Python解析式

In [1]: lst = range(10)

In [2]: %%timeit
   ...: ret = [x ** 2 for x in lst]
   ...: 
100000 loops, best of 3: 5.28 µs per loop

In [3]: %%timeit
   ...: ret = []
   ...: for x in lst:
   ...:     ret.append(x ** 2)
   ...: 
100000 loops, best of 3: 6.09 µs per loop	# 耗时稍高

In [4]: ret = []

In [5]: for x in lst:
   ...:     if x % 2 == 0:
   ...:         ret.append(x)	# 使用for循环
   ...:         

In [6]: ret
Out[6]: [0, 2, 4, 6, 8]

In [7]: ret = [x for x in lst if x % 2 == 0]	# 使用列表解析式

In [8]: ret
Out[8]: [0, 2, 4, 6, 8]

In [9]: (x, y) for x in range(0, 5) for y in range(5, 10)
  File "<ipython-input-9-825e2443da8b>", line 1
    (x, y) for x in range(0, 5) for y in range(5, 10)
             ^
SyntaxError: invalid syntax
# 说明列表解析式一定要使用中括号括起来

In [10]: [(x, y) for x in range(5) for y in range(5, 10)]
Out[10]: 
[(0, 5),
 (0, 6),
 (0, 7),
 (0, 8),
 (0, 9),
 (1, 5),
 (1, 6),
 (1, 7),
 (1, 8),
 (1, 9),
 (2, 5),
 (2, 6),
 (2, 7),
 (2, 8),
 (2, 9),
 (3, 5),
 (3, 6),
 (3, 7),
 (3, 8),
 (3, 9),
 (4, 5),
 (4, 6),
 (4, 7),
 (4, 8),
 (4, 9)]

In [11]: ret = []

In [12]: for x in range(5):
    ...:     for y in range(5, 10):
    ...:         ret.append((x, y))
    ...:         

In [13]: ret
Out[13]: 
[(0, 5),
 (0, 6),
 (0, 7),
 (0, 8),
 (0, 9),
 (1, 5),
 (1, 6),
 (1, 7),
 (1, 8),
 (1, 9),
 (2, 5),
 (2, 6),
 (2, 7),
 (2, 8),
 (2, 9),
 (3, 5),
 (3, 6),
 (3, 7),
 (3, 8),
 (3, 9),
 (4, 5),
 (4, 6),
 (4, 7),
 (4, 8),
 (4, 9)]

In [14]: ret = []

In [15]: for x in lst:
    ...:     if x % 2 == 0:
    ...:         ret.append(x ** 2)
    ...:     else:
    ...:         ret.append(x ** 3)
    ...:         

In [16]: ret
Out[16]: [0, 1, 4, 27, 16, 125, 36, 343, 64, 729]

In [17]: x = 3
# if特殊用法
In [18]: x ** 2 if x % 2 == 0 else x ** 3
Out[18]: 27

In [19]: 3 if True else 4
Out[19]: 3
# 如果采用if特殊用法配合列表解析式 x if cond else y for ...
In [20]: [x ** 2 if x % 2 == 0 else x ** 3 for x in lst]
Out[20]: [0, 1, 4, 27, 16, 125, 36, 343, 64, 729]

In [1]: range(10000)
Out[1]: range(0, 10000)

In [2]: g = (x ** 2 for x in range(100000000000))

In [3]: g
Out[3]: <generator object <genexpr> at 0x7f9f08a5f0a0>

In [4]: next(g)
Out[4]: 0

In [5]: next(g)
Out[5]: 1

In [6]: next(g)
Out[6]: 4

In [1]: lst = [2, 4, 5, 6, 3, 4, 2]

In [2]: s = {x for x in lst}

In [3]: s
Out[3]: {2, 3, 4, 5, 6}	# 可见列表解析式生成的时候会去掉重复，符合集合要求

In [4]: type(s)
Out[4]: set

In [1]: {str(x): x for x in range(5)}
Out[1]: {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4}