2006-11-19 18:17:32 -05:00
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PEP: 3105
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Title: Make print a function
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Version: $Revision$
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Last-Modified: $Date$
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Author: Georg Brandl <g.brandl at gmx.net>
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Status: Draft
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Type: Standards
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Content-Type: text/x-rst
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Created: 19-Nov-2006
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Python-Version: 3.0
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Abstract
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========
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The title says it all -- this PEP proposes a new ``print()`` builtin
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that replaces the ``print`` statement and suggests a specific signature
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for the new function.
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Rationale
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=========
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The ``print`` statement has long appeared on lists of dubious language
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features that are to be removed in Python 3000, such as Guido's "Python
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Regrets" presentation [1]_. As such, the objective of this PEP is not
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new, though it might become much disputed among Python developers.
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The following arguments for a ``print()`` function are distilled from a
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python-3000 message by Guido himself [2]_:
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* ``print`` is the only application-level functionality that has a
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statement dedicated to it. Within Python's world, syntax is generally
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used as a last resort, when something *can't* be done without help from
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the compiler. Print doesn't qualify for such an exception.
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* At some point in application development one quite often feels the need
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to replace ``print`` output by something more sophisticated, like
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logging calls or calls into some other I/O library. With a ``print()``
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function, this is a straightforward string replacement, today it is
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a mess adding all those parentheses and possibly converting ``>>stream``
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style syntax.
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* Having special syntax for ``print`` puts up a much larger barrier for
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evolution, e.g. a hypothetical new ``printf()`` function is not too
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far fetched when it will coexist with a ``print()`` function.
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* There's no easy way to convert ``print`` statements into another call
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if one needs a different separator, not spaces, or none at all.
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Also, there's no easy way *at all* to conveniently print objects with
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some other separator than a space.
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* If ``print()`` is a function, it would be much easier to replace it within
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one module (just ``def print(*args):...``) or even throughout a program
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(e.g. by putting a different function in ``__builtin__.print``). As it is,
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one can do this by writing a class with a ``write()`` method and
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assigning that to ``sys.stdout`` -- that's not bad, but definitely a much
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larger conceptual leap, and it works at a different level than print.
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Specification
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=============
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The signature for ``print()``, taken from various mailings and recently
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posted on the python-3000 list [3]_ is::
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def print(*args, sep=' ', end='\n', file=None)
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A call like::
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print(a, b, c, file=sys.stderr)
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will be equivalent to today's::
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print >>sys.stderr, a, b, c
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while the optional ``sep`` and ``end`` arguments specify what is printed
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between and after the arguments, respectively.
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The ``softspace`` feature (a semi-secret attribute on files currently
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used to tell print whether to insert a space before the first item)
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will be removed. Therefore, there will not be a direct translation for
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today's::
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print "a",
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print
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which will not print a space between the ``"a"`` and the newline.
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Backwards Compatibility
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=======================
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The changes proposed in this PEP will render most of today's ``print``
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statements invalid, only those which incidentally feature parentheses
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around all of their arguments will continue to be valid Python syntax
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in version 3.0.
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Luckily, as it is a statement in Python 2, ``print`` can be detected
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and replaced reliably and non-ambiguously by an automated tool, so
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there should be no major porting problems (provided someone writes the
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mentioned tool).
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References
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==========
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.. [1] http://www.python.org/doc/essays/ppt/regrets/PythonRegrets.pdf
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.. [2] Replacement for print in Python 3.0 (Guido van Rossum)
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2006-11-20 11:31:44 -05:00
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http://mail.python.org/pipermail/python-dev/2005-September/056154.html
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2006-11-19 18:17:32 -05:00
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.. [3] print() parameters in py3k (Guido van Rossum)
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http://mail.python.org/pipermail/python-3000/2006-November/004485.html
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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