PEP 214 completed after channeling and encouragement from Tim Peters.
The referenced SF patch is current with this description. The proposal section is moved to before the justification.
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pep-0214.txt
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pep-0214.txt
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@ -2,10 +2,10 @@ 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: 2.1
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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:
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Post-History: 16-Aug-2000
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Introduction
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@ -21,6 +21,37 @@ Introduction
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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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@ -40,12 +71,11 @@ Justification
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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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oldstdout = sys.stdout
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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 = oldstdout
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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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@ -53,55 +83,10 @@ Justification
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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.
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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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Proposal
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This proposal introduces a syntax change to the print statement,
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which allows the programmer to optionally specify the output file
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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: "print" [">>" expr ","] [ expr ("," expr)* [","] ]
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Where the the expression just after >> must yield an object with a
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write() method (i.e. a file-like object). Thus these two
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statements are equivalent:
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print 'hello world'
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print >> sys.stdout, 'hello world'
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Open Issues
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What should the following do?
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print >> file
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print >> file,
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In the current implementation (see below), the first is a
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SyntaxError and the second prints nothing to file. This is likely
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counterintuitive; the first should print just a newline, making
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these equivalent:
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print >> sys.stdout
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print
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The second should print just a space and no newline to file. It
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doesn't have a non-extended print equivalent, since this is
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illegal:
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print ,
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The closes equivalent is:
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print '',
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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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