
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