What's wrong on using Popen's communicate method?

[jfong at ms4.hinet.net]

I have the test0.py below. I expect to see 'abcd' showing in the notepad window:
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import subprocess as sp
p0 = sp.Popen('notepad.exe', stdin=sp.PIPE)
p0.communicate(input=b'abcd')
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But nothing happens. The notepad is completely empty. What have I missed?

--Jach

PS. I am using python 3.4 on Windows Vista