python-peps / pep-0214.txt

PEP: 214
Title: Extended Print Statement
Version: $Revision$
Last-Modified: $Date$
Author: barry@python.org (Barry Warsaw)
Status: Final
Type: Standards Track
Created: 24-Jul-2000
Python-Version: 2.0
Post-History: 16-Aug-2000


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.


Proposal

    This proposal introduces a syntax extension 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: ... | '>>' test [ (',' test)+ [','] ] )

    where the ellipsis indicates the original print_stmt syntax
    unchanged.  In the extended form, 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'

    As are these two statements:

        print
        print >> sys.stdout

    These two statements are syntax errors:

        print ,
        print >> sys.stdout,


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

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

    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.

    This approach is also very inconvenient for interleaving prints to
    various output streams, and complicates coding in the face of
    legitimate try/except or try/finally clauses.


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].  This approach adds two new opcodes, PRINT_ITEM_TO and
    PRINT_NEWLINE_TO, which simply pop the file like object off the
    top of the stack and use it instead of sys.stdout as the output
    stream.

    (This reference implementation has been adopted in Python 2.0.)


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.


More Justification by the BDFL

    The proposal has been challenged on the newsgroup.  One series of
    challenges doesn't like '>>' and would rather see some other
    symbol.

    Challenge: Why not one of these?

        print in stderr items,.... 
        print + stderr items,.......
        print[stderr] items,.....
        print to stderr items,.....

    Response: If we want to use a special symbol (print <symbol>
    expression), the Python parser requires that it is not already a
    symbol that can start an expression -- otherwise it can't decide
    which form of print statement is used.  (The Python parser is a
    simple LL(1) or recursive descent parser.)

    This means that we can't use the "keyword only in context trick"
    that was used for "import as", because an identifier can start an
    expression.  This rules out +stderr, [sterr], and to stderr.  It
    leaves us with binary operator symbols and other miscellaneous
    symbols that are currently illegal here, such as 'import'.

    If I had to choose between 'print in file' and 'print >> file' I
    would definitely choose '>>'.  In part because 'in' would be a new
    invention (I know of no other language that uses it, while '>>' is
    used in sh, awk, Perl, and C++), in part because '>>', being
    non-alphabetic, stands out more so is more likely to catch the
    reader's attention.

    Challenge: Why does there have to be a comma between the file and
    the rest?

    Response: The comma separating the file from the following expression is
    necessary!  Of course you want the file to be an arbitrary
    expression, not just a single word.  (You definitely want to be
    able to write print >>sys.stderr.)  Without the expression the
    parser would't be able to distinguish where that expression ends
    and where the next one begins, e.g.

        print >>i +1, 2
        print >>a [1], 2
        print >>f (1), 2

    Challenge: Why do you need a syntax extension?  Why not
    writeln(file, item, ...)?

    Response: First of all, this is lacking a feature of the print
    statement: the trailing comma to print which suppresses the final
    newline.  Note that 'print a,' still isn't equivalent to
    'sys.stdout.write(a)' -- print inserts a space between items, and
    takes arbitrary objects as arguments; write() doesn't insert a
    space and requires a single string.

    When you are considering an extension for the print statement,
    it's not right to add a function or method that adds a new feature
    in one dimension (where the output goes) but takes away in another
    dimension (spaces between items, and the choice of trailing
    newline or not).  We could add a whole slew of methods or
    functions to deal with the various cases but that seems to add
    more confusion than necessary, and would only make sense if we
    were to deprecate the print statement altogether.

    I feel that this debate is really about whether print should have
    been a function or method rather than a statement.  If you are in
    the function camp, of course adding special syntax to the existing
    print statement is not something you like.  I suspect the
    objection to the new syntax comes mostly from people who already
    think that the print statement was a bad idea.  Am I right?

    About 10 years ago I debated with myself whether to make the most
    basic from of output a function or a statement; basically I was
    trying to decide between "print(item, ...)" and "print item, ...".
    I chose to make it a statement because printing needs to be taught
    very early on, and is very important in the programs that
    beginners write.  Also, because ABC, which lead the way for so
    many things, made it a statement.  In a move that's typical for
    the interaction between ABC and Python, I changed the name from
    WRITE to print, and reversed the convention for adding newlines
    from requiring extra syntax to add a newline (ABC used trailing
    slashes to indicate newlines) to requiring extra syntax (the
    trailing comma) to suppress the newline.  I kept the feature that
    items are separated by whitespace on output.

    Full example: in ABC,

        WRITE 1
        WRITE 2/

    has the same effect as

        print 1,
        print 2

    has in Python, outputting in effect "1 2\n".

    I'm not 100% sure that the choice for a statement was right (ABC
    had the compelling reason that it used statement syntax for
    anything with side effects, but Python doesn't have this
    convention), but I'm also not convinced that it's wrong.  I
    certainly like the economy of the print statement.  (I'm a rabid
    Lisp-hater -- syntax-wise, not semantics-wise! -- and excessive
    parentheses in syntax annoy me.  Don't ever write return(i) or
    if(x==y): in your Python code! :-)

    Anyway, I'm not ready to deprecate the print statement, and over
    the years we've had many requests for an option to specify the
    file.

    Challenge: Why not > instead of >>?

    Response: To DOS and Unix users, >> suggests "append", while >
    suggests "overwrite"; the semantics are closest to append.  Also,
    for C++ programmers, >> and << are I/O operators.

    Challenge: But in C++, >> is input and << is output!

    Response: doesn't matter; C++ clearly took it from Unix and
    reversed the arrows.  The important thing is that for output, the
    arrow points to the file.

    Challenge: Surely you can design a println() function can do all
    what print>>file can do; why isn't that enough?

    Response: I think of this in terms of a simple programming
    exercise.  Suppose a beginning programmer is asked to write a
    function that prints the tables of multiplication.  A reasonable
    solution is:

        def tables(n):
            for j in range(1, n+1):
                for i in range(1, n+1):
                    print i, 'x', j, '=', i*j
                print

    Now suppose the second exercise is to add printing to a different
    file.  With the new syntax, the programmer only needs to learn one
    new thing: print >> file, and the answer can be like this:

        def tables(n, file=sys.stdout):
            for j in range(1, n+1):
                for i in range(1, n+1):
                    print >> file, i, 'x', j, '=', i*j
                print >> file
        
    With only a print statement and a println() function, the
    programmer first has to learn about println(), transforming the
    original program to using println():

        def tables(n):
            for j in range(1, n+1):
                for i in range(1, n+1):
                    println(i, 'x', j, '=', i*j)
                println()

    and *then* about the file keyword argument:

        def tables(n, file=sys.stdout):
            for j in range(1, n+1):
                for i in range(1, n+1):
                    println(i, 'x', j, '=', i*j, file=sys.stdout)
                println(file=sys.stdout)

    Thus, the transformation path is longer:

        (1) print
        (2) print >> file

    vs.

        (1) print
        (2) println()
        (3) println(file=...)

    Note: defaulting the file argument to sys.stdout at compile time
    is wrong, because it doesn't work right when the caller assigns to
    sys.stdout and then uses tables() without specifying the file.
    This is a common problem (and would occur with a println()
    function too).  The standard solution so far has been:

        def tables(n, file=None):
            if file is None:
                file = sys.stdout
            for j in range(1, n+1):
                for i in range(1, n+1):
                    print >> file, i, 'x', j, '=', i*j
                print >> file

    I've added a feature to the implementation (which I would also
    recommend to println()) whereby if the file argument is None,
    sys.stdout is automatically used.  Thus,

        print >> None, foo bar

    (or, of course, print >> x where x is a variable whose value is
    None) means the same as

        print foo, bar

    and the tables() function can be written as follows:

        def tables(n, file=None):
            for j in range(1, n+1):
                for i in range(1, n+1):
                    print >> file, i, 'x', j, '=', i*j
                print >> file

    [XXX this needs more justification, and a section of its own]


References

    [1] http://docs.python.org/reference/simple_stmts.html#print
    [2] http://sourceforge.net/patch/download.php?id=100970



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