Simplify and clarify resolver platform expansion.

pex/resolver.py

@@ -165,25 +165,24 @@ class Resolver(object):
   class Error(Exception): pass
 
   @staticmethod
-  def _expand_and_maybe_adjust_platform(interpreter, platform=None):
-    # Adjusts `platform` if it is 'current' and does not match the given `interpreter` platform.
-    cur_plat = Platform.current()
+  def _maybe_expand_platform(interpreter, platform=None):
+    # Expands `platform` if it is 'current' and abbreviated.
+    #
+    # IE: If we're on linux and handed a platform of `None`, 'current', or 'linux_x86_64', we expand
+    # the platform to an extended platform matching the given interpreter's abi info, eg:
+    # 'linux_x86_64-cp-27-cp27mu'.
 
     given_platform = Platform.create(platform or 'current')
-    if cur_plat.platform != given_platform.platform:
-      # IE: Say we're on OSX and platform was 'linux-x86_64' or 'linux_x86_64-cp-27-cp27mu'.
+    if given_platform.is_extended:
+      # Always respect an extended platform.
       return given_platform
 
-    if (interpreter.identity.abbr_impl,
-        interpreter.identity.impl_ver,
-        interpreter.identity.abi_tag) == (cur_plat.impl, cur_plat.version, cur_plat.abi):
-      # IE: Say we're on Linux and platform was 'current' or 'linux-x86_64' or
-      # 'linux_x86_64-cp-27-cp27mu'and the current extended platform info matches the given
-      # interpreter exactly.
-      return cur_plat
+    cur_plat = Platform.current()
+    if cur_plat.platform != given_platform.platform:
+      # IE: Say we're on OSX and platform was 'linux-x86_64'.
+      return given_platform
 
-    # Otherwise we need to adjust the platform to match a local interpreter different from the
-    # currently executing interpreter.
+    # Otherwise we need to expand the platform to match the given interpreter.
     adjusted_platform = Platform(platform=cur_plat.platform,
                                  impl=interpreter.identity.abbr_impl,
                                  version=interpreter.identity.impl_ver,
@@ -210,7 +209,7 @@ def _expand_and_maybe_adjust_platform(interpreter, platform=None):
   def __init__(self, allow_prereleases=None, interpreter=None, platform=None,
                pkg_blacklist=None, use_manylinux=None):
     self._interpreter = interpreter or PythonInterpreter.get()
-    self._platform = self._expand_and_maybe_adjust_platform(self._interpreter, platform)
+    self._platform = self._maybe_expand_platform(self._interpreter, platform)
     self._allow_prereleases = allow_prereleases
     self._blacklist = pkg_blacklist.copy() if pkg_blacklist else {}
     self._supported_tags = self._platform.supported_tags(