title: Updating values in numpy array doesn't work with for loops on list
body: <p>Suppose a 2d numpy array as the following:</p>
  
  <p><code>A = np.ones((5,5))</code></p>
  
  <p>Now, I want to update (change) all the values of A using the following code:</p>
  
  <p><code>for row in A:
      for entry in row:
           entry = 999
  </code></p>
  
  <p>After this, the entries en A are 1, i.e. the matrix is the same as the original.</p>
  
  <p>But, If I run this code, the values are changed:</p>
  
  <p><code>for i,row in enumerate(A):
      for j,entry in enumerate(row):
           A[i][j] = 999
  </code>
  Why this is happening?</p>
  
  <p>Is the <code>cell</code> variable a copy and not a pointer?</p>