Best practices

• enumerate
• generators
• genexps
• iteators
• with et contextlib

enumerate

Sous-utilisé !

>>> liste = ('un', 'deux', 'trois')
>>> for index, val in enumerate(liste):
...     print '%d: %s' % (index, val)
...
0: un
1: deux
2: trois

Generators

• yield est devenu une expression (2.5)

  >>> def foo():
  ...     answer = (yield)
  ...     while True:
  ...         answer = yield answer + 1
  ...
  >>> gen = foo()
  >>> gen.next()
  >>> gen.send(1)
  2
  >>> gen.send(45)
  46
  >>> gen.send(89)
  90
  

Generators

• possibilité d'écrire des co-routines
• extension multitask sous PyPI
• greenlet (dispo sans les expr. yield)

Genexps

>>> iter = (x**2 for x in range(10) if x % 2 == 0)
>>> for el in iter:
...     print el
...
0
4
16
36
64

Iterators

utilisez itertools : compilé en C

• islice
• tee
• groupby

islice

• islice, iterator fenêtre

>>> import itertools
>>> def starting_at_five():
...     value = raw_input().strip()
...     while value != '':
...         for el in itertools.islice(value.split(),
...                                    4, None):
...             yield el
...         value = raw_input().strip()
...
>>> iter = starting_at_five()
>>> iter.next()
one two three four five six
'five'
>>> iter.next()
'six'
>>> iter.next()
one two
one two three four five six
'five'
>>> iter.next()
'six'

Tee

• tee: multi-iterator:

  >>> import itertools
  >>> seq = range(1000)
  >>> def with_head(iterable, headsize=1):
  ...     a, b = itertools.tee(iterable)
  ...     return list(itertools.islice(a, headsize)), b
  ...
  >>> with_head(seq)
  ([1], <itertools.tee object at 0x100c698>)
  >>> with_head(seq, 4)
  ([1, 2, 3, 4], <itertools.tee object at 0x100c670>)
  

groupby

• semblable à uniq:

  >>> from itertools import groupby
  >>> def compress(data):
  ...     return ((len(list(group)), name)
  ...             for name, group in groupby(data))
  ...
  >>> def decompress(data):
  ...     return (car * size for size, car in data)
  ...
  >>> list(compress('get uuuuuuuuuuuuuuuuuup'))
  [(1, 'g'), (1, 'e'), (1, 't'), (1, ' '),
   (18, 'u'), (1, 'p')]
  >>> compressed = compress('get uuuuuuuuuuuuuuuuuup')
  >>> ''.join(decompress(compressed))
  'get uuuuuuuuuuuuuuuuuup'
  

with et contextlib

Introduit dans 2.6 mais dispo dans 2.5 avec __future__. Sans with:

>>> f = open(my_file, 'w')
>>> try:
...     f.write('pycon fr, ca roxor')
... finally:
...     f.close()

with et contextlib

Avec with:

>>> from __future__ import with_statement
>>> with open(my_file, 'w') as f:
...      f.write('pycon fr, ca roxor')

contextlib

Création de ses propres contextes:

• implementer __enter__ et __exit__