def delete_columns(mat, columns_to_delete):
	'''
	>>> a = csr_matrix(np.array([[0, 1, 3, 0, 1, 0],
		                           [0, 0, 1, 0, 1, 1]])
	>>> delete_columns(a, [1,2]).todense()
	matrix([[0, 0, 1, 0],
          [0, 0, 1, 1]])

	Parameters
	----------
	mat : csr_matrix
	columns_to_delete : list[int]

	Returns
	-------
	csr_matrix that is stripped of columns indices columns_to_delete
	'''
	column_mask = np.ones(mat.shape[1], dtype=bool)
	column_mask[columns_to_delete] = 0
	return mat.tocsc()[:, column_mask].tocsr()

	>>> a = csr_matrix(np.array([[0, 1, 3, 0, 1, 0],
		                           [0, 0, 1, 0, 1, 1]])
	>>> delete_columns(a, [1,2]).todense()
	matrix([[0, 0, 1, 0],
          [0, 0, 1, 1]])

	Parameters
	----------
	mat : csr_matrix
	columns_to_delete : list[int]

	Returns
	-------
	csr_matrix that is stripped of columns indices columns_to_delete
	
scattertext/CSRMatrixTools.py