Counting array elements in Python

The method len() returns the number of elements in the list.

Syntax:

len(myArray)

Eg:

myArray = [1, 2, 3]
len(myArray)

Output:

3

If you have a multi-dimensional array, len() might not give you the value you are looking for. For instance:

import numpy as np
a = np.arange(10).reshape(2, 5)
print len(a) == 2

This code block will return true, telling you the size of the array is 2. However, there are in fact 10 elements in this 2D array. In the case of multi-dimensional arrays, len() gives you the length of the first dimension of the array i.e.

import numpy as np
len(a) == np.shape(a)[0]

To get the number of elements in a multi-dimensional array of arbitrary shape:

import numpy as np
size = 1
for dim in np.shape(a): size *= dim

len is a built-in function that calls the given container object's __len__ member function to get the number of elements in the object.

Functions encased with double underscores are usually "special methods" implementing one of the standard interfaces in Python (container, number, etc). Special methods are used via syntactic sugar (object creation, container indexing and slicing, attribute access, built-in functions, etc.).

Using obj.__len__() wouldn't be the correct way of using the special method, but I don't see why the others were modded down so much.