Revision 43 as of 2005-12-17 21:25:03

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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

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.

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 [ 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:

Sample implementation

This is a Python 2.4 compatible sample implementation of the approach currently in [ 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
   4     This function does NOT automatically append a line separator (use writeln for that)
   5     """
   7     # Nothing to do if no positional arguments
   8     if not args:
   9         return
  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)
  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))
  22 def writeln(*args, **kwds):
  23     """Functional replacement for the print statement
  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)
   5 # Printing without any spaces
   6 print "%d%d%d" % (1, 2, 3)
   7 writeln(1, 2, 3, sep='')
   9 # Print as comma separated list
  10 print "%d, %d, %d" % (1, 2, 3)
  11 writeln(1, 2, 3, sep=', ')
  13 # Print without a trailing newline
  14 print 1, 2, 3,
  15 write(1, 2, 3)
  17 # Print to a different stream
  18 print >> sys.stderr, 1, 2, 3
  19 writeln(1, 2, 3, stream=sys.stderr)
  21 # Print a simple sequence
  22 print " ".join(map(str, range(10)))
  23 writeln(*range(10))
  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.

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).

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
   4 class Separator:
   5     def __init__(self, sep):
   6         self.sep = sep
   8 SPACE = Separator(' ')
   9 EMPTY = Separator('')
  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)
  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)
  79 obj = object()
  80 refs = sys.getrefcount(obj)
  82 write('obj:', obj, 'refs:', refs)
  83 write(Separator(': '), 'obj', obj,
  84       Separator(', '), 'refs',
  85       Separator(': '), refs,
  86       nl=False)
  87 write()
  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()

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
   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
  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)
  49 obj = object()
  50 refs = sys.getrefcount(obj)
  52 write('obj:', obj, 'refs:', refs)
  53 write('obj:', obj, 'refs:', refs, sep=', ', nl=False)
  54 write()
  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

[ 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)
   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))

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

Another idea

String formatting with %* is a bad idea, imho. Since python is anyway dynamic by nature, why not add built-in string evaluation, as in boo []. for example:

x = "lucy"
write("i love ${x}")


x = 7
write("the answer is ${x * 2}")

if strings (or a special flavor of string, say one marked with backticks*) allowed evaluation of expressions, code will never look like

print "x = ", x, "y = ", y

but rather

write("x = ${x}, y = ${y}")

which is much more readable and easier to maintain. imagine working with 20 '%s' in a single string! it's a disaster. even using the silly %(name) is bad, since you then have to fill a huge dict after your string.

(*) backticks: yes, backticks mean repr(), but did anyone ever hear of them? i think they are depricated anyway. adding a new built-in type, evalstr ("evaluating string"), marked by backticks, is very simple and almost completely backwards compatible. and it works not only in the context of printing output.

write(`hello ${os.getuid()}, the time now is ${time.asctime()}, and you are running on ${}`)

true, it doesnt solve the write/writeln "problem", and i must admit that print as a statement is a pretty useful feature (no parenthesis hassle), but adding evalstrings will make long format string possible and maintainable. plus, it gets us rid of the ugly writef or printf proposals.

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