How to flush the printed statements in IPython

Question

I want to print some debug statements during a loop in my function and I use IPython to call the function. Let an example function be:

def test_print():
    import time
    for i in range(5):
        time.sleep(2)
        print i,  time.time()

The result is like follows:

0 1372337149.84
1 1372337151.84
2 1372337153.85
3 1372337155.85
4 1372337157.85

I expect each row to be printed, then wait for 2 seconds. But the behavior is as follows. I first observe:

0 1372337149.84
1 

Then, after 2 seconds I observe the time stamp of 1 and the id of the next row, which is 2. I observe the last time stamp finally. I couldn't figure out why it behaves like this instead of one row at a time. Any thoughts? Do I need a special flush function to print what is waiting to be printed?

Edomite

It works normally for me. – Bellicose

Using CPython it behaves as expected. For both Linux and Windows. – Consider

I'm using Canopy 1.0.1. It has Python 2.7.3 64 bit in it. – Edomite

I'm using CPython 2.6.5 on Linux and it behaves as expected. – Naima

Sounds like it incorrectly somehow alternates the order of the new line and flushes... something like print '\n', i, ; stdout.flush(); print time.time() – Consider

I think, I noticed similar behaviour with normal python when redirecting stdout to a file, some time ago. Does the flush maybe only occur, if there is no redirection, while ipython does a redirection from python's stdout to its own stdout? – Affliction

Works fine in IPython 0.13.2 too.(linux: py 2.7.4) – Erminiaerminie

@AshwiniChaudhary Probably depends on whether you run it from a CLI or a GUI. If it's run from a GUI, it'll probably redirect stdout to something other than a tty, which will make the stream fully buffered rather than line-buffered. – Outlay

Answer 1

I've never used IPython, but it should suffice to flush stdout after each print statement.

Something like this ought to work...

def test_print():
    import time
    import sys
    for i in range(5):
        time.sleep(2)
        print i,  time.time()
        sys.stdout.flush()

Outlay

But, print adds the new line automatically, so it should have been flushed anyway. – Consider

@Consider In CPython, perhaps. Maybe IPython behaves differently. – Outlay

@Outlay I think this behavior is built into libc. – Consider

@Consider It's a bit more complicated than that, so it'll depend on whether sys.stdout.isatty(). It's also worth noting that python has a -u option to use unbuffered stdout. – Outlay