Inhibit python subprocess from generating .pyc files anymore.

This would case a failure when:
- a .py file is moved
- *.pyc is in svn:ignore, causing the .pyc to not be deleted.
- the new .py modifies its API.
- another .py imports the old .py and uses the new API.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/6854022

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@81942 0039d316-1c4b-4281-b951-d872f2087c98

presubmit_support.py

@@ -1035,14 +1035,20 @@ def DoPresubmitChecks(change,
     A PresubmitOutput object. Use output.should_continue() to figure out
     if there were errors or warnings and the caller should abort.
   """
+  old_environ = os.environ
+  try:
+    # Make sure python subprocesses won't generate .pyc files.
+    os.environ = os.environ.copy()
+    os.environ['PYTHONDONTWRITEBYTECODE'] = '1'
+
     output = PresubmitOutput(input_stream, output_stream)
     if committing:
       output.write("Running presubmit commit checks ...\n")
     else:
       output.write("Running presubmit upload checks ...\n")
     start_time = time.time()
-  presubmit_files = ListRelevantPresubmitFiles(change.AbsoluteLocalPaths(True),
-                                               change.RepositoryRoot())
+    presubmit_files = ListRelevantPresubmitFiles(
+        change.AbsoluteLocalPaths(True), change.RepositoryRoot())
     if not presubmit_files and verbose:
       output.write("Warning, no presubmit.py found.\n")
     results = []
@@ -1101,6 +1107,8 @@ def DoPresubmitChecks(change,
                   "& hate mail to maruel@chromium.org!\n")
       _ASKED_FOR_FEEDBACK = True
     return output
+  finally:
+    os.environ = old_environ
 
 
 def ScanSubDirs(mask, recursive):