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

Benefits of using a function instead of a statement

• Extended call syntax provides better interaction with sequences

• Keyword argument sep allows separator to be changed easily and obviously

• Keyword argument lnterm allows line terminator 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

Getting there from here

The example implementation below shows that creating a function with the desired behaviour is quite straightforward. A problem only arises if we decide we want the builtin to have the name print. This seriously complicates transition, because 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 had a different name (such as println), it would be quite feasible to introduce it during the 2.x series, with the only change in Py3K being the final removal of the print statement.

Sample implementation

This is a Python 2.4 compatible sample implementation, which is why it uses the name println rather than print.

   1 def println(*args, **kwds):
   2     """Functional replacement for the print statement
   3 
   4     >>> println(1, 2, 3)
   5     1 2 3
   6     >>> println(1, 2, 3, sep='')
   7     123
   8     >>> println(1, 2, 3, sep=', ')
   9     1, 2, 3
  10     >>> println(1, 2, 3, lnterm='Alternate line terminator\n')
  11     1 2 3Alternate line terminator
  12     >>> import sys
  13     >>> println(1, 2, 3, stream=sys.stderr)
  14     1 2 3
  15     >>> println(*range(10))
  16     0 1 2 3 4 5 6 7 8 9
  17     >>> println(*(x*x for x in range(10)))
  18     0 1 4 9 16 25 36 49 64 81
  19     """
  20     # Parse the keyword-only optional arguments
  21     defaults = {
  22         "sep": " ",
  23         "lnterm": "\n",
  24         "stream": sys.stdout,
  25     }
  26     for name, default in defaults.items():
  27         item = None
  28         try:
  29             item = kwds[name]
  30         except KeyError:
  31             pass
  32         if item is None:
  33             kwds[name] = default
  34     sep, lnterm, stream = kwds["sep"], kwds["lnterm"], kwds["stream"]
  35     # Perform the print operation without building the whole string
  36     for arg in args[:1]:
  37         stream.write(str(arg))
  38     for arg in args[1:]:
  39         stream.write(sep)
  40         stream.write(str(arg))
  41     stream.write(lnterm)

Code comparisons

These are some comparisons of current print statements with the equivalent code using the builtin.

Standard printing:

   1 print 1, 2, 3
   2 println(1, 2, 3)

Printing without any spaces:

   1 print "%d%d%d" % (1, 2, 3)
   2 println(1, 2, 3, sep='')

Print as comma separated list:

   1 print "%d, %d%, d" % (1, 2, 3)
   2 println(1, 2, 3, sep=', ')

Print without a trailing newline:

   1 print 1, 2, 3,
   2 println(1, 2, 3, lnterm='')

Print to a different stream:

   1 print >> sys.stderr, 1, 2, 3
   2 println(1, 2, 3, stream=sys.stderr)

Print a simple sequence:

   1 print " ".join(map(str, range(10)))
   2 println(*range(10))

Print a generator expression:

   1 print " ".join(str(x*x) for x in range(10))
   2 println(*(x*x for x in range(10)))