flatten是numpy.ndarray.flatten的一个函数，其官方文档是这样描述的：

ndarray.flatten(order='C')

Return a copy of the array collapsed into one dimension.

Parameters:

	order : {‘C’, ‘F’, ‘A’, ‘K’}, optional ‘C’ means to flatten in row-major (C-style) order. ‘F’ means to flatten in column-major (Fortran- style) order. ‘A’ means to flatten in column-major order if a is Fortran contiguous in memory, row-major order otherwise. ‘K’ means to flatten a in the order the elements occur in memory. The default is ‘C’.
Returns:	y : ndarray A copy of the input array, flattened to one dimension.

即返回一个折叠成一维的数组。但是该函数只能适用于numpy对象，即array或者mat，普通的list列表是不行的。

例子：

1、用于array对象

from numpy import *

>>>a=array([[1,2],[3,4],[5,6]])  ###此时a是一个array对象

>>>a

array([[1,2],[3,4],[5,6]])

>>>a.flatten()

array([1,2,3,4,5,6])

2、用于mat对象

>>> a=mat([[1,2,3],[4,5,6]])

>>> a

matrix([[1, 2, 3],

        [4, 5, 6]])
>>> a.flatten()
matrix([[1, 2, 3, 4, 5, 6]])

3、但是该方法不能用于list对象

>>> a=[[1,2,3],[4,5,6],['a','b']]

>>> a

[[1, 2, 3], [4, 5, 6], ['a', 'b']]

>>> a.flatten()                      ###报错

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

AttributeError: 'list' object has no attribute 'flatten'

想要list达到同样的效果可以使用列表表达式：

>>> [y for x in a for y in x]

[1, 2, 3, 4, 5, 6, 'a', 'b']