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python-peps / pep-3111.txt

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PEP: 3111
Title: Simple input built-in in Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Andre Roberge <andre.roberge at gmail.com >
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 13-Sep-2006
Python-Version: 3.0
Post-History: 22-Dec-2006


Abstract
========

Input and output are core features of computer programs.  Currently,
Python provides a simple means of output through the print keyword
and two simple means of interactive input through the input()
and raw_input() built-in functions.

Python 3.0 will introduce various incompatible changes with previous
Python versions[1].  Among the proposed changes, print will become a built-in
function, print(), while input() and raw_input() would be removed completely
from the built-in namespace, requiring importing some module to provide
even the most basic input capability.

This PEP proposes that Python 3.0 retains some simple interactive user
input capability, equivalent to raw_input(), within the built-in namespace.

It was accepted by the BDFL in December 2006 [5].


Motivation
==========

With its easy readability and its support for many programming styles
(e.g. procedural, object-oriented, etc.) among others, Python is perhaps
the best computer language to use in introductory programming classes.
Simple programs often need to provide information to the user (output)
and to obtain information from the user (interactive input).
Any computer language intended to be used in an educational setting should
provide straightforward methods for both output and interactive input.

The current proposals for Python 3.0 [1] include a simple output pathway
via a built-in function named print(), but a more complicated method for
input [e.g. via sys.stdin.readline()], one that requires importing an external
module.  Current versions of Python (pre-3.0) include raw_input() as a
built-in function.  With the availability of such a function, programs that
require simple input/output can be written from day one, without requiring
discussions of importing modules, streams, etc.


Rationale
=========

Current built-in functions, like input() and raw_input(), are found to be
extremely useful in traditional teaching settings. (For more details,
see [2] and the discussion that followed.) 
While the BDFL has clearly stated [3] that input() was not to be kept in
Python 3000, he has also stated that he was not against revising the
decision of killing raw_input().

raw_input() provides a simple mean to ask a question and obtain a response
from a user.  The proposed plans for Python 3.0 would require the replacement
of the single statement::

  name = raw_input("What is your name?")

by the more complicated::

  import sys
  print("What is your name?")
  same = sys.stdin.readline()

However, from the point of view of many Python beginners and educators, the
use of sys.stdin.readline() presents the following problems:

1. Compared to the name "raw_input", the name "sys.stdin.readline()"
is clunky and inelegant.

2. The names "sys" and "stdin" have no meaning for most beginners,
who are mainly interested in *what* the function does, and not *where*
in the package structure it is located.  The lack of meaning also makes
it difficult to remember:
is it "sys.stdin.readline()", or " stdin.sys.readline()"? 
To a programming novice, there is not any obvious reason to prefer
one over the other. In contrast, functions simple and direct names like
print, input, and raw_input, and open are easier to remember.

3. The use of "." notation is unmotivated and confusing to many beginners. 
For example, it may lead some beginners to think "."  is a standard
character that could be used in any identifier.

4. There is an asymmetry with the print function: why is print not called
sys.stdout.print()?


Specification
=============

The existing ``raw_input()`` function will be renamed to ``input()``.

The Python 2 to 3 conversion tool will replace calls to ``input()`` with
``eval(input())`` and ``raw_input()`` with ``input()``.


Naming Discussion
=================

With ``input()`` effectively removed from the language,
the name ``raw_input()`` makes much less sense and alternatives should be
considered.  The various possibilities mentioned in various forums include::

  ask()
  ask_user()
  get_string()
  input()  # initially rejected by BDFL, later accepted
  prompt()
  read()
  user_input()
  get_response()

While it was initially rejected by the BDFL, it has been suggested that the
most direct solution would be to rename "raw_input" to "input" in Python 3000. 
The main objection is that Python 2.x already has a function named "input",
and, even though it is not going to be included in Python 3000,
having a built-in function with the same name but different semantics may
confuse programmers migrating from 2.x to 3000.  Certainly, this is no problem
for beginners, and the scope of the problem is unclear for more experienced
programmers, since raw_input(), while popular with many, is not in
universal use.  In this instance, the good it does for beginners could be
seen to outweigh the harm it does to experienced programmers -
although it could cause confusion for people reading older books or tutorials.

The rationale for accepting the renaming can be found here [4].


References
==========

.. [1] PEP 3100, Miscellaneous Python 3.0 Plans, Kuchling, Cannon
   http://www.python.org/dev/peps/pep-3100/

.. [2] The fate of raw_input() in Python 3000
   http://mail.python.org/pipermail/edu-sig/2006-September/006967.html

.. [3] Educational aspects of Python 3000
   http://mail.python.org/pipermail/python-3000/2006-September/003589.html

.. [4] Rationale for going with the straight renaming
   http://mail.python.org/pipermail/python-3000/2006-December/005249.html

.. [5] BDFL acceptance of the PEP
   http://mail.python.org/pipermail/python-3000/2006-December/005257.html

Copyright
=========

This document has been placed in the public domain.