How can I find the dimensions of a matrix in Python?

As Ayman farhat mentioned you can use the simple method len(matrix) to get the length of rows and get the length of the first row to get the no. of columns using len(matrix[0]) :

>>> a=[[1,5,6,8],[1,2,5,9],[7,5,6,2]]
>>> len(a)
3
>>> len(a[0])
4

Also you can use a library that helps you with matrices "numpy":

>>> import numpy 
>>> numpy.shape(a)
(3,4)

The number of rows of a list of lists would be: len(A) and the number of columns len(A[0]) given that all rows have the same number of columns, i.e. all lists in each index are of the same size.

If you are using NumPy arrays, shape can be used. For example

  >>> a = numpy.array([[[1,2,3],[1,2,3]],[[12,3,4],[2,1,3]]])
  >>> a
  array([[[ 1,  2,  3],
         [ 1,  2,  3]],

         [[12,  3,  4],
         [ 2,  1,  3]]])
 >>> a.shape
 (2, 2, 3)