Standard Streams

Module sys 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 pre-opened Python file objects that are automatically connected to your program’s standard streams when Python starts up. By default, they are all 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

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


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