The document discusses various techniques for iteration in Python. It covers iterating over lists, dictionaries, files and more. It provides examples of iterating properly to avoid errors like modifying a list during iteration. Context managers are also discussed as a clean way to handle resources like file objects. Overall the document shares best practices for writing efficient and robust iteration code in Python.

2. Python: легко и просто.
Красиво решаем повседневные задачи.
Александр Семенюк
a_semianiuk@wargaming.net

3. Python делает всё, чтобы упростить вашу жизнь.
Этот язык предоставляет многообразие инструментов,
позволяющих легко решать многие задачи.

4. if len(mylist) == 0:
raise Exception('Empty list is not acceptable')

5. >>> bool( 0 )
False
>>> bool( [] )
False
>>> bool( {} )
False
>>> bool( None )
False
>>> bool( '' )
False

6. if mylist:
raise Exception('Empty list is not acceptable')

7. class Basket(object):
empty = True
items = []
def __nonzero__(self):
return not self.empty
def __len__(self):
return len(self.items)
basket = Basket()
print bool(basket) # False

8. if mylist == None:
do_something()

9. if mylist == None:
do_something()
if mylist is None:
do_something()

10. $ python -m timeit -s "x = None" "x is None"
10000000 loops, best of 3: 0.0329 usec per loop
$ python -m timeit -s "x = None" "x == None"
10000000 loops, best of 3: 0.0607 usec per loop

11. • Всё в Python – это объект.
• Каждая переменная – ссылка на объект.
• Оператор is сравнивает эти ссылки.
class A:
pass
a = A()
b = a
print a is b
print id(a)
print id(b)
True
33274616
33274616

12. • В Python есть неизменяемые объекты. Их значение не меняется
после создания:
• Числа - 0, 245, 10000000000000001
• Строки - 'Hi people!'
• Кортежи (неизменяемые списки) - (1,2, 'a')
• И, конечно же, None !

13. if mylist:
raise Exception('Empty list is not acceptable')

14. BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StandardError
| +-- BufferError
| +-- ArithmeticError
| | +-- FloatingPointError
| | +-- OverflowError
| | +-- ZeroDivisionError
…

15. class EmptyListError(Exception):
pass
if not mylist:
raise EmptyListError('Empty list is not acceptable')

16. try:
work_with_list([])
except EmptyListError:
# we know what's wrong
try:
work_with_list([])
except Exception:
# what to do?

17. • Python знает, где истина.
• ‘is None’ лучше, чем ‘== None’.
• Всё объект!
• Создавать свои собственные исключения круто.

18. try:
f = open('config.txt')
line = f.readline()
while line:
line = line.strip(' n')
if not line:
continue
if line.startswith('#'):
continue
do_some_staff(line)
line = f.readline()
finally:
f.close()

19. for i in range(len(mylist)):
print mylist[i]

20. for i in range(len(mylist)):
print mylist[i]
for item in mylist:
print item

21. import itertools
for i in itertools.count():
print i

22. for item in reversed(mylist):
print item

23. for item in reversed(mylist):
print item
>>> mylist = [1,2,3]
>>> mylist[::-1]
[3, 2, 1]

24. $ python -m timeit -s "l = [1, 2, 3]" "list(reversed(l))"
1000000 loops, best of 3: 0.995 usec per loop
$ python -m timeit -s "l = [1, 2, 3]" "l[::-1]"
1000000 loops, best of 3: 0.202 usec per loop

25. $ python -m timeit -s "l = [1, 2, 3]" "for i in reversed(l): pass"
1000000 loops, best of 3: 0.28 usec per loop
$ python -m timeit -s "l = [1, 2, 3]" "for i in l[::-1]: pass"
1000000 loops, best of 3: 0.318 usec per loop

26. heroes = ['Batman', 'Spiderman', 'Hulk', 'Invisible Woman']
colors = ['black', 'red', 'green']
for i in range(min(len(heroes), len(colors)):
print list1[i], 'is', list2[i]

27. for hero, color in zip(heroes, colors):
print hero, 'is', color

28. heroes_with_colors = dict(zip(heroes, colors))
print heroes_with_colors
{
'Spiderman': 'red',
'Batman': 'black',
'Hulk': 'green'
}

29. from itertools import izip
for hero, color in izip(heroes, colors):
print hero, 'is', color

30. >>> for i in 'python': i
...
'p'
'y'
't'
'h'
'o'
'n'

31. >>> for i in 'python': i
...
'p'
'y'
't'
'h'
'o'
'n'
>>> for key in heroes_with_colors: key
...
'Spiderman'
'Batman'
'Hulk'

32. >>> for i in 'python': i
...
'p'
'y'
't'
'h'
'o'
'n'
>>> for key in heroes_with_colors: key
...
'Spiderman'
'Batman'
'Hulk'
>>> import re
>>> for match in re.finditer(pattern, string):
... # each match in the string
...

33. >>> for line in myfile:
... print repr(line) # each line in a file
...

34. f = open('config.txt')
try:
for line in f:
line = line.strip(' n')
if not line:
continue
if line.startswith('#'):
continue
do_some_staff(line)
finally:
f.close()

35. def iterate_lines(f):
for line in f:
line = line.strip(' n')
if not line:
continue
if line.startswith('#'):
continue
yield line

36. f = open('config.txt')
try:
for line in iterate_lines(f):
do_some_staff(line)
finally:
f.close()

37. >>> mylist
['a', 'b', 'b', 'c']
>>> for index, item in enumerate(mylist):
... if item == 'b':
... del mylist[index]
...
>>> mylist
['a', 'b', 'c']

38. >>> mydict
{'b': 2}
>>> for key in mydict:
... if key == 'b':
... del mydict[key]
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

39. >>> for key in mydict.keys():
... if key == 'b':
... del mydict[key]
...
>>> mydict
{}

40. f = open('config.txt')
try:
for line in iterate_lines(f):
do_some_staff(line)
finally:
f.close()

41. with EXPR as VAR:
BLOCK

42. with open('config.txt') as f:
process_file(f)

43. try:
dangerous_actions()
except AttributeError:
pass # we don't care

44. from contextlib import contextmanager
@contextmanager
def ignore_errors(*errors):
try:
yield
except errors:
pass

45. >>> with ignore_errors(KeyError, AttributeError):
... raise KeyError
...
>>>

46. with open('config.txt') as f:
for line in iterate_lines(f):
do_some_staff(line)

47. • Итерируйтесь правильно
• Отделяйте административную логику от бизнес логики
• Не изменяйте список, по которому итерируетесь
• Контекстные менеджеры молодцы

48. Большое спасибо за внимание!