Python 3.1 有什么新变化

作者

Raymond Hettinger

This article explains the new features in Python 3.1, compared to 3.0.

PEP 372: 有序字典

常规的 Python 字典会以任意顺序迭代键/值对。 多年以来，有好几位作者编写了可以记住键的初始插入顺序的替代实现。 基于这些实现的经验，现在引入了新的 collections.OrderedDict 类。

The OrderedDict API is substantially the same as regular dictionaries but will iterate over keys and values in a guaranteed order depending on when a key was first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end.

The standard library now supports use of ordered dictionaries in several modules. The configparser module uses them by default. This lets configuration files be read, modified, and then written back in their original order. The _asdict() method for collections.namedtuple() now returns an ordered dictionary with the values appearing in the same order as the underlying tuple indices. The json module is being built-out with an object_pairs_hook to allow OrderedDicts to be built by the decoder. Support was also added for third-party tools like PyYAML.

参见

PEP 372 - 有序字典

PEP 由 Armin Ronacher 和 Raymond Hettinger 撰写，由 Raymond Hettinger 实现。

PEP 378: 千位分隔符的格式说明符

The built-in format() function and the str.format() method use a mini-language that now includes a simple, non-locale aware way to format a number with a thousands separator. That provides a way to humanize a program's output, improving its professional appearance and readability:

>>> format(1234567, ',d')
'1,234,567'
>>> format(1234567.89, ',.2f')
'1,234,567.89'
>>> format(12345.6 + 8901234.12j, ',f')
'12,345.600000+8,901,234.120000j'
>>> format(Decimal('1234567.89'), ',f')
'1,234,567.89'

支持的类型有 int, float, complex 和 decimal.Decimal。

Discussions are underway about how to specify alternative separators like dots, spaces, apostrophes, or underscores. Locale-aware applications should use the existing n format specifier which already has some support for thousands separators.

参见

PEP 378 - 千位分隔符的格式说明符

PEP 由 Raymond Hettinger 撰写，并由 Eric Smith 和 Mark Dickinson 实现

其他语言特性修改

对Python 语言核心进行的小改动：

  • Directories and zip archives containing a __main__.py file can now be executed directly by passing their name to the interpreter. The directory/zipfile is automatically inserted as the first entry in sys.path. (Suggestion and initial patch by Andy Chu; revised patch by Phillip J. Eby and Nick Coghlan; bpo-1739468.)

  • int() 类型增加了一个 bit_length 方法用来返回以二进制代表其参数值所需的比特位数量:

    >>> n = 37
>>> bin(37)
'0b100101'
>>> n.bit_length()
6
>>> n = 2**123-1
>>> n.bit_length()
123
>>> (n+1).bit_length()
124

    （由 Fredrik Johansson, Victor Stinner, Raymond Hettinger 和 Mark Dickinson 在 bpo-3439 中贡献。）

  • format() 字符串中的字段现在可被自动编号:

    >>> 'Sir {} of {}'.format('Gallahad', 'Camelot')
'Sir Gallahad of Camelot'

    之前，字符串应当具有所需的编号字段例如: 'Sir {0} of {1}'。

    （由 Eric Smith在 bpo-5237 中贡献）

  • The string.maketrans() function is deprecated and is replaced by new static methods, bytes.maketrans() and bytearray.maketrans(). This change solves the confusion around which types were supported by the string module. Now, str, bytes, and bytearray each have their own maketrans and translate methods with intermediate translation tables of the appropriate type.

    （由Georg Brandl在 bpo-5675 中贡献）

  • The syntax of the with 语句的语法现在允许单个语句中存在多个上下文管理器:

    >>> with open('mylog.txt') as infile, open('a.out', 'w') as outfile:
...     for line in infile:
...         if '<critical>' in line:
...             outfile.write(line)

    有了这个新语法，contextlib.nested() 函数已不再必要因而现在已被弃用。

    （由 Georg Brandl 和 Mattias Brändström 贡献; appspot issue 53094。）

  • 现在 round(x, n) 当 x 为整数时将返回整数。 之前是返回浮点数:

    >>> round(1123, -2)
1100

    （由 Mark Dickinson在 bpo-4707 贡献）

  • Python now uses David Gay's algorithm for finding the shortest floating point representation that doesn't change its value. This should help mitigate some of the confusion surrounding binary floating point numbers.

    The significance is easily seen with a number like 1.1 which does not have an exact equivalent in binary floating point. Since there is no exact equivalent, an expression like float('1.1') evaluates to the nearest representable value which is 0x1.199999999999ap+0 in hex or 1.100000000000000088817841970012523233890533447265625 in decimal. That nearest value was and still is used in subsequent floating point calculations.

    What is new is how the number gets displayed. Formerly, Python used a simple approach. The value of repr(1.1) was computed as format(1.1, '.17g') which evaluated to '1.1000000000000001'. The advantage of using 17 digits was that it relied on IEEE-754 guarantees to assure that eval(repr(1.1)) would round-trip exactly to its original value. The disadvantage is that many people found the output to be confusing (mistaking intrinsic limitations of binary floating point representation as being a problem with Python itself).

    The new algorithm for repr(1.1) is smarter and returns '1.1'. Effectively, it searches all equivalent string representations (ones that get stored with the same underlying float value) and returns the shortest representation.

    The new algorithm tends to emit cleaner representations when possible, but it does not change the underlying values. So, it is still the case that 1.1 + 2.2 != 3.3 even though the representations may suggest otherwise.

    The new algorithm depends on certain features in the underlying floating point implementation. If the required features are not found, the old algorithm will continue to be used. Also, the text pickle protocols assure cross-platform portability by using the old algorithm.

    （由 Eric Smith 和 Mark Dickinson 在 bpo-1580 贡献）

新增，改进和弃用的模块

  • 增加了一个 collections.Counter 类以支持方便地统计一个序列或可迭代对象中的唯一条目数量。:

    >>> Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
Counter({'blue': 3, 'red': 2, 'green': 1})

    （由 Raymond Hettinger 在 bpo-1696199 中贡献。）

  • Added a new module, tkinter.ttk for access to the Tk themed widget set. The basic idea of ttk is to separate, to the extent possible, the code implementing a widget's behavior from the code implementing its appearance.

    （由 Guilherme Polo 在 bpo-2983 中贡献。）

  • gzip.GzipFile 和 bz2.BZ2File 类现在已支持上下文管理协议:

    >>> # Automatically close file after writing
>>> with gzip.GzipFile(filename, "wb") as f:
...     f.write(b"xxx")

    （由 Antoine Pitrou 贡献。）

  • decimal 模块现在支持基于一个二进制 float 来创建 decimal 对象。 转换是准确的但有时也会令人吃惊:

    >>> Decimal.from_float(1.1)
Decimal('1.100000000000000088817841970012523233890533447265625')

    这个长长的 decimal 结果值显示了 1.1 所保存的实际二进制分数。 这个分数有许多位因为 1.1 无法用二进制来精确表示。

    （由Raymond Hettinger 和 Mark Dickinson贡献。）

  • The itertools module grew two new functions. The itertools.combinations_with_replacement() function is one of four for generating combinatorics including permutations and Cartesian products. The itertools.compress() function mimics its namesake from APL. Also, the existing itertools.count() function now has an optional step argument and can accept any type of counting sequence including fractions.Fraction and decimal.Decimal:

    >>> [p+q for p,q in combinations_with_replacement('LOVE', 2)]
['LL', 'LO', 'LV', 'LE', 'OO', 'OV', 'OE', 'VV', 'VE', 'EE']

>>> list(compress(data=range(10), selectors=[0,0,1,1,0,1,0,1,0,0]))
[2, 3, 5, 7]

>>> c = count(start=Fraction(1,2), step=Fraction(1,6))
>>> [next(c), next(c), next(c), next(c)]
[Fraction(1, 2), Fraction(2, 3), Fraction(5, 6), Fraction(1, 1)]

    （由 Raymond Hettinger 贡献。）

  • collections.namedtuple() now supports a keyword argument rename which lets invalid fieldnames be automatically converted to positional names in the form _0, _1, etc. This is useful when the field names are being created by an external source such as a CSV header, SQL field list, or user input:

    >>> query = input()
SELECT region, dept, count(*) FROM main GROUPBY region, dept

>>> cursor.execute(query)
>>> query_fields = [desc[0] for desc in cursor.description]
>>> UserQuery = namedtuple('UserQuery', query_fields, rename=True)
>>> pprint.pprint([UserQuery(*row) for row in cursor])
[UserQuery(region='South', dept='Shipping', _2=185),
 UserQuery(region='North', dept='Accounting', _2=37),
 UserQuery(region='West', dept='Sales', _2=419)]

    （由 Raymond Hettinger 在 bpo-1818 中贡献。）

  • re.sub(), re.subn() 和 re.split() 函数现在可接受一个 flags 形参。

    （由 Gregory Smith 贡献）

  • The logging module now implements a simple logging.NullHandler class for applications that are not using logging but are calling library code that does. Setting-up a null handler will suppress spurious warnings such as "No handlers could be found for logger foo":

    >>> h = logging.NullHandler()
>>> logging.getLogger("foo").addHandler(h)

    （由 Vinay Sajip 在 bpo-4384 中贡献。）

  • 支持 -m 命令行开关的 runpy 模块现在也支持当提供包名称时通过查找并执行 __main__ 子模块来执行包。

    （由 Andi Vajda 在 bpo-4195 中贡献。）

  • pdb 模块现在可以访问并显示通过 zipimport (或其他符合规范的 PEP 302 加载器) 加载的源代码。

    （由 Alexander Belopolsky 在 bpo-4201 中贡献。）

  • functools.partial 对象现在可以被封存。

（由 Antoine Pitrou 和 Jesse Noller 提议，由 Jack Diederich 实现; bpo-5228。）

  • 为符号增加 pydoc 帮助主题以使得在交互环境下 help('@') 能符合预期的效果。

    （由 David Laban 在 bpo-4739 中贡献。）

  • The unittest module now supports skipping individual tests or classes of tests. And it supports marking a test as an expected failure, a test that is known to be broken, but shouldn't be counted as a failure on a TestResult:

    class TestGizmo(unittest.TestCase):

    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
    def test_gizmo_on_windows(self):
        ...

    @unittest.expectedFailure
    def test_gimzo_without_required_library(self):
        ...

    Also, tests for exceptions have been builtout to work with context managers using the with statement:

    def test_division_by_zero(self):
    with self.assertRaises(ZeroDivisionError):
        x / 0

    In addition, several new assertion methods were added including assertSetEqual(), assertDictEqual(), assertDictContainsSubset(), assertListEqual(), assertTupleEqual(), assertSequenceEqual(), assertRaisesRegexp(), assertIsNone(), and assertIsNotNone().

    （由Benjamin Peterson 和 Antoine Pitrou 贡献。）

  • io 模块新增了三个常量来代表 seek() 方法 SEEK_SET, SEEK_CUR 和 SEEK_END。

  • sys.version_info 元组现在是一个具名元组:

    >>> sys.version_info
sys.version_info(major=3, minor=1, micro=0, releaselevel='alpha', serial=2)

    （由 Ross Light 在 bpo-4285 中贡献。）

  • The nntplib and imaplib modules now support IPv6.

    （由 Derek Morr 在 bpo-1655 和 bpo-1664 中贡献。）

  • The pickle module has been adapted for better interoperability with Python 2.x when used with protocol 2 or lower. The reorganization of the standard library changed the formal reference for many objects. For example, __builtin__.set in Python 2 is called builtins.set in Python 3. This change confounded efforts to share data between different versions of Python. But now when protocol 2 or lower is selected, the pickler will automatically use the old Python 2 names for both loading and dumping. This remapping is turned-on by default but can be disabled with the fix_imports option:

    >>> s = {1, 2, 3}
>>> pickle.dumps(s, protocol=0)
b'c__builtin__\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'
>>> pickle.dumps(s, protocol=0, fix_imports=False)
b'cbuiltins\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'

    An unfortunate but unavoidable side-effect of this change is that protocol 2 pickles produced by Python 3.1 won't be readable with Python 3.0. The latest pickle protocol, protocol 3, should be used when migrating data between Python 3.x implementations, as it doesn't attempt to remain compatible with Python 2.x.

    （由 Alexandre Vassalotti 和 Antoine Pitrou 在 bpo-6137 中贡献。）

  • A new module, importlib was added. It provides a complete, portable, pure Python reference implementation of the import statement and its counterpart, the __import__() function. It represents a substantial step forward in documenting and defining the actions that take place during imports.

    （由 Brett Cannon 贡献。）

性能优化

Major performance enhancements have been added:

  • The new I/O library (as defined in PEP 3116) was mostly written in Python and quickly proved to be a problematic bottleneck in Python 3.0. In Python 3.1, the I/O library has been entirely rewritten in C and is 2 to 20 times faster depending on the task at hand. The pure Python version is still available for experimentation purposes through the _pyio module.

    （由 Amaury Forgeot d'Arc 和 Antoine Pitrou 贡献。）

  • Added a heuristic so that tuples and dicts containing only untrackable objects are not tracked by the garbage collector. This can reduce the size of collections and therefore the garbage collection overhead on long-running programs, depending on their particular use of datatypes.

    （由 Antoine Pitrou 在 bpo-4688 中贡献。）

  • Enabling a configure option named --with-computed-gotos on compilers that support it (notably: gcc, SunPro, icc), the bytecode evaluation loop is compiled with a new dispatch mechanism which gives speedups of up to 20%, depending on the system, the compiler, and the benchmark.

    （由 Antoine Pitrou 以及其他一些参与者在 bpo-4753 中贡献。）

  • The decoding of UTF-8, UTF-16 and LATIN-1 is now two to four times faster.

    （由 Antoine Pitrou 和 Amaury Forgeot d'Arc 在 bpo-4868 中贡献。）

  • The json module now has a C extension to substantially improve its performance. In addition, the API was modified so that json works only with str, not with bytes. That change makes the module closely match the JSON specification which is defined in terms of Unicode.

    （由 Bob Ippolito 在 bpo-4136 中贡献。并由 Antoine Pitrou 和 Benjamin Peterson 转换为Py3.1）

  • Unpickling now interns the attribute names of pickled objects. This saves memory and allows pickles to be smaller.

    （由 Jake McGuire 和 Antoine Pitrou 在 bpo-5084 中贡献。）

IDLE

  • IDLE's format menu now provides an option to strip trailing whitespace from a source file.

    （由 Roger D. Serwy 在 bpo-5150 中贡献。）

构建和 C API 的改变

针对 Python 构建过程和 C API 的改变包括:

  • 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 --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 sys.int_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.int_info
sys.int_info(bits_per_digit=30, sizeof_digit=4)

    （由 Mark Dickinson在 bpo-4258 贡献）

  • The PyLong_AsUnsignedLongLong() function now handles a negative pylong by raising OverflowError instead of TypeError.

    （由 Mark Dickinson 和 Lisandro Dalcrin 在 bpo-5175 中贡献。）

  • Deprecated PyNumber_Int(). Use PyNumber_Long() instead.

    （由 Mark Dickinson在 bpo-4910 贡献）

  • Added a new PyOS_string_to_double() function to replace the deprecated functions PyOS_ascii_strtod() and PyOS_ascii_atof().

    （由 Mark Dickinson 在 bpo-5914 贡献）

  • Added PyCapsule as a replacement for the PyCObject API. The principal difference is that the new type has a well defined interface for passing typing safety information and a less complicated signature for calling a destructor. The old type had a problematic API and is now deprecated.

    （由 Larry Hastings 在 bpo-5630 中贡献。）

移植到 Python 3.1

This section lists previously described changes and other bugfixes that may require changes to your code:

  • The new floating point string representations can break existing doctests. For example:

    def e():
    '''Compute the base of natural logarithms.

    >>> e()
    2.7182818284590451

    '''
    return sum(1/math.factorial(x) for x in reversed(range(30)))

doctest.testmod()

**********************************************************************
Failed example:
    e()
Expected:
    2.7182818284590451
Got:
    2.718281828459045
**********************************************************************

  • The automatic name remapping in the pickle module for protocol 2 or lower can make Python 3.1 pickles unreadable in Python 3.0. One solution is to use protocol 3. Another solution is to set the fix_imports option to False. See the discussion above for more details.