code: def test_DDM_flat():
      dm = DDM([[ZZ(6), ZZ(4)], [ZZ(3), ZZ(1)]], (2, 2), ZZ)
      assert dm.flat() == [ZZ(6), ZZ(4), ZZ(3), ZZ(1)]
  
imports: from sympy.testing.pytest import raises
  from sympy.external.gmpy import GROUND_TYPES
  from sympy.polys import ZZ, QQ
  from sympy.polys.matrices.ddm import DDM
  from sympy.polys.matrices.exceptions import DMShapeError, DMNonInvertibleMatrixError, DMDomainError, DMBadInputError
label_desc: Use image node_id 31 for calling a global function with example usage: test_DDM_flat() without return types