patch fixing a serialization issue with rx,ry and rz gates. (#3837)

The current serialization of parametrized circuits misses the case where a sympy object is a numeric constant, as for exampole sympy.pi. This problem arises for parametrized rx, ry and rz gates

This patch is an attempt at fixing this.

cirq/json_resolver_cache.py

@@ -156,5 +156,8 @@ def two_qubit_matrix_gate(matrix):
         'sympy.Float': lambda approx: sympy.Float(approx),
         'sympy.Integer': sympy.Integer,
         'sympy.Rational': sympy.Rational,
+        'sympy.pi': lambda: sympy.pi,
+        'sympy.E': lambda: sympy.E,
+        'sympy.EulerGamma': lambda: sympy.EulerGamma,
         'complex': complex,
     }

cirq/protocols/json_serialization.py

@@ -235,6 +235,7 @@ def default(self, o):
         # Sympy object? (Must come before general number checks.)
         # TODO: More support for sympy
         # Github issue: https://github.com/quantumlib/Cirq/issues/2014
+
         if isinstance(o, sympy.Symbol):
             return obj_to_dict_helper(o, ['name'], namespace='sympy')
 
@@ -254,6 +255,18 @@ def default(self, o):
                 'q': o.q,
             }
 
+        if isinstance(o, sympy.NumberSymbol):
+            # check if `o` is a numeric symbol,
+            # i.e. one of the transcendental numbers
+            # sympy.pi, sympy.E or sympy.EulerGamma
+            # (note that these are singletons).
+            if o is sympy.pi:
+                return {'cirq_type': 'sympy.pi'}
+            if o is sympy.E:
+                return {'cirq_type': 'sympy.E'}
+            if o is sympy.EulerGamma:
+                return {'cirq_type': 'sympy.EulerGamma'}
+
         # A basic number object?
         if isinstance(o, numbers.Integral):
             return int(o)

cirq/protocols/json_serialization_test.py

@@ -295,6 +295,10 @@ def test_sympy():
     assert_json_roundtrip_works(t * 2)
     assert_json_roundtrip_works(4 * t + 3 * s + 2)
 
+    assert_json_roundtrip_works(sympy.pi)
+    assert_json_roundtrip_works(sympy.E)
+    assert_json_roundtrip_works(sympy.EulerGamma)
+
 
 class SBKImpl(cirq.SerializableByKey):
     """A test implementation of SerializableByKey."""

cirq/protocols/json_test_data/sympy.E.json

@@ -0,0 +1,5 @@
+[
+  {
+    "cirq_type": "sympy.E"
+  }
+]

cirq/protocols/json_test_data/sympy.E.repr

@@ -0,0 +1,3 @@
+[
+sympy.E
+]

cirq/protocols/json_test_data/sympy.EulerGamma.json

@@ -0,0 +1,5 @@
+[
+  {
+    "cirq_type": "sympy.EulerGamma"
+  }
+]

cirq/protocols/json_test_data/sympy.EulerGamma.repr

@@ -0,0 +1,3 @@
+[
+sympy.EulerGamma
+]

cirq/protocols/json_test_data/sympy.pi.json

@@ -0,0 +1,5 @@
+[
+  {
+    "cirq_type": "sympy.pi"
+  }
+]

cirq/protocols/json_test_data/sympy.pi.repr

@@ -0,0 +1,3 @@
+[
+sympy.pi
+]