Copy and view of data mining numpy

When operating on and manipulating arrays, their data is sometimes copied to the new array and sometimes not. This is often a source of confusion for newbies. There are three cases for this:

no copy at all

Simple assignment does not copy array objects or their data, pointing to the same memory location or variable character.

import numpy
# 不完全拷贝
'''
指向同一内存单元或者变量字符
'''
a = numpy.arange(12)
b = a
print(b is a)

b.shape = 3, 4
print(a.shape)


def f(x):
    '''
    Python 传递不定对象作为参考，所以函数调用不拷贝数组。
    :param x: 
    :return: id(x)
    '''
    print(id(x))
print(id(a))
f(a)

True
(3, 4)
45319896
45319896

Views and Shallow Copy

Different array objects share the same data. The view method creates a new array object pointing to the same data.

View and source data, the data uses the same memory, but the organization is different.

import numpy
# 视图和浅复制
'''
数据用的同一内存，但是组织形式不同
'''
a = numpy.arange(12)
a.shape = 3, 4
c = a.view()
print("c: ", c)
print("c is a?: ", c is a)
print("c.base is a?: ", c.base is a)  # C是A所拥有的数据的视图
print("c.flags.owndata?: ", c.flags.owndata)  # C并不拥有数据
print("a.flags.owndata?: ", a.flags.owndata)  # A拥有数据

c.shape = 2, 6
print(a.shape)

c[0, 4] = 123   # 视图数据改变，原矩阵数据改变
print("a: ", a)

a[0, 0] = 12
print("c: ", c)  # 原矩阵数据改变，视图数据改变

"E:\Python 3.6.2\python.exe" F:/PycharmProjects/test.py
c:  [[ 0  1  2  3]
 [ 4  5  6  7]
 [ 8  9 10 11]]
c is a?:  False
c.base is a?:  True
c.flags.owndata?:  False
a.flags.owndata?:  True
(3, 4)
a:  [[  0   1   2   3]
 [123   5   6   7]
 [  8   9  10  11]]
c:  [[ 12   1   2   3 123   5]
 [  6   7   8   9  10  11]]

Process finished with exit code 0

deep copy

This copy method completely copies the array and its data, data and form copy, new memory.

import numpy
# 深复制
a = numpy.arange(12)
a.shape = 3, 4
d = a.copy()
print("d is a?: ", d is a)
print("d.base is a?: ", d.base is a)
d[0, 0] = 99
print("a: ", a)

"E:\Python 3.6.2\python.exe" F:/PycharmProjects/test.py
d is a?:  False
d.base is a?:  False
a:  [[ 0  1  2  3]
 [ 4  5  6  7]
 [ 8  9 10 11]]

Process finished with exit code 0