docstr: If the parameter already exists in the matrix cache, then the label uses that index and the
  matrix cache is unchanged.
docstr_tok: ["If","the","parameter","already","exists","in","the","matrix","cache",",","then","the","label","uses","that","index","and","the","matrix","cache","is","unchanged","."]
code: def test_matrix_already_in_cache_list(self, mat, op):
      """If the parameter already exists in the matrix cache, then the label uses that index and the
      matrix cache is unchanged."""
      cache = {"matrices": [TSWAP, mat]}
      assert op.label(cache=cache) == "U(M1)"
  
      assert len(cache["matrices"]) == 2
  
imports: import pytest
  import numpy
  from scipy.stats import unitary_group
  from gate_data import TSWAP
  import pennylane
  from pennylane.wires import Wires
  from pennylane.operation import DecompositionUndefinedError
label_desc: Use image node_id 6 for calling the TestUnitaryLabels obj's underlying member method code with example usage: obj.test_matrix_already_in_cache_list(mat, op) without return types