This context manager restores the value of the current working directory (cwd) after the enclosed code block completes or raises an exception.  If a directory name is supplied to the context manager then the cwd is changed prior to running the code block. 

from __future__ import with_statement  # only for python 2.5
import contextlib
import os

@contextlib.contextmanager
def chdir(dirname=None):
  curdir = os.getcwd()
  try:
    if dirname is not None:
      os.chdir(dirname)
    yield
  finally:
    os.chdir(curdir)