win: Use the _winreg module for registry access

pylib/gyp/MSVSVersion.py

 # Copyright (c) 2013 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Handle version information related to Visual Stuio."""
 
 import errno
 import os
 import re
 import subprocess
@@ skipping 147 matching lines @@
   try:
     text = _RegistryQueryBase('Sysnative', key, value)
   except OSError, e:
     if e.errno == errno.ENOENT:
       text = _RegistryQueryBase('System32', key, value)
     else:
       raise
   return text
 
 
-def _RegistryGetValue(key, value):
-  """Use reg.exe to obtain the value of a registry key.
+def _RegistryGetValueUsingWinReg(key, value):
+  """Use the _winreg module to obtain the value of a registry key.
 
   Args:
     key: The registry key.
+    value: The particular registry value to read.
+  Return:
+    contents of the registry key's value, or None for failure.  Throws
+    ImportError if _winreg is unavailable.
+  """
+  import _winreg
+  try:
+    root,subkey = key.split('\\', 1)

[scottmg, 2014/11/13 18:05:35]

nit; space after ,

+    assert('HKLM' == root)  # Only need HKLM for now

[scottmg, 2014/11/13 18:05:35]

nits;
- no ()
- root == 'HKLM', rather than yoda-style
- '.' at end of comment.

+    with _winreg.OpenKey(_winreg.HKEY_LOCAL_MACHINE, subkey) as hkey:
+      return _winreg.QueryValueEx(hkey, value)[0]
+  except WindowsError:
+    return None
+
+
+def _RegistryGetValue(key, value):
+  """Use _winreg or reg.exe to obtain the value of a registry key.

[scottmg, 2014/11/13 18:05:35]

add a comment explaining why we need to try both ways

+
+  Args:
+    key: The registry key.
     value: The particular registry value to read.
   Return:
     contents of the registry key's value, or None on failure.
   """
+  try:
+    return _RegistryGetValueUsingWinReg(key, value)
+  except ImportError:
+    pass
+
+  # Fallback to reg.exe if we fail to import _winreg.
   text = _RegistryQuery(key, value)
   if not text:
     return None
   # Extract value.
   match = re.search(r'REG_\w+\s+([^\r]+)\r\n', text)
   if not match:
     return None
   return match.group(1)
 
 
-def _RegistryKeyExists(key):
-  """Use reg.exe to see if a key exists.
-
-  Args:
-    key: The registry key to check.
-  Return:
-    True if the key exists
-  """
-  if not _RegistryQuery(key):
-    return False
-  return True
-
-
 def _CreateVersion(name, path, sdk_based=False):
   """Sets up MSVS project generation.
 
   Setup is based off the GYP_MSVS_VERSION environment variable or whatever is
   autodetected if GYP_MSVS_VERSION is not explicitly specified. If a version is
   passed in that doesn't match a value in versions python will throw a error.
   """
   if path:
     path = os.path.normpath(path)
   versions = {
@@ skipping 192 matching lines @@
   versions = _DetectVisualStudioVersions(version_map[version], 'e' in version)
   if not versions:
     if not allow_fallback:
       raise ValueError('Could not locate Visual Studio installation.')
     if version == 'auto':
       # Default to 2005 if we couldn't find anything
       return _CreateVersion('2005', None)
     else:
       return _CreateVersion(version, None)
   return versions[0]