鏈表推導式

[x for x in x] 

鏈表推導式提供了一個創建鏈表的簡單途徑，無需使用 map()， filter() 以及 lambda。返回鏈表的定義通常要比創建這些鏈表更清晰。每一個鏈表推導式包括在一個for語句之後的表達式，零或多個for或if語句。返回值是由for或if子句之後的表達式得到的元素組成的鏈表。如果想要得到一個元組，必須要加上括號。

>>> freshfruit = [‘  banana‘, ‘  loganberry ‘, ‘passion fruit  ‘]
>>> [weapon.strip() for weapon in
 
 freshfruit]
[‘banana‘, ‘loganberry‘, ‘passion fruit‘]
>>> vec = [2, 4, 6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
[]
>>> [[x,x**2] for x in vec]
[[2, 4], [4, 16], [6, 36]]
 
>>> [x, x**2 for x in vec]      # error - parens required for tuples
  File "<stdin>", line 1, in ?
    [x, x**2 for x in vec]
               ^
SyntaxError: invalid syntax
>>> [(x, x**2) for x in vec]
[(2, 4), (4, 16), (6, 36)]
>>> vec1 = [2, 4, 6]
>>> vec2 = [4, 3, -9]
 
>>> [x*y for x in vec1 for y in vec2]
[8, 6, -18, 16, 12, -36, 24, 18, -54]
>>> [x+y for x in vec1 for y in vec2]
[6, 5, -7, 8, 7, -5, 10, 9, -3]
>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
[8, 12, -54]

為使鏈表推導式匹配for循環的行為，可以在推導之外保留循環變量：

>>> x = 100                     # this gets overwritten
>>> [x**3 for x in range(5)]
[0, 1, 8, 27, 64]
>>> x                           # the final value for range(5)

摘自
Python handbook
Guido van Rossum 
Fred L. Drake, Jr., editor

python 鏈表表達式 map、filter易讀版