Clarification through Code Examples

I've been asking myself the same question, so I came up with these two snippets, which clarify how sys.stdin and input() differ by emulating the latter with the former:

import sys

def my_input(prompt=''):
  print(prompt, end='') # prompt with no newline
  for line in sys.stdin:
    if '\n' in line: # We want to read only the first line and stop there
      break
  return line.rstrip('\n')

Here is a more condensed version:

import sys

def my_input(prompt=''):
  print(prompt, end='')
  return sys.stdin.readline().rstrip('\n')

Both these snippets differ from the input() function in that they do not detect the End Of File (see below).

Clarification through Documentation

This is how the official documentation describes the function input():

input([prompt])

If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, EOFError is raised.

And here's how sys.stdin is described:

sys.stdin

File object used by the interpreter for standard input.
stdin is used for all interactive input (including calls to input());
These streams (sys.stdin, sys.stdout and sys.stderr) are regular text files like those returned by the open() function. [...]

So whereas input() is a function, sys.stdin is an object (a File object). As such, it has a number of attributes, which you can explore in the interpreter, with:

> dir(sys.stdin)

['_CHUNK_SIZE',
 '__class__',
 '__del__',
 '__delattr__',
 '__dict__',
 '__dir__',

  ...

 'truncate',
 'writable',
 'write',
 'write_through',
 'writelines']

and which you can display individually, for instance:

> sys.stdin.mode
r

It also has methods, such as readline(), which "reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline." (1)

Full implementation

This last method allows us to fully emulate the input() function, including its EOF Exception error:

def my_input(prompt=''):
  print(prompt, end='')
  line = sys.stdin.readline()
  if line == '': # readline() returns an empty string only if EOF has been reached
    raise EOFError('EOF when reading a line')
  else:
    return line.rstrip('\n')

So you have used Python's "pre built in functions", presumably like this:

file_object = open('filename')
for something in file_object:
    some stuff here

This reads the file by invoking an iterator on the file object which happens to return the next line from the file.

You could instead use:

file_object = open('filename')
lines = file_object.readlines()

which reads the lines from the current file position into a list.

Now, sys.stdin is just another file object, which happens to be opened by Python before your program starts. What you do with that file object is up to you, but it is not really any different to any other file object, its just that you don't need an open.

for something in sys.stdin:
    some stuff here

will iterate through standard input until end-of-file is reached. And so will this:

lines = sys.stdin.readlines()

Your first question is really about different ways of using a file object.

Second, where is it reading from? It is reading from file descriptor 0 (zero). On Windows it is file handle 0 (zero). File descriptor/handle 0 is connected to the console or tty by default, so in effect it is reading from the keyboard. However it can be redirected, often by a shell (like bash or cmd.exe) using syntax like this:

myprog.py < input_file.txt 

That alters file descriptor zero to read a file instead of the keyboard. On UNIX or Linux this uses the underlying call dup2(). Read your shell documentation for more information about redirection (or maybe man dup2 if you are brave).

stdin and stdout are file-like objects provided by the OS. In general, when a program is run in an interactive session, stdin is keyboard input and stdout is the user's tty, but the shell can be used to redirect them from normal files or piped output from and input to other programs.

input() is used to prompt the user for typed input. In the case of something like a programming puzzle, it's normally assumed that stdin is redirected from a data file, and when the input format is given it's usually best to use sys.stdin.read() rather than prompting for input with input(). input() is intended for interactive user input, it can display a prompt (on sys.stdout) and use the GNU readline library (if present) to allow line editing, etc.

print() is, indeed, the most common way of writing to stdout. There's no need to do anything special to specify the output stream. print() writes to sys.stdout if no alternate file is given to it as a file= parameter.

Use the fileinput module:

import fileinput

for line in fileinput.input():
    pass

fileinput will loop through all the lines in the input specified as file names given in command-line arguments, or the standard input if no arguments are provided.

Note: line will contain a trailing newline; to remove it use line.rstrip().