Understanding stdin stdout stderr [duplicate]

Question

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• Confused about stdin, stdout and stderr? 10 answers

I'm trying to understand stdin stdout and stderr.

I see them used in people's code all the time and I can't understand exactly what they are. I am assuming that they have something to do with input/output but have been searching for an explanation online and can't find one. Does anybody know of a good link with an explanation or if it is simple enough to explain it would be a great help to me.

Since I am learning Python 3, examples in that would be helpful.

Answer 1

sys.stdin
sys.stdout
sys.stderr

File objects used by the interpreter for standard input, output and errors:

• stdin is used for all interactive input (including calls to input());

• stdout is used for the output of print() and expression statements and for the prompts of input();

• The interpreter’s own prompts and its error messages go to stderr.

For your more understanding:

>>> import sys
>>> for i in (sys.stdin, sys.stdout, sys.stderr):
...     print i
... 
<open file '<stdin>', mode 'r' at 0x103451150>
<open file '<stdout>', mode 'w' at 0x1034511e0>
<open file '<stderr>', mode 'w' at 0x103451270>

mode r means reading and mode w means writing

Answer 2

Does this explain it well enough?

sys.stdin
sys.stdout
sys.stderr
File objects corresponding to the interpreter’s standard input, output and error streams.

stdin is used for all interpreter input except for scripts but including calls to input() and raw_input().

stdout is used for the output of print and expression statements and for the prompts of input() and raw_input().

The interpreter’s own prompts and (almost all of) its error messages go to stderr.
stdout and stderr needn’t be built-in file objects: any object is acceptable as long as it has a write() method that takes a string argument.
(Changing these objects doesn’t affect the standard I/O streams of processes executed by os.popen(), os.system() or the exec*() family of functions in the os module.)

To clairify

if I tell the python shell to do a print:

print 'what is your name?'

"what is your name" will go to stdout, whatever that is. If you haven't made any redirects that's by default the terminal you're using. You can interact with the standard streams in various different ways, for example:

sys.stdout.flush()

Tells the python shell to force any buffered information that I've printed to go to stdout right away.