Strange redirection effect with raw_input

Question

According to the manual, raw_input writes to stdout. I have this little program (test_raw_input.py):

# Test if rawinput writes to stdout or stderr
raw_input('This is my prompt > ')

And no matter how I run this:

$ python test_raw_input.py > xxx

or

$ python test_raw_input.py 2> xxx

The prompt always ends up in xxx. Why is this happening?

• kennytm: FYI, it will be written to stdout if you redirect both stdout and stderr python test_raw_input.py > xxx 2> yyy.
• blueFast: Exactly, that is my point. > is stdout, 2> is stderr. The manual says raw_input writes to stdout, so only the first version should put the prompt in xxx. The second version, using 2>, should show the prompt in the terminal, since stdout is not redirected to xxx. But this is not the case: even when only redirecting stderr, the prompt is put into xxx. Why?

Answer 1

From your response to KennyTM I gather you understand

python test_raw_input.py > xxx

and it's only the second usage that you don't understand:

python test_raw_input.py 2> xxx

I think you are running into the behavior described here http://mail.python.org/pipermail/python-dev/2008-January/076446.html, which resulted in bug report http://bugs.python.org/issue1927, which has a comment saying it wasn't fixed yet last september.

However, there is a workaround: from https://groups.google.com/forum/?fromgroups=#!topic/chennaipy/R_VJYNdel-o, if you

import readline

before using raw_input, the behavior will be as you expect.

• blueFast: Thanks, that's interesting. This begs the question: if raw_input is (incorrectly) outputting to stderr, why does the stdout redirection also work? That's why I titled the question "strange effect", because it seems that raw_input redirects to either stdout or stderr, but I do not understand exactly how this works.
• Confusion: Good point. The shell redirections are visible to the Python interpreter, so they can influence the behavior of the code. I haven't looked at the C, but I'm guessing the code behaves differently depending on whether stdout or stderr has been redirected. It may useful to add this to the bug report: the people handling the issue may not be aware of this behavior.