Document CIRCUIT_TYPE and hide other typevars/aliases in circuits.py (quantumlib#5229)

Fixes quantumlib#5150 (assuming this renders nicely on the docsite; how can I check that locally?)

This adds an underscore prefix to hide some type aliases and type vars that are not part of the public interface of the module. Also adds a docstring to the `CIRCUIT_TYPE` variable, which is used in a few other places.

Author: maffoo

cirq-core/cirq/circuits/circuit.py

@@ -52,6 +52,7 @@
 
 import cirq._version
 from cirq import _compat, devices, ops, protocols, qis
+from cirq._doc import document
 from cirq.circuits._bucket_priority_queue import BucketPriorityQueue
 from cirq.circuits.circuit_operation import CircuitOperation
 from cirq.circuits.insert_strategy import InsertStrategy
@@ -65,9 +66,30 @@
 if TYPE_CHECKING:
     import cirq
 
-T_DESIRED_GATE_TYPE = TypeVar('T_DESIRED_GATE_TYPE', bound='ops.Gate')
+
+_TGate = TypeVar('_TGate', bound='cirq.Gate')
+
 CIRCUIT_TYPE = TypeVar('CIRCUIT_TYPE', bound='AbstractCircuit')
-INT_TYPE = Union[int, np.integer]
+document(
+    CIRCUIT_TYPE,
+    """Type variable for an AbstractCircuit.
+
+    This can be used when writing generic functions that operate on circuits.
+    For example, suppose we define the following function:
+
+        def foo(circuit: CIRCUIT_TYPE) -> CIRCUIT_TYPE:
+            ...
+
+    This lets the typechecker know that this function takes any kind of circuit
+    and returns the same type, that is, if passed a `cirq.Circuit` it will return
+    `cirq.Circuit`, and similarly if passed `cirq.FrozenCircuit` it will return
+    `cirq.FrozenCircuit`. This is particularly useful for things like the
+    transformer API, since it can preserve more type information than if we typed
+    the function as taking and returning `cirq.AbstractCircuit`.
+    """,
+)
+
+_INT_TYPE = Union[int, np.integer]
 
 _DEVICE_DEP_MESSAGE = 'Attaching devices to circuits will no longer be supported.'
 
@@ -752,8 +774,8 @@ def findall_operations(
                     yield index, op
 
     def findall_operations_with_gate_type(
-        self, gate_type: Type[T_DESIRED_GATE_TYPE]
-    ) -> Iterable[Tuple[int, 'cirq.GateOperation', T_DESIRED_GATE_TYPE]]:
+        self, gate_type: Type[_TGate]
+    ) -> Iterable[Tuple[int, 'cirq.GateOperation', _TGate]]:
         """Find the locations of all gate operations of a given type.
 
         Args:
@@ -767,7 +789,7 @@ def findall_operations_with_gate_type(
         result = self.findall_operations(lambda operation: isinstance(operation.gate, gate_type))
         for index, op in result:
             gate_op = cast(ops.GateOperation, op)
-            yield index, gate_op, cast(T_DESIRED_GATE_TYPE, gate_op.gate)
+            yield index, gate_op, cast(_TGate, gate_op.gate)
 
     def has_measurements(self):
         return protocols.is_measurement(self)
@@ -1818,20 +1840,20 @@ def __radd__(self, other):
     # Needed for numpy to handle multiplication by np.int64 correctly.
     __array_priority__ = 10000
 
-    def __imul__(self, repetitions: INT_TYPE):
+    def __imul__(self, repetitions: _INT_TYPE):
         if not isinstance(repetitions, (int, np.integer)):
             return NotImplemented
         self._moments *= int(repetitions)
         return self
 
-    def __mul__(self, repetitions: INT_TYPE):
+    def __mul__(self, repetitions: _INT_TYPE):
         if not isinstance(repetitions, (int, np.integer)):
             return NotImplemented
         if self._device == cirq.UNCONSTRAINED_DEVICE:
             return Circuit(self._moments * int(repetitions))
         return Circuit(self._moments * int(repetitions), device=self._device)
 
-    def __rmul__(self, repetitions: INT_TYPE):
+    def __rmul__(self, repetitions: _INT_TYPE):
         if not isinstance(repetitions, (int, np.integer)):
             return NotImplemented
         return self * int(repetitions)
@@ -2750,27 +2772,26 @@ def _list_repr_with_indented_item_lines(items: Sequence[Any]) -> str:
     return f'[\n{indented}\n]'
 
 
-TIn = TypeVar('TIn')
-TOut = TypeVar('TOut')
-TKey = TypeVar('TKey')
+_TIn = TypeVar('_TIn')
+_TOut = TypeVar('_TOut')
+_TKey = TypeVar('_TKey')
 
 
 @overload
 def _group_until_different(
-    items: Iterable[TIn],
-    key: Callable[[TIn], TKey],
-) -> Iterable[Tuple[TKey, List[TIn]]]:
+    items: Iterable[_TIn], key: Callable[[_TIn], _TKey]
+) -> Iterable[Tuple[_TKey, List[_TIn]]]:
     pass
 
 
 @overload
 def _group_until_different(
-    items: Iterable[TIn], key: Callable[[TIn], TKey], val: Callable[[TIn], TOut]
-) -> Iterable[Tuple[TKey, List[TOut]]]:
+    items: Iterable[_TIn], key: Callable[[_TIn], _TKey], val: Callable[[_TIn], _TOut]
+) -> Iterable[Tuple[_TKey, List[_TOut]]]:
     pass
 
 
-def _group_until_different(items: Iterable[TIn], key: Callable[[TIn], TKey], val=lambda e: e):
+def _group_until_different(items: Iterable[_TIn], key: Callable[[_TIn], _TKey], val=lambda e: e):
     """Groups runs of items that are identical according to a keying function.
 
     Args: