# 列表生成式

``````>>> range(1, 11)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
``````

``````>>> L = []
>>> for x in range(1, 11):
...    L.append(x * x)
...
>>> L
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
``````

``````>>> [x * x for x in range(1, 11)]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
``````

for循环后面还可以加上if判断，这样我们就可以筛选出仅偶数的平方：

``````>>> [x * x for x in range(1, 11) if x % 2 == 0]
[4, 16, 36, 64, 100]
``````

``````>>> [m + n for m in 'ABC' for n in 'XYZ']
['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']
``````

``````>>> import os # 导入os模块，模块的概念后面讲到
>>> [d for d in os.listdir('.')] # os.listdir可以列出文件和目录
['.emacs.d', '.ssh', '.Trash', 'Adlm', 'Applications', 'Desktop', 'Documents', 'Downloads', 'Library', 'Movies', 'Music', 'Pictures', 'Public', 'VirtualBox VMs', 'Workspace', 'XCode']
``````

`for`循环其实可以同时使用两个甚至多个变量，比如`dict``iteritems()`可以同时迭代key和value：

``````>>> d = {'x': 'A', 'y': 'B', 'z': 'C' }
>>> for k, v in d.iteritems():
...     print k, '=', v
...
y = B
x = A
z = C
``````

``````>>> d = {'x': 'A', 'y': 'B', 'z': 'C' }
>>> [k + '=' + v for k, v in d.iteritems()]
['y=B', 'x=A', 'z=C']
``````

``````>>> L = ['Hello', 'World', 'IBM', 'Apple']
>>> [s.lower() for s in L]
['hello', 'world', 'ibm', 'apple']
``````

### 小结

``````>>> L = ['Hello', 'World', 18, 'Apple', None]
>>> [s.lower() for s in L]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute 'lower'
``````

``````>>> x = 'abc'
>>> y = 123
>>> isinstance(x, str)
True
>>> isinstance(y, str)
False
``````