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 recommended for managing resources like file handles during iteration. The document emphasizes separating administrative from business logic and using tools like generators and context managers.

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!