Shell re-direction

[Mike Gould]

Hi all,
I have a very strange problem with the output run from commands run via
os.system(). If the python script is run without re-direction the
output appears as expected, but if I re-direct the output from the
script the output gets re-ordered.

For example given the following script:

import os
print '1'
os.system('echo 2')
print '3'

$ python test.py
1
2
3
$ python test.py | cat
2
1
3

Can anyone explain what's going on here?

Thanks
Mike

[Jeff Epler]

buffering.

In the first case, there is either no buffering, or line buffering on
sys.stdout, so you see the lines in order.

In the second case, there is a buffer of a few hundred or thousand bytes
for stdout in the python process, and you see the two lines of python
output together (in this case, when the python process exits and flushes
all its buffers).

You can use the "flush" method on file objects to "clear out" these
buffers.

Jeff

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