[Tutor] optional parameter in list comprehension

[prasad rao]

hello!    I am a novice looking up to the tutor as my Guide.
    I got a problem   while using optional parameter.

#! user/bin/env python
import os

def myfiles(directory,extension=None):
    for x in [os.path.join(x,i) for x,y,z in os.walk(directory)\
    for i in z if i.endswith(extension)]:print x

if __name__=='__main__':
  import sys
  myfiles(sys.argv[1],sys.argv[2])

>>> myfiles.myfiles('C:\\python26')

Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    myfiles.myfiles('C:\\python26')
  File "C:\Python26\lib\site-packages\myfiles.py", line 6, in myfiles
    for i in z if i.endswith(extension)]:print x
TypeError: expected a character buffer object
>>> myfiles.myfiles('C:\\python26','doc')
C:\python26\Doc\examples.doc

How can I rectify this?

Thanks

Prasad
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