[Python] Allow use of '__slots__' in Extensible classes

interfaces/cython/cantera/reaction.pxd

@@ -257,9 +257,11 @@ cdef class CustomRate(ReactionRate):
     cdef Func1 _rate_func  # prevent premature garbage collection
 
 cdef class ExtensibleRate(ReactionRate):
+    cdef public list _delegates
     cdef set_cxx_object(self, CxxReactionRate* rate=*)
 
 cdef class ExtensibleRateData:
+    cdef public list _delegates
     cdef set_cxx_object(self, CxxReactionDataDelegator* rate)
 
 cdef class InterfaceRateBase(ArrheniusRateBase):

interfaces/cython/cantera/reactor.pxd

@@ -215,6 +215,7 @@ cdef class FlowReactor(Reactor):
     pass
 
 cdef class ExtensibleReactor(Reactor):
+    cdef public _delegates
     cdef CxxReactorAccessor* accessor
 
 cdef class ReactorSurface:

samples/python/kinetics/custom_reactions.py

@@ -33,6 +33,7 @@
 # construct reactions based on ExtensibleRate: replace 2nd reaction with equivalent
 # ExtensibleRate
 class ExtensibleArrheniusData(ct.ExtensibleRateData):
+    __slots__ = ("T",)
     def __init__(self):
         self.T = None
 
@@ -46,6 +47,7 @@ def replace_update(self, gas):
 
 @ct.extension(name="extensible-Arrhenius", data=ExtensibleArrheniusData)
 class ExtensibleArrhenius(ct.ExtensibleRate):
+    __slots__ = ("A", "b", "Ea_R")
     def after_set_parameters(self, params, units):
         self.A = params["A"]
         self.b = params["b"]

test/python/user_ext.py

@@ -1,12 +1,16 @@
 import cantera as ct
 
 class SquareRateData(ct.ExtensibleRateData):
+    __slots__ = ("Tsquared",)
+
     def replace_update(self, gas):
         self.Tsquared = gas.T**2
         return True
 
 @ct.extension(name="square-rate", data=SquareRateData)
 class SquareRate(ct.ExtensibleRate):
+    __slots__ = ("A",)
+
     def after_set_parameters(self, node, units):
         self.A = node["A"]