Allow initialization of MeasurementKeys with empty strings (quantumlib#4445)

- An empty string is still a string and is used occasionally as a
placeholder in user code.  We should support creation of measurement
keys with empty strings.

Author: dstrain115

cirq-core/cirq/ops/measurement_gate_test.py

@@ -52,8 +52,8 @@ def test_measure_init(num_qubits):
         cirq.MeasurementGate(5, 'a', invert_mask=(True,) * 6)
     with pytest.raises(ValueError, match='len.* !='):
         cirq.MeasurementGate(5, 'a', qid_shape=(1, 2))
-    with pytest.raises(ValueError, match='cannot be empty'):
-        cirq.MeasurementGate(2, qid_shape=(1, 2))
+    with pytest.raises(ValueError, match='valid string'):
+        cirq.MeasurementGate(2, qid_shape=(1, 2), key=None)
     with pytest.raises(ValueError, match='Specify either'):
         cirq.MeasurementGate()
 

cirq-core/cirq/value/measurement_key.py

@@ -41,8 +41,8 @@ class MeasurementKey:
     path: Tuple[str, ...] = dataclasses.field(default_factory=tuple)
 
     def __post_init__(self):
-        if not self.name:
-            raise ValueError("Measurement key name cannot be empty")
+        if not isinstance(self.name, str):
+            raise ValueError("Measurement key name must be a valid string.")
         if MEASUREMENT_KEY_SEPARATOR in self.name:
             raise ValueError(
                 f'Invalid key name: {self.name}\n{MEASUREMENT_KEY_SEPARATOR} is not allowed in '

cirq-core/cirq/value/measurement_key_test.py

@@ -20,8 +20,12 @@
 def test_empty_init():
     with pytest.raises(TypeError, match='required positional argument'):
         _ = cirq.MeasurementKey()
-    with pytest.raises(ValueError, match='cannot be empty'):
-        _ = cirq.MeasurementKey('')
+    with pytest.raises(ValueError, match='valid string'):
+        _ = cirq.MeasurementKey(None)
+    with pytest.raises(ValueError, match='valid string'):
+        _ = cirq.MeasurementKey(4.2)
+    # Initialization of empty string should be allowed
+    _ = cirq.MeasurementKey('')
 
 
 def test_nested_key():