Add a method to specify ExpressionTuple evaluation function

etuples/core.py

@@ -3,7 +3,7 @@
 import warnings
 from collections import deque
 from collections.abc import Generator, Sequence
-from typing import Any, Callable
+from typing import Callable
 
 etuple_repr = reprlib.Repr()
 etuple_repr.maxstring = 100
@@ -178,8 +178,16 @@ def eval_obj(self):
         )
         return trampoline_eval(self._eval_step())
 
-    def _eval_apply(self, op: Callable, op_args: inspect.BoundArguments) -> Any:
-        return op(*op_args.args, **op_args.kwargs)
+    def _eval_apply_fn(self, op: Callable) -> Callable:
+        """Return the callable used to evaluate the expression tuple.
+
+        The expression tuple's operator can be any `Callable`, i.e. either
+        a function or an instance of a class that defines `__call__`. In
+        the latter case, one can evalute the expression tuple using a
+        method other than `__call__` by overloading this method.
+
+        """
+        return op
 
     def _eval_step(self):
         if len(self._tuple) == 0:
@@ -210,7 +218,7 @@ def _eval_step(self):
                     evaled_args.append(i)
 
             try:
-                op_sig = inspect.signature(op)
+                op_sig = inspect.signature(self._eval_apply_fn(op))
             except ValueError:
                 # This handles some builtin function types
                 _evaled_obj = op(*(evaled_args + [kw.value for kw in evaled_kwargs]))
@@ -220,7 +228,7 @@ def _eval_step(self):
                 )
                 op_args.apply_defaults()
 
-                _evaled_obj = self._eval_apply(op, op_args)
+                _evaled_obj = self._eval_apply_fn(op)(*op_args.args, **op_args.kwargs)
 
             if isinstance(_evaled_obj, Generator):
                 self._evaled_obj = _evaled_obj

tests/test_core.py

@@ -68,6 +68,22 @@ def test_ExpressionTuple(capsys):
         ExpressionTuple((print, "hi")).eval_obj
 
 
+def test_eval_apply_fn():
+    class Add(object):
+        def __call__(self):
+            return None
+
+        def add(self, x, y):
+            return x + y
+
+    class AddExpressionTuple(ExpressionTuple):
+        def _eval_apply_fn(self, op):
+            return op.add
+
+    op = Add()
+    assert AddExpressionTuple((op, 1, 2)).evaled_obj == 3
+
+
 def test_etuple():
     """Test basic `etuple` functionality."""