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

3、但是该方法不能用于list对象

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

4、用在矩阵

>>> a = [[1,3],[2,4],[3,5]]>>> a = mat(a)>>> y = a.flatten()>>> ymatrix([[1, 3, 2, 4, 3, 5]])>>> y = a.flatten().A>>> yarray([[1, 3, 2, 4, 3, 5]])>>> shape(y)(1, 6)>>> shape(y[0])(6,)>>> y = a.flatten().A[0]>>> yarray([1, 3, 2, 4, 3, 5])

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