feat: add QubitPauliOperator.get_dict (#1755)

* add dict export to QPO

* add a test

* update changelog

* update docstring with suggestion

Co-authored-by: Alec Edgington <[email protected]>

* shorten docstring for pylint

* update assert

* update changelog

* fix sympy equality check

* fix float

---------

Co-authored-by: Alec Edgington <[email protected]>

pytket/docs/changelog.rst

@@ -7,6 +7,7 @@ Unreleased
 Features:
 
 * Add Python 3.13 support to the ZX module.
+* Add :py:meth:`QubitPauliOperator.get_dict` method.
 
 Fixes:
 

pytket/pytket/utils/operators.py

@@ -216,6 +216,16 @@ def subs(self, symbol_dict: dict[Symbol, complex]) -> None:
         for key, value in self._dict.items():
             self._dict[key] = value.subs(symbol_dict)
 
+    def get_dict(self) -> dict[QubitPauliString, Expr]:
+        """Generate a dict representation of QubitPauliOperator,
+        mapping each :py:class:`QubitPauliString` in the support
+        to its corresponding value.
+
+        :return: A dict of Pauli strings and their coefficients
+          as key-value pairs
+        """
+        return self._dict
+
     def to_list(self) -> list[dict[str, Any]]:
         """Generate a list serialized representation of QubitPauliOperator,
          suitable for writing to JSON.

pytket/tests/utils_test.py

@@ -22,7 +22,7 @@
 from hypothesis import HealthCheck, given, settings, strategies
 from hypothesis.strategies._internal import SearchStrategy
 from simulator import TketSimBackend, TketSimShotBackend  # type: ignore
-from sympy import symbols
+from sympy import I, symbols
 
 from pytket.backends.backend import Backend
 from pytket.circuit import Circuit, OpType, Qubit
@@ -90,6 +90,14 @@ def test_all_paulis() -> None:
     assert len(list(circs)) == 3
 
 
+def test_dict_export() -> None:
+    qps1 = QubitPauliString(Qubit(0), Pauli.Y)
+    qps2 = QubitPauliString(Qubit(0), Pauli.X)
+    op = QubitPauliOperator({qps1: 1j, qps2: 0.5})
+    # Do equality check with a sympy "I" for the imaginary part
+    assert op.get_dict() == {qps1: 1.0 * I, qps2: 0.5}
+
+
 def test_shots_to_counts() -> None:
     shot_table = np.asarray([[0, 0], [0, 1], [0, 0]])
     counts = counts_from_shot_table(shot_table)