Index: Doc/library/platform.rst
===================================================================
--- Doc/library/platform.rst	(revision 85046)
+++ Doc/library/platform.rst	(working copy)
@@ -17,6 +17,13 @@
 --------------
 
 
+.. data:: isposix
+
+   A boolean True when the platform is a posix system, False otherwise.
+
+   .. versionadded:: 3.3
+
+
 .. function:: architecture(executable=sys.executable, bits='', linkage='')
 
    Queries the given executable (defaults to the Python interpreter binary) for
Index: Lib/platform.py
===================================================================
--- Lib/platform.py	(revision 85046)
+++ Lib/platform.py	(working copy)
@@ -115,6 +115,8 @@
 
 ### Globals & Constants
 
+isposix = 'posix' in sys.builtin_module_names
+
 # Determine the platform's /dev/null device
 try:
     DEV_NULL = os.devnull
Index: Lib/test/test_platform.py
===================================================================
--- Lib/test/test_platform.py	(revision 85046)
+++ Lib/test/test_platform.py	(working copy)
@@ -39,6 +39,9 @@
     def test_system(self):
         res = platform.system()
 
+    def test_isposix(self):
+        res = platform.isposix
+
     def test_node(self):
         res = platform.node()