Difference between input and sys.stdin

The streams standards are stdin , stdout and stderr , and represent files connected to the input, output and output of errors that a process has by default. Our application should not assume anything about them, or that the input is done by keyboard, or that the output goes to a terminal, ... nothing, only that they behave like text files that are read and / or write line to line.

The input function has two tasks:

• display a message (in stdout )
• read a line of text (of stdin )

This behavior can be simulated in the following way:

import sys

sys.stdout.write("Dame un número:")
sys.stdout.flush()
n = sys.stdin.readline()

which is equivalent to

n = input("Dame un número:")

Obviously, it is much more convenient to use input than standard streams (The same goes for using print instead of stdout ). So, what's the use of streams?

As I said before, streams can be seen as files and can be exchanged with them where necessary. It is a simple system to create test batteries of code fragments. It is also used to process the output or input of child processes (something that goes out of topic).

A non-trivial example:

import sys
from io import StringIO

class WrapInput:
    def __init__(self, entrada):
        self.entrada = entrada

    def wrapper(self, f, *args, **kw):
        (sys.stdin, old_stdin) = (StringIO(self.entrada), sys.stdin)
        res = f(*args,**kw)
        sys.stdin = old_stdin
        return res

n = WrapInput("2").wrapper(input, "Dame un número: ")   
print("El número es {}".format(n))

Do not wait for you to enter the number by keyboard, read the "2" stored in StringIO .

Using input or stdin is equivalent. It's more convenient to use input in almost any condition, but with stdin you can do much more magic.