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    Maybe file-objects should have `write()`- and `writeln()`-methods similar to the built-in functions? ''-- TS''

This page discusses the benefits of replacing the current print statement with an equivalent builtin. The write and writeln functions presented below do everything the print statement does without requiring any hacking of the grammar, and also make a number of things significantly easier.

Guido has made it clear he wants to get rid of the print statement in ["Python3.0"]. This page considers why we would want to go that way, and how we can actually get there. It should be turned into a PEP eventually.

Benefits of using a function instead of a statement

  • Extended call syntax provides better interaction with sequences
  • Keyword argument sep allows item separator to be changed easily and obviously

  • Keyword argument linesep could optionally allow line separator to be changed easily and obviously

  • Keyword argument stream allows easy and obvious redirection

  • The builtin can be replaced for application wide customisation (e.g. per-thread logging)
  • Interacts well with PEP 309's partial function application, and the rest of Python's ability to handle functions
    • BDFL comments:

      • don't waste your time on sequence printing
      • I'm not excited about sep and linesep keyword args
      • add to benefits: easier transition to other function/method calls
      • if it were me, I'd use 'to=' or 'file=' rather than 'stream=' (too long)

Guido's own arguments

There is a theoretical argument: print is the only application-level functionality that has a statement dedicated to it. Within Python's world, syntax is generally used as a last resort, when something can't be done without help from the compiler. Print doesn't qualify for such an exception (quite the opposite actually).

But more important to me are my own experiences exploring the boundaries of print.

  • I quite often come to a point in the evolution of a program where I need to change all print statements into logging calls, or calls into some other I/O or UI library. If print were a function, this would be a straightforward string replacement; as it is, finding where to add the parentheses is often a pain (the end isn't always on the same line

    as the start). It's even worse if there are already ">>stream" options present. Trailing commas also make this more complicated than it needs to be.

  • Having special syntax puts up a much larger barrier for evolution of a feature. For examle, adding printf (or changing print to printf) is a much bigger deal now that print is a statement than if it had been a built-in function: trial implementations are much more work, there are only a few people who know how to modify Python's bytecode compiler, etc. (Having printf() be a function and print remain a statement is of course a possibility, but only adds more confusion and makes printf() a second-class citizen, thereby proving my point.)
  • There is a distinct non-linearity in print's ease of use once you decide that you don't want to have spaces between items; you either have to switch to using sys.stdout.write(), or you have to collect all items in a string. This is not a simple transformation, consider what it takes to get rid of the spaces before the commas in this simple example:
    • print "x =", x, ", y =", y, ", z =", z

    If it was a built-in function, having a built-in companion function that did a similar thing without inserting spaces and adding a newline would be the logical thing to do (or adding keyword parameters to control that behavior; but I prefer a second function); but with only print as it currently stands, you'd have to switch to something like
    • print "x = " + str(x) + ", y = " + str(x) + ", z = " + str(z)

    or
    • print "x = %s, y = %s, z = %s" % (x, y, z)

    neither of which is very attractive. (And don't tell me that the

    spaces are no big deal -- they aren't in this example, but they are in other situations.)

  • If it were a function, it would be much easier to replace it within

    one module (just def print(*args):...) or even throughout a program (e.g. by putting a different function in __builtin__.print). As it is, you can do this by writing a class with a write() method and assigning that to sys.stdout -- that's not bad, but definitely a much larger conceptual leap, and it works at a different level than print.

Summarizing, my main problems with print as a statement are the transformations -- when print doesn't cut it, you have to switch to something entirely different. If it were a function the switch would feel much smoother. I find that important: things that are conceptually related should be syntactically related (within the realm of common sense, as always).

Getting there from here

The example implementation below shows that creating a function with the desired behaviour is quite straightforward. However, calling the builtin print is a problem due to the fact that print is a reserved word in Python 2.x. Since the print statement will be around until Py3K allows us to break backwards compatibility, devising a transition plan that lets programmers 'get ready early' for the Py3K transition becomes a significant challenge.

If, on the other hand, the builtin has a different name, it is quite feasible to introduce it during the 2.x series. In [http://www.python.org/peps/pep-3000.html PEP 3000], it is suggested that the print statement be replaced by two builtins: write and writeln. These names are used in the example below. By using alternative names, and providing the builtins in the 2.x series, it is possible to 'future-proof' code against the removal of the print statement in Py3k.

This technique of having two printing operations is not uncommon - Java has both print and println methods, and C# has Write and WriteLine. The main problem with the approach is that the writeln form will actually be more commonly used, but has the longer, less obvious name of the two proposed functions. This perception of relative use is based on a comparison of relative usage levels of the two current forms of the print statement (i.e., with and without the trailing comma) by some of the developers on python-dev.

Some other names for the builtins which have been suggested are:

  • print - excellent name, but causes transition problems as described above

  • println - avoids the transition problems, reflects default behaviour of adding a line, matches Java method name

  • printline - alternative to println, that avoids the somewhat cryptic abbreviation

  • writeline - alternative to writeln that avoids the somewhat cryptic abbreviation

  • say - short alternative to println invented in Perl 6 (which uses print for no-newline output)

  • out - not a verb, and converting to it may be problematic due to shadowing by variable names

  • output - nice symmetry with input, but using the term as a verb is not typical

  • prnt - easily edited into print later on

  • write - decent name, but confusing when compared to write() method

    • Maybe file-objects should have write()- and writeln()-methods similar to the built-in functions? -- TS

Sample implementation

This is a Python 2.4 compatible sample implementation of the approach currently in [http://www.python.org/peps/pep-3000.html PEP 3000]. This version of writeln doesn't provide a linesep keyword argument in order to keep things simple. Some other variations are covered further down this Wiki page.

   1 def write(*args, **kwds):
   2     """Functional replacement for the print statement
   3 
   4     This function does NOT automatically append a line separator (use writeln for that)
   5     """
   6 
   7     # Nothing to do if no positional arguments
   8     if not args:
   9         return
  10 
  11     def parse_kwds(sep=" ", stream=sys.stdout):
  12         """ Helper function to parse keyword arguments """
  13         return sep, stream
  14     sep, stream = parse_kwds(**kwds)
  15 
  16     # Perform the print operation without building the whole string
  17     stream.write(str(args[0]))
  18     for arg in args[1:]:
  19         stream.write(sep)
  20         stream.write(str(arg))
  21         
  22 def writeln(*args, **kwds):
  23     """Functional replacement for the print statement
  24 
  25     >>> writeln(1, 2, 3)
  26     1 2 3
  27     >>> writeln(1, 2, 3, sep='')
  28     123
  29     >>> writeln(1, 2, 3, sep=', ')
  30     1, 2, 3
  31     >>> import sys
  32     >>> writeln(1, 2, 3, stream=sys.stderr)
  33     1 2 3
  34     >>> writeln(*range(10))
  35     0 1 2 3 4 5 6 7 8 9
  36     >>> writeln(*(x*x for x in range(10)))
  37     0 1 4 9 16 25 36 49 64 81
  38     """
  39     # Perform the print operation without building the whole string
  40     write(*args, **kwds)
  41     write("\n", **kwds)

Code comparisons

These are some comparisons of current print statements with the equivalent code using the builtins write and writeln.

   1 # Standard printing
   2 print 1, 2, 3
   3 writeln(1, 2, 3)
   4 
   5 # Printing without any spaces
   6 print "%d%d%d" % (1, 2, 3)
   7 writeln(1, 2, 3, sep='')
   8 
   9 # Print as comma separated list
  10 print "%d, %d, %d" % (1, 2, 3)
  11 writeln(1, 2, 3, sep=', ')
  12 
  13 # Print without a trailing newline
  14 print 1, 2, 3,
  15 write(1, 2, 3)
  16 
  17 # Print to a different stream
  18 print >> sys.stderr, 1, 2, 3
  19 writeln(1, 2, 3, stream=sys.stderr)
  20 
  21 # Print a simple sequence
  22 print " ".join(map(str, range(10)))
  23 writeln(*range(10))
  24 
  25 # Print a generator expression
  26 print " ".join(str(x*x) for x in range(10))
  27 writeln(*(x*x for x in range(10)))

Newline / No-newline

Another possibility to deal with the newline / no-newline cases would be to have a single function which would take an extra keyword argument "linesep" or "end" (or perhaps some slight magic: an empty string as the last argument), so to print without newline, you would do

   1 # Print without a trailing newline
   2 print 1, 2, 3,
   3 writeln(1, 2, 3, end='')
   4 # or (shorthand)
   5 writeln(1, 2, 3, '')

The default case should be to insert a newline.

  • I quite like the single function idea (early versions of this Wiki page used only a single function), but giving it a good name is challenging. The version without the keyword argument is a definite non-starter, though, as there is far too much risk of quirky behaviour when printing a string variable which just happens to contain the empty string. - Nick Coghlan

    • BDFL comments: I definitely am not keen on the single function with keyword args. IMO all you need is a companion function that inserts no separator and no newline; the desired separators are then easily given explicitly. Oh, and you will never get away with using the final empty string to mean "no newline". This would be very confusing for someone who printed a variable like so: print("The value is:", x) when the variable happens to be empty.

Iterating Iterables

Another potentially interesting improvement could be for the function to iterate all iterables, in order to be able to use generator expressions without having to use the star syntax and to avoid the creation of a temporary sequence. This would allow:

   1 # Print a generator expression
   2 print " ".join(str(x*x) for x in range(10))
   3 writeln(x*x for x in range(10))
   4 # Or optionally
   5 writeln((x*x for x in range(10)), iter=1)

This behaviour could be optionally triggered by a keyword argument "iter". Another possibility would be to always do the iteration and to force the caller to str() the generator if he wants to print it without iteration (happens rarely).

  • Nailing down this kind of behaviour is trickier than one might think. The python-dev discussion of the Python 2.5 candidate library function [http://mail.python.org/pipermail/python-dev/2005-March/052215.html itertools.walk] goes over some of the potential problems. We've survived without fancy iterator handling in the print statement - let's avoid adding anything we don't have a demonstrated need for (the extended call syntax stuff comes 'for free' with the conversion to using a function). - Nick Coghlan

    • BDFL comments: bah. implicitly exhausting iterables has side effects, which is a bad idea for a print function. It would not be a good idea if commenting out a print() call changes the behavior of the program.

Another Strawman

Here's my own strawman implementation of write() and writef() using semantics I think are pretty useful. I'll post to python-dev about the details. - Barry Warsaw

   1 import sys
   2 from string import Template
   3 
   4 class Separator:
   5     def __init__(self, sep):
   6         self.sep = sep
   7 
   8 SPACE = Separator(' ')
   9 EMPTY = Separator('')
  10 
  11 
  12 def writef(fmt, *args, **kws):
  13     if 'to' in kws:
  14         to = kws.get('to')
  15         del kws['to']
  16     else:
  17         to = sys.stdout
  18     if 'nl' in kws:
  19         nl = kws.get('nl')
  20         del kws['nl']
  21         if nl is True:
  22             nl = '\n'
  23         elif nl is False:
  24             nl = ''
  25     else:
  26         nl = '\n'
  27     if isinstance(fmt, Template):
  28         if args:
  29             raise TypeError('invalid positional arguments')
  30         s = fmt.substitute(kws)
  31     else:
  32         if kws:
  33             raise TypeError('invalid keyword arguments')
  34         s = fmt % args
  35     to.write(s)
  36     to.write(nl)
  37 
  38 
  39 def write(*args, **kws):
  40     if 'to' in kws:
  41         to = kws.get('to')
  42         del kws['to']
  43     else:
  44         to = sys.stdout
  45     if 'nl' in kws:
  46         nl = kws.get('nl')
  47         del kws['nl']
  48         if nl is True:
  49             nl = '\n'
  50         elif nl is False:
  51             nl = ''
  52     else:
  53         nl = '\n'
  54     if 'sep' in kws:
  55         sep = kws.get('sep')
  56         del kws['sep']
  57     else:
  58         sep = ' '
  59     if kws:
  60         raise TypeError('invalid keyword arguments')
  61     it = iter(args)
  62     # Suppress leading separator, but consume all Separator instances
  63     for s in it:
  64         if isinstance(s, Separator):
  65             sep = args[0].sep # Should this be s.sep?
  66         else:
  67             # Don't write a leading separator
  68             to.write(str(s))
  69             break
  70     for s in it:
  71         if isinstance(s, Separator):
  72             sep = s.sep
  73         else:
  74             to.write(sep)
  75             to.write(str(s))
  76     to.write(nl)
  77 
  78 
  79 obj = object()
  80 refs = sys.getrefcount(obj)
  81 
  82 write('obj:', obj, 'refs:', refs)
  83 write(Separator(': '), 'obj', obj,
  84       Separator(', '), 'refs',
  85       Separator(': '), refs,
  86       nl=False)
  87 write()
  88 
  89 writef('obj: %s, refs: %s', obj, refs)
  90 writef(Template('obj: $obj, refs: $refs, obj: $obj'),
  91        obj=obj, refs=refs,
  92        to=sys.stderr,
  93        nl=False)
  94 write()
  • For the code comparisons shown earlier, simply put write where writeln is currently used, and add the keyword argument nl=False for the no trailing newline case. I quite like this approach. - Nick Coghlan

    • BDFL comments: I like the write/writef parallel; would like it even more if it was print/printf. But please drop the Separator thing. The use case isn't common enough to burden people with the possibility. Also, we need to spend more time researching the formatting language. (See a post in python-dev by Steven Bethard: "string formatting options and removing basestring.__mod__".

Another variant - `format` builtin

Barry's writef builtin cuts down a little on the typing, but is somewhat inflexible in that it only supports string % or string.Template formatting when printing directly to a stream. It also causes problems by preventing the use of to or nl as keywords in the format string. A separate format builtin would deal with both of those problems, at the expense of some extra typing when using it. Such a builtin would also help with avoiding some of the tuple related quirks of the string mod operator, as well as making it easy to write code that supports both types of string formatting. The version below is based on Barry's, but eliminates the Separator concept, and replaces writef with format - Nick Coghlan

   1 import sys
   2 from string import Template
   3 
   4 # Real implementation would avoid blocking use of 'fmt'
   5 # as an element of the formatting string
   6 def format(fmt, *args, **kws):
   7     if isinstance(fmt, Template):
   8         if args:
   9             raise TypeError('invalid positional arguments')
  10         s = fmt.substitute(kws)
  11     else:
  12         if kws:
  13             s = fmt % kws
  14         else:
  15             s = fmt % args
  16     return s
  17 
  18 
  19 def write(*args, **kws):
  20     if 'to' in kws:
  21         to = kws.get('to')
  22         del kws['to']
  23     else:
  24         to = sys.stdout
  25     if 'nl' in kws:
  26         nl = kws.get('nl')
  27         del kws['nl']
  28         if nl is True:
  29             nl = '\n'
  30         elif nl is False:
  31             nl = ''
  32     else:
  33         nl = '\n'
  34     if 'sep' in kws:
  35         sep = kws.get('sep')
  36         del kws['sep']
  37     else:
  38         sep = ' '
  39     if kws:
  40         raise TypeError('invalid keyword arguments')
  41     for s in args[:1]:
  42         to.write(str(s))
  43     for s in args[1:]:
  44         to.write(sep)
  45         to.write(str(s))
  46     to.write(nl)
  47 
  48 
  49 obj = object()
  50 refs = sys.getrefcount(obj)
  51 
  52 write('obj:', obj, 'refs:', refs)
  53 write('obj:', obj, 'refs:', refs, sep=', ', nl=False)
  54 write()
  55 
  56 write(format('obj: %s, refs: %s', obj, refs))
  57 write(format('obj: %(obj)s, refs: %(refs)s', obj=obj, refs=refs))
  58 write(format(Template('obj: $obj, refs: $refs, obj: $obj'),
  59               obj=obj, refs=refs),
  60        to=sys.stderr,
  61        nl=False)
  62 write()

Displaying iterators

I'm looking into an approach which adds explicit support for displaying iterators into the string mod operator. The intent is that "%''j" % (my_seq,) will become roughly equivalent to ''.join(map(str, my_seq)). - Nick Coghlan

[http://sourceforge.net/tracker/?func=detail&aid=1281573&group_id=5470&atid=305470 SF Patch #1281573] for anyone who wants to play with it. Only strings are supported so far (no Unicode), but it illustrates the concept quite well.

   1 # Print a simple sequence
   2 print " ".join(map(str, range(10)))
   3 print "%' 'j" % range(10)
   4 
   5 # Print a generator expression
   6 print " ".join(str(x*x) for x in range(10))
   7 print "%' 'j" % (x*x for x in range(10))
  • BDFL comments: again, please don't do this.

Scrap C-Style Formatting

What's one more strawman, right? :) My approach is tailor-made for gettext (although I'm no expert in gettext usage). Keywords become the default and positionals disappear completely.

>>> print('x = {x}, y = {y}, z = {z}', x=x, y=y, z=z)

There's some redundancy in the the keyword arguments (unfortunately), but it helps insulate the format string from the code that uses it. It removes the problems of separator vs no separator It allows it to be self-documenting for the gettext translators, with no problems in reordering or reformatting. You could even give extra arguments that aren't always used (but they wouldn't be self-documenting I suppose).

Further options are using locals():

>>> print('x = {x}, y = {y}, z = {z}', **locals())

but only if you don't mind exposing them (debatable). If you need something besides %s (the default) then go as follows:

>>> print('x = {r:x}, y = {f9.8:y}, z = {i:z}', x=x, y=y, z=z)

Or maybe even something that allows arbitrary arguments to be passed to the formatter. - Adam Olsen

PrintAsFunction (last edited 2011-08-14 09:25:04 by eth595)

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