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****************************
What's New in Python 2.7
****************************
:Author: A.M. Kuchling (amk at amk.ca)
:Release: |release|
:Date: |today|
.. Fix accents on Kristjan Valur Jonsson, Fuerstenau, Tarek Ziade.
.. $Id$
Rules for maintenance:
* Anyone can add text to this document. Do not spend very much time
on the wording of your changes, because your text will probably
get rewritten to some degree.
* The maintainer will go through Misc/NEWS periodically and add
changes; it's therefore more important to add your changes to
Misc/NEWS than to this file.
* This is not a complete list of every single change; completeness
is the purpose of Misc/NEWS. Some changes I consider too small
or esoteric to include. If such a change is added to the text,
I'll just remove it. (This is another reason you shouldn't spend
too much time on writing your addition.)
* If you want to draw your new text to the attention of the
maintainer, add 'XXX' to the beginning of the paragraph or
section.
* It's OK to just add a fragmentary note about a change. For
example: "XXX Describe the transmogrify() function added to the
socket module." The maintainer will research the change and
write the necessary text.
* You can comment out your additions if you like, but it's not
necessary (especially when a final release is some months away).
* Credit the author of a patch or bugfix. Just the name is
sufficient; the e-mail address isn't necessary.
* It's helpful to add the bug/patch number in a parenthetical comment.
XXX Describe the transmogrify() function added to the socket
module.
(Contributed by P.Y. Developer; :issue:`12345`.)
This saves the maintainer some effort going through the SVN logs
when researching a change.
This article explains the new features in Python 2.7.
No release schedule has been decided yet for 2.7.
.. Compare with previous release in 2 - 3 sentences here.
add hyperlink when the documentation becomes available online.
Python 3.1
================
Much as Python 2.6 incorporated features from Python 3.0,
version 2.7 is influenced by features from 3.1.
XXX mention importlib; anything else?
One porting change: the :option:`-3` switch now automatically
enables the :option:`-Qwarn` switch that causes warnings
about using classic division with integers and long integers.
.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. ========================================================================
PEP 372: Adding an ordered dictionary to collections
====================================================
XXX write this
Several modules will now use :class:`OrderedDict` by default. The
:mod:`ConfigParser` module uses :class:`OrderedDict` for the list
of sections and the options within a section.
The :meth:`namedtuple._asdict` method returns an :class:`OrderedDict`
as well.
Other Language Changes
======================
Some smaller changes made to the core Python language are:
* :meth:`str.format` method now supports automatic numbering of the replacement
fields. This makes using :meth:`str.format` more closely resemble using
``%s`` formatting::
>>> '{}:{}:{}'.format(2009, 04, 'Sunday')
'2009:4:Sunday'
>>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
'2009:4:Sunday'
The auto-numbering takes the fields from left to right, so the first ``{...}``
specifier will use the first argument to :meth:`str.format`, the next
specifier will use the next argument, and so on. You can't mix auto-numbering
and explicit numbering -- either number all of your specifier fields or none
of them -- but you can mix auto-numbering and named fields, as in the second
example above. (Contributed by Eric Smith; :issue`5237`.)
* The :func:`int` and :func:`long` types gained a ``bit_length``
method that returns the number of bits necessary to represent
its argument in binary::
>>> n = 37
>>> bin(37)
'0b100101'
>>> n.bit_length()
6
>>> n = 2**123-1
>>> n.bit_length()
123
>>> (n+1).bit_length()
124
(Contributed by Fredrik Johansson and Victor Stinner; :issue:`3439`.)
* Conversions from long integers and regular integers to floating
point now round differently, returning the floating-point number
closest to the number. This doesn't matter for small integers that
can be converted exactly, but for large numbers that will
unavoidably lose precision, Python 2.7 will now approximate more
closely. For example, Python 2.6 computed the following::
>>> n = 295147905179352891391
>>> float(n)
2.9514790517935283e+20
>>> n - long(float(n))
65535L
Python 2.7's floating-point result is larger, but much closer to the
true value::
>>> n = 295147905179352891391
>>> float(n)
2.9514790517935289e+20
>>> n-long(float(n)
... )
-1L
(Implemented by Mark Dickinson; :issue:`3166`.)
* The :class:`bytearray` type's :meth:`translate` method will
now accept ``None`` as its first argument. (Fixed by Georg Brandl;
:issue:`4759`.)
.. ======================================================================
Optimizations
-------------
Several performance enhancements have been added:
.. * A new :program:`configure` option, :option:`--with-computed-gotos`,
compiles the main bytecode interpreter loop using a new dispatch
mechanism that gives speedups of up to 20%, depending on the system
and benchmark. The new mechanism is only supported on certain
compilers, such as gcc, SunPro, and icc.
* The garbage collector now performs better when many objects are
being allocated without deallocating any. A full garbage collection
pass is only performed when the middle generation has been collected
10 times and when the number of survivor objects from the middle
generation exceeds 10% of the number of objects in the oldest
generation. The second condition was added to reduce the number
of full garbage collections as the number of objects on the heap grows,
avoiding quadratic performance when allocating very many objects.
(Suggested by Martin von Loewis and implemented by Antoine Pitrou;
:issue:`4074`.)
* The garbage collector tries to avoid tracking simple containers
which can't be part of a cycle. In Python 2.7, this is now true for
tuples and dicts containing atomic types (such as ints, strings,
etc.). Transitively, a dict containing tuples of atomic types won't
be tracked either. This helps reduce the cost of each
garbage collection by decreasing the number of objects to be
considered and traversed by the collector.
(Contributed by Antoine Pitrou; :issue:`4688`.)
* Integers are now stored internally either in base 2**15 or in base
2**30, the base being determined at build time. Previously, they
were always stored in base 2**15. Using base 2**30 gives
significant performance improvements on 64-bit machines, but
benchmark results on 32-bit machines have been mixed. Therefore,
the default is to use base 2**30 on 64-bit machines and base 2**15
on 32-bit machines; on Unix, there's a new configure option
:option:`--enable-big-digits` that can be used to override this default.
Apart from the performance improvements this change should be
invisible to end users, with one exception: for testing and
debugging purposes there's a new structseq ``sys.long_info`` that
provides information about the internal format, giving the number of
bits per digit and the size in bytes of the C type used to store
each digit::
>>> import sys
>>> sys.long_info
sys.long_info(bits_per_digit=30, sizeof_digit=4)
(Contributed by Mark Dickinson; :issue:`4258`.)
Another set of changes made long objects a few bytes smaller: 2 bytes
smaller on 32-bit systems and 6 bytes on 64-bit.
(Contributed by Mark Dickinson; :issue:`5260`.)
* The division algorithm for long integers has been made faster
by tightening the inner loop, doing shifts instead of multiplications,
and fixing an unnecessary extra iteration.
Various benchmarks show speedups of between 50% and 150% for long
integer divisions and modulo operations.
(Contributed by Mark Dickinson; :issue:`5512`.)
* The implementation of ``%`` checks for the left-side operand being
a Python string and special-cases it; this results in a 1-3%
performance increase for applications that frequently use ``%``
with strings, such as templating libraries.
(Implemented by Collin Winter; :issue:`5176`.)
* List comprehensions with an ``if`` condition are compiled into
faster bytecode. (Patch by Antoine Pitrou, back-ported to 2.7
by Jeffrey Yasskin; :issue:`4715`.)
.. ======================================================================
New, Improved, and Deprecated Modules
=====================================
As in every release, Python's standard library received a number of
enhancements and bug fixes. Here's a partial list of the most notable
changes, sorted alphabetically by module name. Consult the
:file:`Misc/NEWS` file in the source tree for a more complete list of
changes, or look through the Subversion logs for all the details.
* The :mod:`bz2` module's :class:`BZ2File` now supports the context
management protocol, so you can write ``with bz2.BZ2File(...) as f: ...``.
(Contributed by Hagen Fuerstenau; :issue:`3860`.)
* New class: the :class:`Counter` class in the :mod:`collections` module is
useful for tallying data. :class:`Counter` instances behave mostly
like dictionaries but return zero for missing keys instead of
raising a :exc:`KeyError`:
.. doctest::
:options: +NORMALIZE_WHITESPACE
>>> from collections import Counter
>>> c = Counter()
>>> for letter in 'here is a sample of english text':
... c[letter] += 1
...
>>> c
Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
'p': 1, 'r': 1, 'x': 1})
>>> c['e']
5
>>> c['z']
0
There are two additional :class:`Counter` methods: :meth:`most_common`
returns the N most common elements and their counts, and :meth:`elements`
returns an iterator over the contained element, repeating each element
as many times as its count::
>>> c.most_common(5)
[(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
>>> c.elements() ->
'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
's', 's', 'r', 't', 't', 'x'
Contributed by Raymond Hettinger; :issue:`1696199`.
The :class:`namedtuple` class now has an optional *rename* parameter.
If *rename* is true, field names that are invalid because they've
been repeated or that aren't legal Python identifiers will be
renamed to legal names that are derived from the field's
position within the list of fields:
>>> from collections import namedtuple
>>> T = namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True)
>>> T._fields
('field1', '_1', '_2', 'field2')
(Added by Raymond Hettinger; :issue:`1818`.)
The :class:`deque` data type now exposes its maximum length as the
read-only :attr:`maxlen` attribute. (Added by Raymond Hettinger.)
* In Distutils, :func:`distutils.sdist.add_defaults` now uses
*package_dir* and *data_files* to create the MANIFEST file.
:mod:`distutils.sysconfig` will now read the :envvar:`AR`
environment variable.
It is no longer mandatory to store clear-text passwords in the
:file:`.pypirc` file when registering and uploading packages to PyPI. As long
as the username is present in that file, the :mod:`distutils` package will
prompt for the password if not present. (Added by Tarek Ziade,
based on an initial contribution by Nathan Van Gheem; :issue:`4394`.)
A Distutils setup can now specify that a C extension is optional by
setting the *optional* option setting to true. If this optional is
supplied, failure to build the extension will not abort the build
process, but instead simply not install the failing extension.
(Contributed by Georg Brandl; :issue:`5583`.)
* New method: the :class:`Decimal` class gained a
:meth:`from_float` class method that performs an exact conversion
of a floating-point number to a :class:`Decimal`.
Note that this is an **exact** conversion that strives for the
closest decimal approximation to the floating-point representation's value;
the resulting decimal value will therefore still include the inaccuracy,
if any.
For example, ``Decimal.from_float(0.1)`` returns
``Decimal('0.1000000000000000055511151231257827021181583404541015625')``.
(Implemented by Raymond Hettinger; :issue:`4796`.)
* The :class:`Fraction` class will now accept two rational numbers
as arguments to its constructor.
(Implemented by Mark Dickinson; :issue:`5812`.)
* New function: the :mod:`gc` module's :func:`is_tracked` returns
true if a given instance is tracked by the garbage collector, false
otherwise. (Contributed by Antoine Pitrou; :issue:`4688`.)
* The :mod:`gzip` module's :class:`GzipFile` now supports the context
management protocol, so you can write ``with gzip.GzipFile(...) as f: ...``.
(Contributed by Hagen Fuerstenau; :issue:`3860`.)
It's now possible to override the modification time
recorded in a gzipped file by providing an optional timestamp to
the constructor. (Contributed by Jacques Frechet; :issue:`4272`.)
* The :class:`io.FileIO` class now raises an :exc:`OSError` when passed
an invalid file descriptor. (Implemented by Benjamin Peterson;
:issue:`4991`.)
* New function: ``itertools.compress(*data*, *selectors*)`` takes two
iterators. Elements of *data* are returned if the corresponding
value in *selectors* is true::
itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
A, C, E, F
New function: ``itertools.combinations_with_replacement(*iter*, *r*)``
returns all the possible *r*-length combinations of elements from the
iterable *iter*. Unlike :func:`combinations`, individual elements
can be repeated in the generated combinations::
itertools.combinations_with_replacement('abc', 2) =>
('a', 'a'), ('a', 'b'), ('a', 'c'),
('b', 'b'), ('b', 'c'), ('c', 'c')
Note that elements are treated as unique depending on their position
in the input, not their actual values.
The :class:`itertools.count` function now has a *step* argument that
allows incrementing by values other than 1. :func:`count` also
now allows keyword arguments, and using non-integer values such as
floats or :class:`Decimal` instances. (Implemented by Raymond
Hettinger; :issue:`5032`.)
:func:`itertools.combinations` and :func:`itertools.product` were
previously raising :exc:`ValueError` for values of *r* larger than
the input iterable. This was deemed a specification error, so they
now return an empty iterator. (Fixed by Raymond Hettinger; :issue:`4816`.)
* The :mod:`json` module was upgraded to version 2.0.9 of the
simplejson package, which includes a C extension that makes
encoding and decoding faster.
(Contributed by Bob Ippolito; :issue:`4136`.)
To support the new :class:`OrderedDict` type, :func:`json.load`
now has an optional *object_pairs_hook* parameter that will be called
with any object literal that decodes to a list of pairs.
(Contributed by Raymond Hettinger; :issue:`5381`.)
* The :mod:`multiprocessing` module's :class:`Manager*` classes
can now be passed a callable that will be called whenever
a subprocess is started, along with a set of arguments that will be
passed to the callable.
(Contributed by lekma; :issue:`5585`.)
* The :mod:`pydoc` module now has help for the various symbols that Python
uses. You can now do ``help('<<')`` or ``help('@')``, for example.
(Contributed by David Laban; :issue:`4739`.)
* The :mod:`re` module's :func:`split`, :func:`sub`, and :func:`subn`
now accept an optional *flags* argument, for consistency with the
other functions in the module. (Added by Gregory P. Smith.)
* New function: the :mod:`subprocess` module's
:func:`check_output` runs a command with a specified set of arguments
and returns the command's output as a string when the command runs without
error, or raises a :exc:`CalledProcessError` exception otherwise.
::
>>> subprocess.check_output(['df', '-h', '.'])
'Filesystem Size Used Avail Capacity Mounted on\n
/dev/disk0s2 52G 49G 3.0G 94% /\n'
>>> subprocess.check_output(['df', '-h', '/bogus'])
...
subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1
(Contributed by Gregory P. Smith.)
* New function: :func:`is_declared_global` in the :mod:`symtable` module
returns true for variables that are explicitly declared to be global,
false for ones that are implicitly global.
(Contributed by Jeremy Hylton.)
* The ``sys.version_info`` value is now a named tuple, with attributes
named ``major``, ``minor``, ``micro``, ``releaselevel``, and ``serial``.
(Contributed by Ross Light; :issue:`4285`.)
* The :mod:`threading` module's :meth:`Event.wait` method now returns
the internal flag on exit. This means the method will usually
return true because :meth:`wait` is supposed to block until the
internal flag becomes true. The return value will only be false if
a timeout was provided and the operation timed out.
(Contributed by XXX; :issue:`1674032`.)
* The :mod:`unittest` module was enhanced in several ways.
The progress messages will now show 'x' for expected failures
and 'u' for unexpected successes when run in verbose mode.
(Contributed by Benjamin Peterson.)
Test cases can raise the :exc:`SkipTest` exception to skip a test.
(:issue:`1034053`.)
The error messages for :meth:`assertEqual`,
:meth:`assertTrue`, and :meth:`assertFalse`
failures now provide more information. If you set the
:attr:`longMessage` attribute of your :class:`TestCase` classes to
true, both the standard error message and any additional message you
provide will be printed for failures. (Added by Michael Foord; :issue:`5663`.)
The :meth:`assertRaises` and :meth:`failUnlessRaises` methods now
return a context handler when called without providing a callable
object to run. For example, you can write this::
with self.assertRaises(KeyError):
raise ValueError
(Implemented by Antoine Pitrou; :issue:`4444`.)
The methods :meth:`addCleanup` and :meth:`doCleanups` were added.
:meth:`addCleanup` allows you to add cleanup functions that
will be called unconditionally (after :meth:`setUp` if
:meth:`setUp` fails, otherwise after :meth:`tearDown`). This allows
for much simpler resource allocation and deallocation during tests.
:issue:`5679`
A number of new methods were added that provide more specialized
tests. Many of these methods were written by Google engineers
for use in their test suites; Gregory P. Smith, Michael Foord, and
GvR worked on merging them into Python's version of :mod:`unittest`.
* :meth:`assertIsNone` and :meth:`assertIsNotNone` take one
expression and verify that the result is or is not ``None``.
* :meth:`assertIs` and :meth:`assertIsNot` take two values and check
whether the two values evaluate to the same object or not.
(Added by Michael Foord; :issue:`2578`.)
* :meth:`assertGreater`, :meth:`assertGreaterEqual`,
:meth:`assertLess`, and :meth:`assertLessEqual` compare
two quantities.
* :meth:`assertMultiLineEqual` compares two strings, and if they're
not equal, displays a helpful comparison that highlights the
differences in the two strings.
* :meth:`assertRegexpMatches` checks whether its first argument is a
string matching a regular expression provided as its second argument.
* :meth:`assertRaisesRegexp` checks whether a particular exception
is raised, and then also checks that the string representation of
the exception matches the provided regular expression.
* :meth:`assertIn` and :meth:`assertNotIn` tests whether
*first* is or is not in *second*.
* :meth:`assertSameElements` tests whether two provided sequences
contain the same elements.
* :meth:`assertSetEqual` compares whether two sets are equal, and
only reports the differences between the sets in case of error.
* Similarly, :meth:`assertListEqual` and :meth:`assertTupleEqual`
compare the specified types and explain the differences.
More generally, :meth:`assertSequenceEqual` compares two sequences
and can optionally check whether both sequences are of a
particular type.
* :meth:`assertDictEqual` compares two dictionaries and reports the
differences. :meth:`assertDictContainsSubset` checks whether
all of the key/value pairs in *first* are found in *second*.
* A new hook, :meth:`addTypeEqualityFunc` takes a type object and a
function. The :meth:`assertEqual` method will use the function
when both of the objects being compared are of the specified type.
This function should compare the two objects and raise an
exception if they don't match; it's a good idea for the function
to provide additional information about why the two objects are
matching, much as the new sequence comparison methods do.
:func:`unittest.main` now takes an optional ``exit`` argument.
If False ``main`` doesn't call :func:`sys.exit` allowing it to
be used from the interactive interpreter. :issue:`3379`.
:class:`TestResult` has new :meth:`startTestRun` and
:meth:`stopTestRun` methods; called immediately before
and after a test run. :issue:`5728` by Robert Collins.
* The :func:`is_zipfile` function in the :mod:`zipfile` module will now
accept a file object, in addition to the path names accepted in earlier
versions. (Contributed by Gabriel Genellina; :issue:`4756`.)
:mod:`zipfile` now supports archiving empty directories and
extracts them correctly. (Fixed by Kuba Wieczorek; :issue:`4710`.)
.. ======================================================================
.. whole new modules get described in subsections here
importlib: Importing Modules
------------------------------
Python 3.1 includes the :mod:`importlib` package, a re-implementation
of the logic underlying Python's :keyword:`import` statement.
:mod:`importlib` is useful for implementors of Python interpreters and
to user who wish to write new importers that can participate in the
import process. Python 2.7 doesn't contain the complete
:mod:`importlib` package, but instead has a tiny subset that contains
a single function, :func:`import_module`.
``import_module(*name*, *package*=None)`` imports a module. *name* is
a string containing the module or package's name. It's possible to do
relative imports by providing a string that begins with a ``.``
character, such as ``..utils.errors``. For relative imports, the
*package* argument must be provided and is the name of the package that
will be used as the anchor for
the relative import. :func:`import_module` both inserts the imported
module into ``sys.modules`` and returns the module object.
Here are some examples::
>>> from importlib import import_module
>>> anydbm = import_module('anydbm') # Standard absolute import
>>> anydbm
<module 'anydbm' from '/p/python/Lib/anydbm.py'>
>>> # Relative import
>>> sysconfig = import_module('..sysconfig', 'distutils.command')
>>> sysconfig
<module 'distutils.sysconfig' from '/p/python/Lib/distutils/sysconfig.pyc'>
:mod:`importlib` was implemented by Brett Cannon and introduced in
Python 3.1.
ttk: Themed Widgets for Tk
--------------------------
Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform's widgets. This widget
set was originally called Tile, but was renamed to Ttk (for "themed Tk")
on being added to Tcl/Tck release 8.5.
XXX write a brief discussion and an example here.
The :mod:`ttk` module was written by Guilherme Polo and added in
:issue:`2983`. An alternate version called ``Tile.py``, written by
Martin Franklin and maintained by Kevin Walzer, was proposed for
inclusion in :issue:`2618`, but the authors argued that Guilherme
Polo's work was more comprehensive.
.. ======================================================================
Build and C API Changes
=======================
Changes to Python's build process and to the C API include:
* If you use the :file:`.gdbinit` file provided with Python,
the "pyo" macro in the 2.7 version will now work when the thread being
debugged doesn't hold the GIL; the macro will now acquire it before printing.
(Contributed by Victor Stinner; :issue:`3632`.)
* :cfunc:`Py_AddPendingCall` is now thread-safe, letting any
worker thread submit notifications to the main Python thread. This
is particularly useful for asynchronous IO operations.
(Contributed by Kristjan Valur Jonsson; :issue:`4293`.)
* Global symbols defined by the :mod:`ctypes` module are now prefixed
with ``Py`, or with ``_ctypes``. (Implemented by Thomas
Heller; :issue:`3102`.)
* The :program:`configure` script now checks for floating-point rounding bugs
on certain 32-bit Intel chips and defines a :cmacro:`X87_DOUBLE_ROUNDING`
preprocessor definition. No code currently uses this definition,
but it's available if anyone wishes to use it.
(Added by Mark Dickinson; :issue:`2937`.)
.. ======================================================================
Port-Specific Changes: Windows
-----------------------------------
* The :mod:`msvcrt` module now contains some constants from
the :file:`crtassem.h` header file:
:data:`CRT_ASSEMBLY_VERSION`,
:data:`VC_ASSEMBLY_PUBLICKEYTOKEN`,
and :data:`LIBRARIES_ASSEMBLY_NAME_PREFIX`.
(Contributed by David Cournapeau; :issue:`4365`.)
* The new :cfunc:`_beginthreadex` API is used to start threads, and
the native thread-local storage functions are now used.
(Contributed by Kristjan Valur Jonsson; :issue:`3582`.)
.. ======================================================================
Port-Specific Changes: Mac OS X
-----------------------------------
* The ``/Library/Python/2.7/site-packages`` is now appended to
``sys.path``, in order to share added packages between the system
installation and a user-installed copy of the same version.
(Changed by Ronald Oussoren; :issue:`4865`.)
Other Changes and Fixes
=======================
* When importing a module from a :file:`.pyc` or :file:`.pyo` file
with an existing :file:`.py` counterpart, the :attr:`co_filename`
attributes of the resulting code objects are overwritten when the
original filename is obsolete. This can happen if the file has been
renamed, moved, or is accessed through different paths. (Patch by
Ziga Seilnacht and Jean-Paul Calderone; :issue:`1180193`.)
* The :file:`regrtest.py` script now takes a :option:`--randseed=`
switch that takes an integer that will be used as the random seed
for the :option:`-r` option that executes tests in random order.
The :option:`-r` option also now reports the seed that was used
(Added by Collin Winter.)
* The :file:`regrtest.py` script now takes a :option:`-j` switch
that takes an integer specifying how many tests run in parallel. This
allows to shorten the total runtime on multi-core machines.
This option is compatible with several other options, including the
:option:`-R` switch which is known to produce long runtimes.
(Added by Antoine Pitrou, :issue:`6152`.)
.. ======================================================================
Porting to Python 2.7
=====================
This section lists previously described changes and other bugfixes
that may require changes to your code:
* Because of an optimization for the :keyword:`with` statement, the special
methods :meth:`__enter__` and :meth:`__exit__` must belong to the object's
type, and cannot be directly attached to the object's instance. This
affects new-style classes (derived from :class:`object`) and C extension
types. (:issue:`6101`.)
.. ======================================================================
.. _acks27:
Acknowledgements
================
The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: no one yet.
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