154 lines
4.8 KiB
Plaintext
154 lines
4.8 KiB
Plaintext
PEP: 214
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Title: Extended Print Statement
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Version: $Revision$
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Author: bwarsaw@beopen.com (Barry A. Warsaw)
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Python-Version: 2.0
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Status: Draft
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Created: 24-Jul-2000
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Post-History: 16-Aug-2000
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Introduction
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This PEP describes a syntax to extend the standard `print'
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statement so that it can be used to print to any file-like object,
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instead of the default sys.stdout. This PEP tracks the status and
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ownership of this feature. It contains a description of the
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feature and outlines changes necessary to support the feature.
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This PEP summarizes discussions held in mailing list forums, and
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provides URLs for further information, where appropriate. The CVS
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revision history of this file contains the definitive historical
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record.
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Proposal
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This proposal introduces a syntax extension to the print
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statement, which allows the programmer to optionally specify the
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output file target. An example usage is as follows:
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print >> mylogfile, 'this message goes to my log file'
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Formally, the syntax of the extended print statement is
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print_stmt: ... | '>>' test [ (',' test)+ [','] ] )
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where the ellipsis indicates the original print_stmt syntax
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unchanged. In the extended form, the expression just after >>
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must yield an object with a write() method (i.e. a file-like
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object). Thus these two statements are equivalent:
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print 'hello world'
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print >> sys.stdout, 'hello world'
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As are these two statements:
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print
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print >> sys.stdout
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These two statements are syntax errors:
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print ,
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print >> sys.stdout,
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Justification
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`print' is a Python keyword and introduces the print statement as
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described in section 6.6 of the language reference manual[1].
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The print statement has a number of features:
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- it auto-converts the items to strings
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- it inserts spaces between items automatically
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- it appends a newline unless the statement ends in a comma
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The formatting that the print statement performs is limited; for
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more control over the output, a combination of sys.stdout.write(),
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and string interpolation can be used.
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The print statement by definition outputs to sys.stdout. More
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specifically, sys.stdout must be a file-like object with a write()
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method, but it can be rebound to redirect output to files other
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than specifically standard output. A typical idiom is
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sys.stdout = mylogfile
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try:
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print 'this message goes to my log file'
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finally:
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sys.stdout = sys.__stdout__
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The problem with this approach is that the binding is global, and
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so affects every statement inside the try: clause. For example,
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if we added a call to a function that actually did want to print
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to stdout, this output too would get redirected to the logfile.
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This approach is also very inconvenient for interleaving prints to
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various output streams, and complicates coding in the face of
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legitimate try/except or try/finally clauses.
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Reference Implementation
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A reference implementation, in the form of a patch against the
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Python 2.0 source tree, is available on SourceForge's patch
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manager[2]. This approach adds two new opcodes, PRINT_ITEM_TO and
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PRINT_NEWLINE_TO, which simply pop the file like object off the
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top of the stack and use it instead of sys.stdout as the output
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stream.
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Alternative Approaches
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An alternative to this syntax change has been proposed (originally
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by Moshe Zadka) which requires no syntax changes to Python. A
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writeln() function could be provided (possibly as a builtin), that
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would act much like extended print, with a few additional
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features.
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def writeln(*args, **kws):
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import sys
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file = sys.stdout
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sep = ' '
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end = '\n'
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if kws.has_key('file'):
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file = kws['file']
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del kws['file']
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if kws.has_key('nl'):
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if not kws['nl']:
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end = ' '
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del kws['nl']
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if kws.has_key('sep'):
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sep = kws['sep']
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del kws['sep']
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if kws:
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raise TypeError('unexpected keywords')
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file.write(sep.join(map(str, args)) + end)
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writeln() takes a three optional keyword arguments. In the
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context of this proposal, the relevant argument is `file' which
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can be set to a file-like object with a write() method. Thus
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print >> mylogfile, 'this goes to my log file'
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would be written as
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writeln('this goes to my log file', file=mylogfile)
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writeln() has the additional functionality that the keyword
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argument `nl' is a flag specifying whether to append a newline or
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not, and an argument `sep' which specifies the separator to output
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in between each item.
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References
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[1] http://www.python.org/doc/current/ref/print.html
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[2] http://sourceforge.net/patch/download.php?id=100970
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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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End:
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