Remove use of namedtupled defaults kwarg.

* It is incompatible with python < 3.7.

Fixes #6

python/npcomp/tracing/emitters.py

@@ -17,16 +17,16 @@ class TraceValueType(Enum):
   NDARRAY = 1
 
 
-class TraceValue(
-    namedtuple("TraceValue", ["value", "type"],
-               defaults=(TraceValueType.NDARRAY,))):
+class TraceValue(namedtuple("TraceValue", ["value", "type"])):
   __slots__ = ()
   """A Python value and the trace type that it should correspond to."""
 
 
+TraceValue.__new__.__defaults__ = (TraceValueType.NDARRAY,)
+
+
 class TraceInvocation(
-    namedtuple("TraceInvocation", ["inputs", "kwargs", "protocol", "method"],
-               defaults=(Protocol.ARRAY_FUNC, "__call__"))):
+    namedtuple("TraceInvocation", ["inputs", "kwargs", "protocol", "method"])):
   """An invocation of a single functions.
 
   This abstracts over both ufuncs and array_funcs, differentiating by the
@@ -35,10 +35,12 @@ class TraceInvocation(
   __slots__ = ()
 
 
+TraceInvocation.__new__.__defaults__ = (Protocol.ARRAY_FUNC, "__call__")
+
+
 class EmissionRequest(
     namedtuple("EmissionRequest",
-               ["input_ssa_values", "dialect_helper", "extra"],
-               defaults=(None,))):
+               ["input_ssa_values", "dialect_helper", "extra"])):
   """Represents the result of processing inputs from an invocation.
 
   The `input_ssa_values` are mlir.ir.Value instances corresponding to
@@ -53,10 +55,12 @@ class EmissionRequest(
   __slots__ = ()
 
 
+EmissionRequest.__new__.__defaults__ = (None,)
+
+
 class TraceValueMap(
     namedtuple("TraceValueMap",
-               ["input_trace_values", "result_trace_value_types", "extra"],
-               defaults=(None,))):
+               ["input_trace_values", "result_trace_value_types", "extra"])):
   """The result of mapping an invocation to corresponding op structure.
 
   This type associates:
@@ -69,6 +73,9 @@ class TraceValueMap(
   __slots__ = ()
 
 
+TraceValueMap.__new__.__defaults__ = (None)
+
+
 class FuncEmitter:
   """An emitter for an op-like function invocation."""