Learning Numpy - Simple Tutorial For Beginners - NumPy Copies & Views - Part 19

While executing the functions, some of them return a copy of the input array, while some return the view. When the contents are physically stored in another location, it is called Copy. If on the other hand, a different view of the same memory content is provided, we call it as View.

No Copy

Simple assignments do not make the copy of array object. Instead, it uses the same id() of the original array to access it. The id() returns a universal identifier of Python object, similar to the pointer in C.

Furthermore, any changes in either gets reflected in the other. For example, the changing shape of one will change the shape of the other too.

import numpy as np
a = np.arange(6)

print ('Our array is:')
print (a)
print ('\n')

print ('Applying id() function:')
print (id(a))
print ('\n')

print ('a is assigned to b:')
b = a
print (b)
print ('\n')

print ('b has same id():')
print (id(b))
print ('\n')

print ('Change shape of b:')
b.shape = 3,2
print (b)
print ('\n')

print ('Shape of a also gets changed:')
print (a)
print ('\n')

Our array is:
[0 1 2 3 4 5]


Applying id() function:
139825477915392


a is assigned to b:
[0 1 2 3 4 5]


b has same id():
139825477915392


Change shape of b:
[[0 1]
 [2 3]
 [4 5]]


Shape of a also gets changed:
[[0 1]
 [2 3]
 [4 5]]

View or Shallow Copy

NumPy has ndarray.view() method which is a new array object that looks at the same data of the original array. Unlike the earlier case, change in dimensions of the new array doesn’t change dimensions of the original.

import numpy as np
# To begin with, a is 3X2 array 
a = np.arange(6).reshape(3,2)

print ('Array a:')
print (a)
print ('\n')

print ('Create view of a:')
b = a.view()
print (b)
print ('\n')

print ('id() for both the arrays are different:')
print ('id() of a:')
print (id(a))
print ('\n')
print ('id() of b:')
print (id(b))
print ('\n')

# Change the shape of b. It does not change the shape of a 
b.shape = 2,3

print ('Shape of b:')
print (b)
print ('\n')

print ('Shape of a:')
print (a)
print ('\n')

Array a:
[[0 1]
 [2 3]
 [4 5]]


Create view of a:
[[0 1]
 [2 3]
 [4 5]]


id() for both the arrays are different:
id() of a:
139825054061152


id() of b:
139825054061632


Shape of b:
[[0 1 2]
 [3 4 5]]


Shape of a:
[[0 1]
 [2 3]
 [4 5]]

Example 2

import numpy as np
a = np.array([[10,10], [2,3], [4,5]])

print ('Our array is:')
print (a)
print ('\n')

print ('Create a slice:')
s = a[:, :2]
print (s)
print ('\n')

Our array is:
[[10 10]
 [ 2  3]
 [ 4  5]]


Create a slice:
[[10 10]
 [ 2  3]
 [ 4  5]]

Deep Copy

The ndarray.copy() function creates a deep copy. It is a complete copy of the array and its data, and doesn’t share with the original array.

import numpy as np
a = np.array([[10,10], [2,3], [4,5]])

print ('Array a is:')
print (a)
print ('\n')

print ('Create a deep copy of a:')
b = a.copy()
print ('Array b is:' )
print (b)
print ('\n')

#b does not share any memory of a 
print ('Can we write b is a')
print (b is a)
print ('\n')

print ('Change the contents of b:')
b[0,0] = 100
print ('\n')

print ('Modified array b:')
print (b)
print ('\n')

print ('a remains unchanged:')
print (a)
print ('\n')

Array a is:
[[10 10]
 [ 2  3]
 [ 4  5]]


Create a deep copy of a:
Array b is:
[[10 10]
 [ 2  3]
 [ 4  5]]


Can we write b is a
False


Change the contents of b:


Modified array b:
[[100  10]
 [  2   3]
 [  4   5]]


a remains unchanged:
[[10 10]
 [ 2  3]
 [ 4  5]]

Refernces

• Learning Numpy - Array From Existing Data Part 6
• Learning Numpy - Array Creation Routines Part 5
• Learning Numpy - Array Attributes Part 4
• Learning Numpy - Data Types Part 3