Python 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit (Intel)] on win32
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>>> import os
>>> path  = os.getcwd()
>>> filenames = os.listdir(path)
>>> for filename in filenames:
...     os.rename(os.path.join(path, filename), os.path.join(path, filename.replace(' ', '-')))
...
>>>