Programming Python, 3rd Edition

Standard Streams

The sys module is also the place where the standard input, output, and error streams of your Python programs live:

>>>for f in (sys.stdin, sys.stdout, sys.stderr): print f
...
<open file '<stdin>', mode 'r' at 762210>
<open file '<stdout>', mode 'w' at 762270>
<open file '<stderr>', mode 'w' at 7622d0>

The standard streams are simply preopened Python file objects that are automatically connected to your program’s standard streams when Python starts up. By default, all of them are tied to the console window where Python (or a Python program) was started. Because the print statement and raw_input functions are really nothing more than user-friendly interfaces to the standard output and input streams, they are similar to using stdout and stdin in sys directly:

>>>print 'hello stdout world'
hello stdout world

>>> sys.stdout.write('hello stdout world' + '\n')
hello stdout world

>>> raw_input('hello stdin world>')
hello stdin world>spam
'spam'

>>> print 'hello stdin world>',; sys.stdin.readline( )[:-1]
hello stdin world>eggs

'eggs'

Standard Streams on Windows

Windows users: if you click a .py Python program’s filename in a Windows file explorer to start it (or launch it with os.system), a DOS console box automatically pops up to serve as the program’s standard stream. If your program makes windows of its own, you can avoid this console pop-up window by naming your program’s source-code file with a .pyw extension, not with a .py extension. The .pyw extension simply means a ...