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:
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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’.
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| Returns: |
y : ndarray
A copy of the input array, flattened to one dimension.
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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
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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])
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2、用于mat对象
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>>> a=mat([[1,2,3],[4,5,6]])
>>> a
matrix([[1, 2, 3],
[4, 5, 6]])<br>>>> a.flatten()<br>matrix([[1, 2, 3, 4, 5, 6]])<br>
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3、但是该方法不能用于list对象
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>>> 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'
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想要list达到同样的效果可以使用列表表达式：
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>>> [y for x in a for y in x]
[1, 2, 3, 4, 5, 6, 'a', 'b']
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