Fix using mutable Extensions() instances as default parameters (#294)

* reapply issue-283 changes

* remove unnecessary change

* another typo

* removing parens as suggested - tests still pass

Author: Ashalynd

CONTRIBUTING.md

@@ -36,6 +36,12 @@ To do this, you must first install `protobuf-compiler` and `virtualenv`:
 sudo apt-get install protobuf-compiler virtualenv
 ```
 
+If you are using python3, you must also install `python3-tk`:
+
+```bash
+sudo apt-get install python3-tk
+```
+
 Next, from the root directory of your clone of cirq's repository, run the
 continuous integration scripts:
 

cirq/circuits/circuit.py

@@ -333,10 +333,10 @@ def __str__(self):
 
     def to_unitary_matrix(
             self,
-            qubit_order_key: Callable[[QubitId], Any]=None,
+            qubit_order_key: Callable[[QubitId], Any] = None,
             qubits_that_should_be_present: Iterable[QubitId] = (),
-            ignore_terminal_measurements=True,
-            ext: Extensions=None) -> np.ndarray:
+            ignore_terminal_measurements = True,
+            ext: Extensions = None) -> np.ndarray:
         """Converts the circuit into a unitary matrix, if possible.
 
         Args:
@@ -395,7 +395,7 @@ def is_ignorable_measurement(moment_index, operation):
 
     def to_text_diagram(
             self,
-            ext: Extensions = Extensions(),
+            ext: Extensions = None,
             use_unicode_characters: bool = True,
             transpose: bool = False,
             qubit_order_key: Callable[[QubitId], Any] = None) -> str:
@@ -419,6 +419,9 @@ def to_text_diagram(
         if qubit_order_key is None:
             qubit_order_key = _str_lexicographic_respecting_int_order
 
+        if ext is None:
+            ext = Extensions()
+
         qubits = {
             q
             for moment in self.moments for op in moment.operations

cirq/circuits/expand_composite.py

@@ -33,14 +33,14 @@ class ExpandComposite(PointOptimizer):
     """
 
     def __init__(self,
-                 composite_gate_extension: Extensions = Extensions()) -> None:
+                 composite_gate_extension: Extensions = None) -> None:
         """Construct the optimization pass.
 
         Args:
             composite_gate_extension: An extension that that can be used
                 to supply or override a CompositeGate decomposition.
         """
-        self.extension = composite_gate_extension
+        self.extension = composite_gate_extension or Extensions()
 
     def optimization_at(self, circuit, index, op):
         decomposition = self._decompose(op)

cirq/google/merge_interactions.py

@@ -34,10 +34,10 @@ class MergeInteractions(PointOptimizer):
     def __init__(self,
                  tolerance: float = 1e-8,
                  allow_partial_czs: bool = True,
-                 extensions: Extensions = Extensions()) -> None:
+                 extensions: Extensions = None) -> None:
         self.tolerance = tolerance
         self.allow_partial_czs = allow_partial_czs
-        self.extensions = extensions
+        self.extensions = extensions or Extensions()
 
     def optimization_at(self, circuit, index, op):
         if len(op.qubits) != 2: