Difference between input and sys.stdin

The standard streams are stdin, stdoutand stderr, and they represent files connected to the default input, output, and error output of a process. Our application must not assume anything about them, neither that the input is done by keyboard, nor that the output goes to a terminal,...nothing, just that they behave like text files that are read and/or written line by line. line.

The function inputhas two tasks:

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

This behavior can be simulated as follows:

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 comfortable to use inputthan the standard streams (The same goes for using printinstead of stdout). So what is the point of using streams?

As I said before, the streams can be seen as files and can be exchanged with them where necessary. It is a simple system for creating test suites from code snippets. It is also used to process the output or input of child processes (something that is off 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))

It doesn't wait for you to enter the number on the keyboard, it reads the one "2"stored in StringIO.

Use inputor stdinis equivalent. It's more comfortable to use inputin almost any condition, but stdinyou can do a lot more magic with it.