python-peps/peps/pep-3105.rst

PEP: 3105
Title: Make print a function
Version: $Revision$
Last-Modified: $Date$
Author: Georg Brandl <georg@python.org>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 19-Nov-2006
Python-Version: 3.0
Post-History:


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, and of those, only the ones printing a single
parenthesized value will continue to do the same thing.  For example,
in 2.x::

    >>> print ("Hello")
    Hello
    >>> print ("Hello", "world")
    ('Hello', 'world')

whereas in 3.0::

    >>> print ("Hello")
    Hello
    >>> print ("Hello", "world")
    Hello world

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).


Implementation
==============

The proposed changes were implemented in the Python 3000 branch in the
Subversion revisions 53685 to 53704. Most of the legacy code in the
library has been converted too, but it is an ongoing effort to catch
every print statement that may be left in the distribution.


References
==========

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

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

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


Copyright
=========

This document has been placed in the public domain.


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