Differences between revisions 16 and 18 (spanning 2 versions)
Revision 16 as of 2005-09-03 05:27:20
Size: 5211
Editor: NickCoghlan
Comment: Add print builtin to 'no trailing newline' example
Revision 18 as of 2005-09-03 13:48:45
Size: 5208
Editor: BrockFiler
Comment:
Deletions are marked like this. Additions are marked like this.
Line 1: Line 1:
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. 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.
Line 5: Line 7:
 * 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 `sep` allows item separator to be changed easily and obviously
 * Keyword argument `linesep` could optionally allow line separator to be changed easily and obviously
Line 9: Line 11:
 * Interacts well with PEP 309's partial function application  * Interacts well with PEP 309's partial function application, and the rest of Python's ability to handle functions
Line 12: Line 14:
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. 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.
Line 14: Line 16:
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. 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.
Line 16: Line 18:
Some names which have been suggested are: 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:
Line 20: Line 24:
 * `writeline` - similar to `writeln`, but avoids the somewhat cryptic abbreviation
Line 23: Line 28:
An alternative transition strategy would be to add `println` during the Python 2.x series, then, in Py3k, remove the `print` statement and replace it with a `print` builtin which did not append a trailing line separator, and did not support the `lnterm` keyword. The use of `println` with `lnterm = ''` would then simply be a transition strategy - normal Py3k usage would be to use `print` if you didn't want the newline added, and `println` if you did. The `lnterm` argument to `println` would typically only be used to get a different line separator, rather than to eliminate the line separator entirely.

This technique of having two methods is not uncommon - Java has both `print` and `println` methods, and C# has `Write` and `WriteLine`.
Line 28: Line 29:
This is a Python 2.4 compatible sample implementation, which is why it uses the name `println` rather than `print`. 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.
Line 31: Line 32:
def println(*args, **kwds): def write(*args, **kwds):
Line 34: Line 35:
    >>> println(1, 2, 3)
    1 2 3
    >>> println(1, 2, 3, sep='')
    123
    >>> println(1, 2, 3, sep=', ')
    1, 2, 3
    >>> println(1, 2, 3, lnterm='Alternate line terminator\n')
    1 2 3Alternate line terminator
    >>> import sys
    >>> println(1, 2, 3, stream=sys.stderr)
    1 2 3
    >>> println(*range(10))
    0 1 2 3 4 5 6 7 8 9
    >>> println(*(x*x for x in range(10)))
    0 1 4 9 16 25 36 49 64 81
    This function does NOT automatically append a line separator (use writeln for that)
Line 50: Line 37:
    # Nothing to do if no positional arguments
    if not args:
        return
Line 51: Line 41:
    defaults = {     kwd_values = {
Line 53: Line 43:
        "lnterm": "\n",
Line 56: Line 45:
    for name, default in defaults.items():
        item = None
        try:
            item = kwds[name]
        except KeyError:
            pass
        if item is None:
            kwds[name] = default
    sep, lnterm, stream = kwds["sep"], kwds["lnterm"], kwds["stream"]
    kwd_values.update(kwds)
    if len(kwd_values) is not 2:
        raise TypeError("write() and writeln() only accept sep and stream as keyword arguments")
    sep, stream = kwd_values["sep"], kwd_values["stream"]
Line 66: Line 50:
    for arg in args[:1]:
    
stream.write(str(arg))
    stream.write(str(args[0]))
Line 71: Line 54:
    stream.write(lnterm)
def writeln(*args, **kwds):
    """Functional replacement for the print statement

    >>> writeln(1, 2, 3)
    1 2 3
    >>> writeln(1, 2, 3, sep='')
    123
    >>> writeln(1, 2, 3, sep=', ')
    1, 2, 3
    >>> import sys
    >>> writeln(1, 2, 3, stream=sys.stderr)
    1 2 3
    >>> writeln(*range(10))
    0 1 2 3 4 5 6 7 8 9
    >>> writeln(*(x*x for x in range(10)))
    0 1 4 9 16 25 36 49 64 81
    """
    # Perform the print operation without building the whole string
    write(*args, **kwds)
    write("\n", **kwds)
Line 75: Line 78:
These are some comparisons of current print statements with the equivalent code using the builtin. These are some comparisons of current `print` statements with the equivalent code using the builtins `write` and `writeln`.
Line 77: Line 80:
'''Standard printing:'''
Line 79: Line 81:
# Standard printing
Line 80: Line 83:
println(1, 2, 3) writeln(1, 2, 3)

# Printing without any spaces
print "%d%d%d" % (1, 2, 3)
writeln(1, 2, 3, sep='')

# Print as comma separated list
print "%d, %d, %d" % (1, 2, 3)
writeln(1, 2, 3, sep=', ')

# Print without a trailing newline
print 1, 2, 3,
write(1, 2, 3)

# Print to a different stream
print >> sys.stderr, 1, 2, 3
writeln(1, 2, 3, stream=sys.stderr)

# Print a simple sequence
print " ".join(map(str, range(10)))
writeln(*range(10))

# Print a generator expression
print " ".join(str(x*x) for x in range(10))
writeln(*(x*x for x in range(10)))
Line 82: Line 109:

'''Printing without any spaces:'''
{{{#!python
print "%d%d%d" % (1, 2, 3)
println(1, 2, 3, sep='')
}}}

'''Print as comma separated list:'''
{{{#!python
print "%d, %d%, d" % (1, 2, 3)
println(1, 2, 3, sep=', ')
}}}

'''Print without a trailing newline:'''
{{{#!python
print 1, 2, 3,
println(1, 2, 3, lnterm='')
print(1, 2, 3) # If Py3k gains a print builtin to replace the print statement
}}}

'''Print to a different stream:'''
{{{#!python
print >> sys.stderr, 1, 2, 3
println(1, 2, 3, stream=sys.stderr)
}}}

'''Print a simple sequence:'''
{{{#!python
print " ".join(map(str, range(10)))
println(*range(10))
}}}

'''Print a generator expression:'''
{{{#!python
print " ".join(str(x*x) for x in range(10))
println(*(x*x for x in range(10)))
}}}

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)

Unable to edit the page? See the FrontPage for instructions.