python-peps/pep-0214.txt

PEP: 214
Title: Extended Print Statement
Version: $Revision$
Owner: bwarsaw@beopen.com (Barry A. Warsaw)
Python-Version: 2.0
Status: Draft



Introduction

    This PEP describes a syntax to extend the standard `print'
    statement so that it can be used to print to any file-like object,
    instead of the default sys.stdout.  This PEP tracks the status and
    ownership of this feature.  It contains a description of the
    feature and outlines changes necessary to support the feature.
    This PEP summarizes discussions held in mailing list forums, and
    provides URLs for further information, where appropriate.  The CVS
    revision history of this file contains the definitive historical
    record.



Justification

    `print' is a Python keyword and introduces the print statement as
    described in section 6.6 of the language reference manual[1].
    The print statement has a number of features:

    - it auto-converts the items to strings
    - it inserts spaces between items automatically
    - it appends a newline unless the statement ends in a comma

    The formatting that the print statement performs is limited; for
    more control over the output, a combination of sys.stdout.write(),
    and string interpolation can be used.

    The print statement by definition outputs to sys.stdout.  More
    specifically, sys.stdout must be a file-like object with a write()
    method, but it can be rebound to redirect output to files other
    than specifically standard output.  A typical idiom is

	oldstdout = sys.stdout
        sys.stdout = mylogfile
	try:
	    print 'this message goes to my log file'
	finally:
	    sys.stdout = oldstdout

    The problem with this approach is that the binding is global, and
    so affects every statement inside the try: clause.  For example,
    if we added a call to a function that actually did want to print
    to stdout, this output too would get redirected to the logfile.



Proposal

    This proposal introduces a syntax change to the print statement,
    which allows the programmer to optionally specify the output file
    target.  An example usage is as follows:

        print >> mylogfile, 'this message goes to my log file'

    Formally, the syntax of the extended print statement is

        print_stmt: "print" [">>" expr ","] [ expr ("," expr)* [","] ]

    Where the the expression just after >> must yield an object with a
    write() method (i.e. a file-like object).  Thus these two
    statements are equivalent:

	print 'hello world'
        print >> sys.stdout, 'hello world'



Reference Implementation

    A reference implementation, in the form of a patch against the
    Python 2.0 source tree, is available on SourceForge's patch
    manager[2].  The approach this patch takes is to introduce two new
    opcodes, one which temporarily rebinds sys.stdout to the specified
    file object, performs the print as normal, and then bind
    sys.stdout back to sys.__stdout__ (which is the real physical
    standard out and should not be changed).  In some ways this is
    equivalent to the try/finally idiom above, except that the
    rebinding of sys.stdout is in effect only for the duration of the
    print statement itself.

    An alternative approach is possible, where only one new opcode is
    added.  This opcode would be exactly like the existing PRINT_ITEM
    opcode except that it would find the target file object at the top
    of the stack, and use this file instead of digging it out of
    sys.stdout.



Alternative Approaches

    An alternative to this syntax change has been proposed (originally
    by Moshe Zadka) which requires no syntax changes to Python.  A
    writeln() function could be provided (possibly as a builtin), that
    would act much like extended print, with a few additional
    features.

	def writeln(*args, **kws):
	    import sys
	    file = sys.stdout
	    sep = ' '
	    end = '\n'
	    if kws.has_key('file'):
		file = kws['file']
		del kws['file']
	    if kws.has_key('nl'):
		if not kws['nl']:
		    end = ' '
		del kws['nl']
	    if kws.has_key('sep'):
		sep = kws['sep']
		del kws['sep']
	    if kws:
		raise TypeError('unexpected keywords')
	    file.write(sep.join(map(str, args)) + end)

    writeln() takes a three optional keyword arguments.  In the
    context of this proposal, the relevant argument is `file' which
    can be set to a file-like object with a write() method.  Thus

        print >> mylogfile, 'this goes to my log file'

    would be written as

        writeln('this goes to my log file', file=mylogfile)

    writeln() has the additional functionality that the keyword
    argument `nl' is a flag specifying whether to append a newline or
    not, and an argument `sep' which specifies the separator to output
    in between each item.



References

    [1] http://www.python.org/doc/current/ref/print.html
    [2] http://sourceforge.net/patch/download.php?id=100970



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