need explanation

[Steven D'Aprano]

On Mon, 21 Jan 2013 10:06:41 -0600, kwakukwatiah wrote:

> please I need some explanation on sys.stdin  and sys.stdout, and piping
> out

"stdin" and "stdout" (and also stderr) are three special, standard, 
system files used by console programs that read and write text. That's 
nearly all of them.

"stdin" is short for "standard input". Likewise for "standard output" and 
"standard error".

When you give the Python command:

print "Hello World"

the string "Hello World" is written to stdout, which then displays it in 
the console.

"stderr" is similar, except that it is used for error messages. And stdin 
is used for input, rather than output.

So, in Python, I can do this:

py> import sys
py> sys.stdout.write("Hello world\n")
Hello world

But of course normally you would just use print.

Using sys.stdout, sys.stdin and sys.stderr in Python is usually 
considered moderately advanced. Beginners do not usually need to care 
about them.


These three special files do *not* live on the disk. There is no disk 
file called "stdout" unless you create one yourself, and if you do, it 
won't be special, it will just be an ordinary file with the name "stdout".

These standard files are used in Unix and Linux, and less so in Windows, 
for console applications. For example, under Linux I might write this 
command:

[steve at ando ~]$ touch foo
[steve at ando ~]$ ls foo
foo

The output of the `ls` command is written to stdout, which displays it on 
the console. But I can *redirect* that output to a real file on disk:

[steve at ando ~]$ ls foo > /tmp/a
[steve at ando ~]$ cat /tmp/a
foo


Errors don't go to stdout, they go to stderr:

[steve at ando ~]$ ls bar > /tmp/a
ls: bar: No such file or directory


Because there is no file called "bar", the `ls` command writes an error 
message to stderr. Even though I am redirecting stdout, I am not touching 
stderr, so it prints to the console.

Of course there is a way to redirect stderr as well:


[steve at ando ~]$ ls bar 2> /tmp/a
[steve at ando ~]$ cat /tmp/a
ls: bar: No such file or directory


Similarly, you can redirect stdin, or you can use a pipe | to turn the 
output of one command into the input of another command. This is mostly 
useful when using something like command.com in Windows, not so common in 
Python.


-- 
Steven