Check for Python 3 by spawning it and checking the output of '--version'

Closes #76.

Author: msabramo

lib/configure.js

@@ -12,6 +12,7 @@ var fs = require('graceful-fs')
   , which = require('which')
   , semver = require('semver')
   , mkdirp = require('mkdirp')
+  , exec = require('child_process').exec
   , win = process.platform == 'win32'
 
 exports.usage = 'Generates ' + (win ? 'MSVC project files' : 'a Makefile') + ' for the current module'
@@ -38,8 +39,7 @@ function configure (gyp, argv, callback) {
         return
       }
       log.verbose('`which` succeeded for `' + python + '`', execPath)
-      // TODO: ensure compatible Python version
-      getNodeDir()
+      checkPythonVersion()
     })
   }
 
@@ -58,20 +58,40 @@ function configure (gyp, argv, callback) {
         if (err.code == 'ENOENT') {
           failNoPython()
         } else {
-          callbackk(err)
+          callback(err)
         }
         return
       }
       python = pythonPath
-      getNodeDir()
+      checkPythonVersion()
     })
   }
 
+  function checkPythonVersion () {
+    exec(python + ' --version', function (err, stdout, stderr) {
+      if (err) {
+        return callback(err)
+      }
+      var version = stderr.trim().replace(/^python /i, '')
+      if (semver.lt(version, '3.0.0')) {
+        getNodeDir()
+      } else {
+        failPython3()
+      }
+    });
+  }
+
   function failNoPython () {
     callback(new Error('Can\'t find Python executable "' + python
           + '", you can set the PYTHON env variable.'))
   }
 
+  function failPython3 () {
+    callback(new Error('Python executable "' + python
+          + '" is Python 3, which is not supported.\n'
+          + 'You can set the PYTHON env variable to point to a Python 2 interpreter.'))
+  }
+
   function getNodeDir () {
 
     // 'python' should be set by now