python-peps/peps/pep-0214.rst

PEP: 214
Title: Extended Print Statement
Author: Barry Warsaw <barry@python.org>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
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 wouldn'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