popen2 with large input

[cherico]

from popen2 import popen2

r, w = popen2 ( 'tr "[A-Z]" "[a-z]"' )
w.write ( t ) # t is a text file of around 30k bytes
w.close ()
text = r.readlines ()
print text
r.close ()

This simple script halted on 

w.write ( t )

Anyone knows what the problem is?