Understanding stdin stdout stderr

Question

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.

Solution 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

Solution 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.

Comments

[Alex Mollberg]

ok so input on the command line is assigned to sys.stdout in the program itself?

[Mike]

@AlexMollberg - Not exactly. Input you give on the command line is given to the running programs' stdin stream. Things you print go to stdout... for example the line print "hello" doesn't tell the program where to print it to, it just goes to stdout which if you haven'tmade any redirects is the terminal (displayed on the monitor)

[Alex Mollberg]

ok this makes sense now. thank you

[JinSnow]

I'm confuse: any buffered information that I've printed to go to stdout right away but printed informations goes in stdout by defaullt, right? If so, what the point of sys.stdout.flush()?

[Mike]

printed information goes to stdout by default, but only when it's ready to go. Print operations are time expensive and thus aren't done instantly, but the information is buffered instead and then it prints when the system determines it should. This could be during a lull in the system or when there is "enough" in the buffer to go. The way to force it out is with a flush operation.