1. Provide a fallback mechanism for determining ABI flags if config vars

   are unavailable, but issue a warning if this is used.
2. Explicitly handle the case where the unicode detection finds wide
   unicode but this is a 3.3+ build (necessary due to #1)
3. Fix tests broken due to #2.

pip/pep425tags.py

@@ -55,15 +55,42 @@ def get_impl_version_info():
         return sys.version_info[0], sys.version_info[1]
 
 
+def get_flag(var, fallback, expected=True, warn=True):
+    """Use a fallback method for determining SOABI flags if the needed config
+    var is unset or unavailable."""
+    val = get_config_var(var)
+    if val is None:
+        if warn:
+            warnings.warn("Config variable '{0}' is unset, Python ABI tag may "
+                          "be incorrect".format(var), RuntimeWarning, 2)
+        return fallback()
+    return val == expected
+
+
 def get_abi_tag():
     """Return the ABI tag based on SOABI (if available) or emulate SOABI
     (CPython 2, PyPy)."""
     soabi = get_config_var('SOABI')
     impl = get_abbr_impl()
-    if not soabi and impl in ('cp', 'pp'):
-        d = 'd' if get_config_var('Py_DEBUG') else ''
-        m = 'm' if get_config_var('WITH_PYMALLOC') else ''
-        u = 'u' if get_config_var('Py_UNICODE_SIZE') == 4 else ''
+    if not soabi and impl in ('cp', 'pp') and hasattr(sys, 'maxunicode'):
+        d = ''
+        m = ''
+        u = ''
+        if get_flag('Py_DEBUG',
+                    lambda: hasattr(sys, 'gettotalrefcount'),
+                    warn=(impl == 'cp')):
+            d = 'd'
+        if get_flag('WITH_PYMALLOC',
+                    lambda: impl == 'cp',
+                    warn=(impl == 'cp')):
+            m = 'm'
+        if get_flag('Py_UNICODE_SIZE',
+                    lambda: sys.maxunicode == 0x10ffff,
+                    expected=4,
+                    warn=(impl == 'cp' and
+                          sys.version_info < (3, 3))) \
+                and sys.version_info < (3, 3):
+            u = 'u'
         abi = '%s%s%s%s%s' % (impl, get_impl_ver(), d, m, u)
     elif soabi and soabi.startswith('cpython-'):
         abi = 'cp' + soabi.split('-')[1]

tests/unit/test_wheel.py

@@ -1,5 +1,6 @@
 """Tests for wheel binary packages and .dist-info."""
 import os
+import sys
 
 import pytest
 from mock import patch, Mock
@@ -317,26 +318,28 @@ def abi_tag_unicode(self, flags, config_vars):
         config_vars.update({'SOABI': None})
         base = pip.pep425tags.get_abbr_impl() + pip.pep425tags.get_impl_ver()
 
-        config_vars.update({'Py_UNICODE_SIZE': 2})
-        mock_gcf = self.mock_get_config_var(**config_vars)
-        with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
-            abi_tag = pip.pep425tags.get_abi_tag()
-            assert abi_tag == base + flags
-
-        config_vars.update({'Py_UNICODE_SIZE': 4})
-        mock_gcf = self.mock_get_config_var(**config_vars)
-        with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
-            abi_tag = pip.pep425tags.get_abi_tag()
-            assert abi_tag == base + flags + 'u'
-
-        # On Python >= 3.3, UCS-4 is essentially permanently enabled, and
-        # Py_UNICODE_SIZE is None. SOABI on these builds does not include the
-        # 'u' so manual SOABI detection should not do so either.
-        config_vars.update({'Py_UNICODE_SIZE': None})
-        mock_gcf = self.mock_get_config_var(**config_vars)
-        with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
-            abi_tag = pip.pep425tags.get_abi_tag()
-            assert abi_tag == base + flags
+        if sys.version_info < (3, 3):
+            config_vars.update({'Py_UNICODE_SIZE': 2})
+            mock_gcf = self.mock_get_config_var(**config_vars)
+            with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
+                abi_tag = pip.pep425tags.get_abi_tag()
+                assert abi_tag == base + flags
+
+            config_vars.update({'Py_UNICODE_SIZE': 4})
+            mock_gcf = self.mock_get_config_var(**config_vars)
+            with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
+                abi_tag = pip.pep425tags.get_abi_tag()
+                assert abi_tag == base + flags + 'u'
+
+        else:
+            # On Python >= 3.3, UCS-4 is essentially permanently enabled, and
+            # Py_UNICODE_SIZE is None. SOABI on these builds does not include
+            # the 'u' so manual SOABI detection should not do so either.
+            config_vars.update({'Py_UNICODE_SIZE': None})
+            mock_gcf = self.mock_get_config_var(**config_vars)
+            with patch('pip.pep425tags.sysconfig.get_config_var', mock_gcf):
+                abi_tag = pip.pep425tags.get_abi_tag()
+                assert abi_tag == base + flags
 
     def test_broken_sysconfig(self):
         """