PEP: 3105
Title: Make print a function
Version: $Revision$
Last-Modified: $Date$
Author: Georg Brandl <g.brandl at gmx.net>
Status: Draft
Type: Standards
Content-Type: text/x-rst
Created: 19-Nov-2006
Python-Version: 3.0


Abstract
========

The title says it all -- this PEP proposes a new ``print()`` builtin
that replaces the ``print`` statement and suggests a specific signature
for the new function.


Rationale
=========

The ``print`` statement has long appeared on lists of dubious language
features that are to be removed in Python 3000, such as Guido's "Python
Regrets" presentation [1]_. As such, the objective of this PEP is not
new, though it might become much disputed among Python developers.

The following arguments for a ``print()`` function are distilled from a
python-3000 message by Guido himself [2]_:

* ``print`` is the only application-level functionality that has a
  statement dedicated to it. Within Python's world, syntax is generally
  used as a last resort, when something *can't* be done without help from
  the compiler. Print doesn't qualify for such an exception.

* At some point in application development one quite often feels the need
  to replace ``print`` output by something more sophisticated, like
  logging calls or calls into some other I/O library. With a ``print()``
  function, this is a straightforward string replacement, today it is
  a mess adding all those parentheses and possibly converting ``>>stream``
  style syntax.

* Having special syntax for ``print`` puts up a much larger barrier for
  evolution, e.g. a hypothetical new ``printf()`` function is not too
  far fetched when it will coexist with a ``print()`` function.

* There's no easy way to convert ``print`` statements into another call
  if one needs a different separator, not spaces, or none at all.
  Also, there's no easy way *at all* to conveniently print objects with
  some other separator than a space.

* If ``print()`` is a function, it would be much easier to replace it within
  one module (just ``def print(*args):...``) or even throughout a program
  (e.g. by putting a different function in ``__builtin__.print``). As it is,
  one can do this by writing a class with a ``write()`` method and
  assigning that to ``sys.stdout`` -- that's not bad, but definitely a much
  larger conceptual leap, and it works at a different level than print.


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

The signature for ``print()``, taken from various mailings and recently
posted on the python-3000 list [3]_ is::

    def print(*args, sep=' ', end='\n', file=None)

A call like::

    print(a, b, c, file=sys.stderr)

will be equivalent to today's::

    print >>sys.stderr, a, b, c

while the optional ``sep`` and ``end`` arguments specify what is printed
between and after the arguments, respectively.

The ``softspace`` feature (a semi-secret attribute on files currently
used to tell print whether to insert a space before the first item)
will be removed. Therefore, there will not be a direct translation for
today's::

    print "a",
    print

which will not print a space between the ``"a"`` and the newline.


Backwards Compatibility
=======================

The changes proposed in this PEP will render most of today's ``print``
statements invalid, only those which incidentally feature parentheses
around all of their arguments will continue to be valid Python syntax
in version 3.0.

Luckily, as it is a statement in Python 2, ``print`` can be detected
and replaced reliably and non-ambiguously by an automated tool, so
there should be no major porting problems (provided someone writes the
mentioned tool).


References
==========

.. [1] http://www.python.org/doc/essays/ppt/regrets/PythonRegrets.pdf

.. [2] Replacement for print in Python 3.0 (Guido van Rossum)
       http://mail.python.org/pipermail/python-dev/2005-September/056154.html

.. [3] print() parameters in py3k (Guido van Rossum)
       http://mail.python.org/pipermail/python-3000/2006-November/004485.html


Copyright
=========

This document has been placed in the public domain.


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