Cirq x exponent symbol (#7400)

Raise TypeError when cirq.X or other EigenGate-derived gate is raised to
a string exponent, e.g., in `cirq.X ** "text"`.
Raise TypeError also for other unsupported exponent types.

Fixes #2936

Author: ACE07-Sev

cirq-core/cirq/ops/eigen_gate.py

@@ -103,9 +103,15 @@ def __init__(
                 `cirq.unitary(cirq.rx(pi))` equals -iX instead of X.
 
         Raises:
+            TypeError: If the supplied exponent is a string.
             ValueError: If the supplied exponent is a complex number with an
                 imaginary component.
         """
+        if not isinstance(exponent, (numbers.Number, sympy.Expr)):
+            raise TypeError(
+                "Gate exponent must be a number or sympy expression. "
+                f"Invalid type: {type(exponent).__name__!r}"
+            )
         if isinstance(exponent, complex):
             if exponent.imag:
                 raise ValueError(f"Gate exponent must be real. Invalid Value: {exponent}")
@@ -286,7 +292,12 @@ def _period(self) -> float | None:
         real_periods = [abs(2 / e) for e in exponents if e != 0]
         return _approximate_common_period(real_periods)
 
-    def __pow__(self, exponent: float | sympy.Symbol) -> EigenGate:
+    def __pow__(self, exponent: value.TParamVal) -> EigenGate:
+        if not isinstance(exponent, (numbers.Number, sympy.Expr)):
+            raise TypeError(
+                "Gate exponent must be a number or sympy expression. "
+                f"Invalid type: {type(exponent).__name__!r}"
+            )
         new_exponent = protocols.mul(self._exponent, exponent, NotImplemented)
         if new_exponent is NotImplemented:
             return NotImplemented  # pragma: no cover

cirq-core/cirq/ops/eigen_gate_test.py

@@ -248,10 +248,19 @@ def test_pow() -> None:
     assert ZGateDef(exponent=0.25, global_shift=0.5) ** 2 == ZGateDef(
         exponent=0.5, global_shift=0.5
     )
-    with pytest.raises(ValueError, match="real"):
+    with pytest.raises(ValueError, match="Gate exponent must be real."):
         assert ZGateDef(exponent=0.5) ** 0.5j
     assert ZGateDef(exponent=0.5) ** (1 + 0j) == ZGateDef(exponent=0.5)
 
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert ZGateDef(exponent=0.5) ** "text"
+
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert ZGateDef(exponent="text")
+
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert ZGateDef(exponent=sympy.Symbol('a')) ** "text"
+
 
 def test_inverse() -> None:
     assert cirq.inverse(CExpZinGate(0.25)) == CExpZinGate(-0.25)

cirq-core/cirq/ops/pauli_gates_test.py

@@ -225,3 +225,10 @@ def test_powers() -> None:
     assert isinstance(cirq.X**1, cirq.Pauli)
     assert isinstance(cirq.Y**1, cirq.Pauli)
     assert isinstance(cirq.Z**1, cirq.Pauli)
+
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert cirq.X ** 'text'
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert cirq.Y ** 'text'
+    with pytest.raises(TypeError, match="Gate exponent must be a number or sympy expression."):
+        assert cirq.Z ** 'text'

cirq-core/cirq/sim/simulator_base_test.py

@@ -222,7 +222,9 @@ def test_noise_applied_measurement_gate():
 def test_parameterized_copies_all_but_last():
     sim = CountingSimulator()
     n = 4
-    rs = sim.simulate_sweep(cirq.Circuit(cirq.X(q0) ** 'a'), [{'a': i} for i in range(n)])
+    rs = sim.simulate_sweep(
+        cirq.Circuit(cirq.X(q0) ** sympy.Symbol('a')), [{'a': i} for i in range(n)]
+    )
     for i in range(n):
         r = rs[i]
         assert r._final_simulator_state.gate_count == 1