This document summarizes the key changes and new features introduced in Python 3, including: - Strings are now unicode by default and implicit conversions between str and bytes are prohibited. - Integers are unified under int and long is removed. - New features like formatted string literals, function annotations, nonlocal and keyword-only arguments, extended iterable unpacking, comprehensions, exception chaining, and yield from. - New collection abstract base classes and data structures like OrderedDict and Counter. - Metaclasses can now control class creation and new metaclass like OrderedClass is introduced. - Under the hood improvements including a new GIL and stable ABI version tagged shared object files.

1. Python 3
Андрей Светлов
@andrew_svetlov
andrew.svetlov@gmail.com
http://asvetlov.blogspot.com

2. Releases
2.6 — Oct 2008
3.0 — Dec 2008
3.1 — Jul 2009
2.7 — Jul 2010
3.2 — Feb 2011
3.3 — Aug 2012

3. Строки
Только unicode
Utf-8 — кодировка по умолчанию
Неявные преобразования str↔bytes
запрещены.

4. Числа
Unified int
Long — отсутствует
Дерево числовых типов
Decimal — теперь на C

5. Форматирование строк
"{a:s} {b:.2f}".format(a='строка', b=3.14159)
"{c[0]}-{c[1]}".format(c=(1, 2))
"{0.x}, {0.y}".format(coord)

6. Function annotations
def f(a: int, b: float) -> str:
return "{}:{}".format(a, b)
>>> f.__annotations__
{'a': builtins.int, 'b': builtins.float,
'return': builtins.str}

7. Nonlocal
def f():
a=0
def g():
nonlocal a
a += 1
g()
return a

8. Keyword only
>>> def f(a, b=0, *, c, d='Smith'):
... pass
>>> f(1, c='John', d='Doe')
>>> f(1, c='Mary')
>>> f(1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: f() needs keyword-only argument c

9. Extended iterable unpacking
a, *rest = range(5)
a, *rest, b = range(5)

10. Comprehensions
{key(k): value(k) for k in range(5)}
{f(k) for k in range(5)}
{1, 2, 3, 4, 5}

11. ABC and new `super()`
class Base(metaclass=abc.ABCMeta):
@abc.abstractmethod
def f(self, a):
"""Comprehensive doc"""
pass
class A(Base):
def f(self, a):

12. Exception chain
def f():
try:
1/0
except Exception as ex:
raise RuntimeError("Oops") from ex
f()

13. Exception chain 2
Traceback (most recent call last):
File "<string>", line 3, in f
1/0
ZeroDivisionError: division by zero
The above exception was the direct cause of
the following exception:
Traceback (most recent call last):

14. Абстрактные типы для коллекций
Hashable
Iterable
Sized
Container
Callable
Sequence
etc.

15. OrderedDict
>>> d = {'banana': 3, 'apple':4, 'pear': 1, 'orange':
2}
# dictionary sorted by key
>>> OrderedDict(sorted(d.items(), key=lambda t:
t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange',
2), ('pear', 1)])

16. Counter
>>> cnt = Counter()
>>> for word in ['red', 'blue', 'red',
… 'green', 'blue', 'blue']:
... cnt[word] += 1
>>> cnt
Counter({'blue': 3, 'red': 2, 'green': 1})

17. Yield from
>>> def g(x):
... yield from range(x, 0, -1)
... yield from range(x)
...
>>> list(g(5))
[5, 4, 3, 2, 1, 0, 1, 2, 3, 4]

18. Новые метаклассы
class OrderedClass(type):
@classmethod
def __prepare__(metacls, name, bases,
**kwds):
return collections.OrderedDict()
def __new__(cls, name, bases, classdict):
result = type.__new__(cls, name, bases,
dict(classdict))
result.members = tuple(classdict)

19. Незаметные вкусности
Новый GIL
Importlib
Stable ABI
PYC repository dirs
ABI version tagged .so files
Packaging

20. Вопросы?
@andrew_svetlov
andrew.svetlov@gmail.com
http://asvetlov.blogspot.com