Add type hints on base Linker and stop using undef

theano/gof/cc.py

@@ -1881,9 +1881,9 @@ class OpWiseCLinker(link.LocalLinker):
         self.fgraph = None
         self.fallback_on_perform = fallback_on_perform
         self.nice_errors = nice_errors
-        self.allow_gc = allow_gc
         if schedule:
             self.schedule = schedule
+        super().__init__(allow_gc=allow_gc)
 
     def accept(self, fgraph, no_recycling=None, profile=None):
         """

theano/gof/link.py

@@ -711,9 +711,9 @@ class PerformLinker(LocalLinker):
         if allow_gc is None:
             allow_gc = theano.config.allow_gc
         self.fgraph = None
-        self.allow_gc = allow_gc
         if schedule:
             self.schedule = schedule
+        super().__init__(allow_gc=allow_gc)
 
     def accept(self, fgraph, no_recycling=None, profile=None):
         """

theano/gof/vm.py

@@ -771,7 +771,6 @@ class VM_Linker(link.LocalLinker):
         if allow_gc is None:
             allow_gc = config.allow_gc
         self.fgraph = None
-        self.allow_gc = allow_gc
         self.use_cloop = use_cloop
         self.callback = callback
         self.callback_input = callback_input
@@ -783,6 +782,7 @@ class VM_Linker(link.LocalLinker):
         self.updated_vars = {}
         if schedule:
             self.schedule = schedule
+        super().__init__(allow_gc=allow_gc)
 
     def accept(self, fgraph, no_recycling=None, profile=None):
         """Check if fgraph is the first FunctionGraph that has ever been

theano/link/basic.py

+import typing
 from copy import copy
 
-from theano.gof import utils
-from theano.gof.utils import undef
+from theano.utils import deprecated
 
 
 class Linker:
     """
-    WRITEME
+    Base type for all linkers.
 
+    A linker takes a FunctionGraph and turns it into a callable.
     """
 
-    def clone(self, allow_gc=undef):
+    def __init__(self, *, allow_gc: typing.Optional[bool] = None):
+        self._allow_gc = allow_gc
+        super().__init__()
+
+    @property
+    def allow_gc(self) -> typing.Optional[bool]:
+        """Determines if the linker may allow garbage collection.
+
+        None means undefined.
+        """
+        return self._allow_gc
+
+    def clone(self, allow_gc: typing.Optional[bool] = None):
         new = copy(self)
-        if allow_gc is not undef:
-            new.allow_gc = allow_gc
+        if allow_gc is not None:
+            new._allow_gc = allow_gc
         return new
 
     def make_thunk(self):
@@ -38,9 +51,11 @@ class Linker:
         print e.data # 3.0 iff inplace == True (else unknown)
 
         """
-        raise utils.MethodNotDefined("make_thunk", type(self), self.__class__.__name__)
+        raise NotImplementedError(
+            f"make_thunk method of {type(self)} is not implemented."
+        )
 
-    # DELETEME #
+    @deprecated("Marked for deletion. Only tests use it.")
     def make_function(self, unpack_single=True, **kwargs):
         """
         Returns a function that takes values corresponding to the inputs of the
@@ -62,6 +77,8 @@ class Linker:
         length 1 will be returned.
 
         """
+        from theano.gof import utils
+
         thunk, inputs, outputs = self.make_thunk(**kwargs)
 
         def execute(*args):