python-peps/pep-3105.txt

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2006-11-19 18:17:32 -05:00
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)
2006-11-20 11:31:44 -05:00
http://mail.python.org/pipermail/python-dev/2005-September/056154.html
2006-11-19 18:17:32 -05:00
.. [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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