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
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
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
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 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)) 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)
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
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
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:
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
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.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
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()