Trying to plot two variable function, calculating function loop gives wrong shape

I have some function f, which is some compliciated integral taking two parameters x and y . I want to make contour plot in this parameter space (i.e. over some arrays X and Y), so I use loops over those two paremeters.
My code looks like this:

def Z_values(X,Y,f):
  matrix=[]
  columns=[]
  for x in X:
    for y in Y:
       a=f(x,y)
       columns.append(a)
    matrix.append(columns)
    columns.clear()
return matrix

plt.contour(X,Y,Z_values(X,Y,f))
plt.show()

I get following error:
TypeError: Input z must be at least a (2, 2) shaped array, but has shape (4, 0))
I don’t understand what I do wrong in the loop. What is wrong?

Edit: wrong copypasted error.

>Solution :

You cannot create columns outside the loop and clear it in the loop. In such case the matrix actually contains the reference points to the same column and it’s getting cleared again and again. So the function Z_values returns list of empty list.

Corrected code as below:

def Z_values(X,Y,f):
    matrix=[]
    for x in X:
        columns=[]
        for y in Y:
            a=f(x,y)
            columns.append(a)
        matrix.append(columns)
    return matrix

plt.contour(X,Y,Z_values(X,Y,lambda x,y: np.exp(x**2+y**2)))
plt.show()

I would suggest you to learn some basic C/C++ programming to understand the concept of reference/pointer and how they are related to memory addresses where the data are actually stored. This will help you avoid this kind of mistake.