code: def test_XXM_to_flat_nz(DM):
      M = DM([[1, 2, 0], [0, 0, 0], [0, 0, 3]])
      elements = [ZZ(1), ZZ(2), ZZ(3)]
      indices = ((0, 0), (0, 1), (2, 2))
      assert M.to_flat_nz() == (elements, (indices, M.shape))
  
imports: from sympy.external.gmpy import GROUND_TYPES
  from sympy import ZZ, QQ, GF, ZZ_I, symbols
  from sympy.polys.matrices.exceptions import DMBadInputError, DMDomainError, DMNonSquareMatrixError, DMNonInvertibleMatrixError, DMShapeError
  from sympy.polys.matrices.domainmatrix import DM, DomainMatrix, DDM, SDM, DFM
  from sympy.testing.pytest import raises, skip
  import pytest
label_desc: Use image node_id 26 for calling a global function with example usage: test_XXM_to_flat_nz(DM) without return types