リスト内包表記

Pythonにはリスト内容表記がある。for～in～を使ってこうやって書く：

>>> [x*x for x in [1,2,3,4,5]]
[1, 4, 9, 16, 25]

表記こそ違うけどやってることはHaskellのリスト内包表記とおなじだ。

forは複数つけることもできる：

>>> [(x,y) for x in [1,2,3,4,5] for y in [1,2,3,4,5]]
[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5),
 (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5),
 (5, 1), (5, 2), (5, 3), (5, 4), (5, 5)]

これは次の二重のforと同じ。

>>> r = []
>>> for x in [1,2,3,4,5]:
...     for y in [1,2,3,4,5]:
...         r.append((x,y))
...
>>> r
[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5),
 (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5),
 (5, 1), (5, 2), (5, 3), (5, 4), (5, 5)]

さらに、ifを使って条件を制限できる：

>>> [(x,y) for x in [1,2,3,4,5] for y in [1,2,3,4,5] if x <= y]
[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 2), (2, 3), (2, 4), (2, 5), (3, 3),
 (3, 4), (3, 5), (4, 4), (4, 5), (5, 5)]