Differences between revisions 17 and 18
Revision 17 as of 2005-09-03 07:10:09
Size: 5388
Editor: NickCoghlan
Comment: Rewrite based on PEP 3000 and execution of sample code
Revision 18 as of 2005-09-03 13:48:45
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Editor: BrockFiler
Comment:
Deletions are marked like this. Additions are marked like this.
Line 45: Line 45:
    for name, default in kwd_values.items():
        try:
            item = kwds[name]
        except KeyError:
            continue
        del kwds[name]
        if item is not None:
            kwd_values[name] = item
    if kwds:
        raise TypeError("%s is an invalid keyword argument for this function" % kwds.keys()[0])
    kwd_values.update(kwds)
    if len(kwd_values) is not 2:
        raise TypeError("write() and writeln() only accept sep and stream as keyword arguments")

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 ["Python 3.0"]. This page considers why we want to go that way, and how we can actually get there. It will probably be turned into a PEP at some point.

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

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.htm 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.

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

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

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

  • printline - similar to println, but avoids the somewhat cryptic abbreviation

  • writeline - similar to writeln, but avoids the somewhat cryptic abbreviation

  • 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

Sample implementation

This is a Python 2.4 compatible sample implementation. This version of writeln doesn't provide a linesep keyword argument in order to keep things simple.

   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     # Nothing to do if no positional arguments
   7     if not args:
   8         return
   9     # Parse the keyword-only optional arguments
  10     kwd_values = {
  11         "sep": " ",
  12         "stream": sys.stdout,
  13     }
  14     kwd_values.update(kwds)
  15     if len(kwd_values) is not 2:
  16         raise TypeError("write() and writeln() only accept sep and stream as keyword arguments")
  17     sep, stream = kwd_values["sep"], kwd_values["stream"]
  18     # Perform the print operation without building the whole string
  19     stream.write(str(args[0]))
  20     for arg in args[1:]:
  21         stream.write(sep)
  22         stream.write(str(arg))
  23 
  24 def writeln(*args, **kwds):
  25     """Functional replacement for the print statement
  26 
  27     >>> writeln(1, 2, 3)
  28     1 2 3
  29     >>> writeln(1, 2, 3, sep='')
  30     123
  31     >>> writeln(1, 2, 3, sep=', ')
  32     1, 2, 3
  33     >>> import sys
  34     >>> writeln(1, 2, 3, stream=sys.stderr)
  35     1 2 3
  36     >>> writeln(*range(10))
  37     0 1 2 3 4 5 6 7 8 9
  38     >>> writeln(*(x*x for x in range(10)))
  39     0 1 4 9 16 25 36 49 64 81
  40     """
  41     # Perform the print operation without building the whole string
  42     write(*args, **kwds)
  43     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)))

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

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