Fix validate_activation crash on builtin functions

validate_activation in neat/activations.py had an internal contradiction:
the isinstance check on line 104 explicitly admitted BuiltinFunctionType,
but line 109 then accessed function.__code__.co_argcount, which crashes
with AttributeError on C-implemented CPython builtins because they do
not have a __code__ attribute.

Calling config.genome_config.add_activation('my_abs', abs) — the exact
pattern documented in docs/customization.rst — raised:
  AttributeError: 'builtin_function_or_method' object has no attribute
  '__code__'
instead of the documented InvalidActivationFunction.

The sister function validate_aggregation in neat/aggregations.py already
handled this case correctly using inspect.signature with a
TypeError/ValueError fallback for builtins. Rewrite validate_activation
to mirror that pattern: try inspect.signature first, fall back to
accepting BuiltinFunctionType when signature inspection fails, and use
signature.bind(object()) to verify the callable accepts exactly one
positional argument.

This is also a strict upgrade over an __code__-based check: inspect
can actually introspect most CPython builtins (abs, len, sum, round,
divmod all return Signature objects) and can therefore correctly
reject wrong-arity builtins like divmod, which the old check could
never have caught.

Delete the misleading "# avoid deprecated use of inspect" comment. The
inspect module is not deprecated — that comment appears to be a fossil
from when this code used inspect.getargspec (which was removed in
Python 3.11). inspect.signature is the current, non-deprecated API
and is already used successfully by validate_aggregation in the same
codebase.

Add 8 regression tests in tests/test_coverage_gaps.py under a new
TestValidateActivation class. validate_activation previously had zero
test coverage, which is why this bug escaped notice. Tests cover:
plain functions, lambdas, builtins with introspectable signatures
(abs — the original regression), builtins without introspectable
signatures (max, min — fallback path), wrong-arity pure functions,
zero-arg functions, wrong-arity builtins (divmod), and non-callable
objects. Full suite: 629 passed (was 621), 6 skipped, 0 failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: CodeReclaimers

neat/activations.py

@@ -4,6 +4,7 @@
 and code for adding new user-defined ones
 """
 
+import inspect
 import math
 import types
 
@@ -106,8 +107,22 @@ def validate_activation(function):
                        types.LambdaType)):
         raise InvalidActivationFunction("A function object is required.")
 
-    if function.__code__.co_argcount != 1:  # avoid deprecated use of `inspect`
-        raise InvalidActivationFunction("A single-argument function is required.")
+    try:
+        signature = inspect.signature(function)
+    except (TypeError, ValueError) as exc:
+        # CPython builtins (e.g. max, min) may lack an introspectable
+        # signature. Mirror validate_aggregation: accept them and let
+        # any arity errors surface at use time.
+        if isinstance(function, types.BuiltinFunctionType):
+            return
+        raise InvalidActivationFunction(
+            "Unable to inspect activation callable signature.") from exc
+
+    try:
+        signature.bind(object())
+    except TypeError as exc:
+        raise InvalidActivationFunction(
+            "A single-argument function is required.") from exc
 
 
 class ActivationFunctionSet:

tests/test_coverage_gaps.py

@@ -16,6 +16,7 @@
 import neat
 from neat.activations import (
     elu_activation, lelu_activation, selu_activation,
+    validate_activation, InvalidActivationFunction,
 )
 from neat.aggregations import AggregationFunctionSet, validate_aggregation, InvalidAggregationFunction
 from neat.config import ConfigParameter, UnknownConfigItemError
@@ -86,6 +87,66 @@ def test_selu_zero(self):
         assert selu_activation(0.0) == 0.0
 
 
+# ──────────────────────────────────────────────────────────────────────
+# validate_activation signature handling (mirrors validate_aggregation)
+# ──────────────────────────────────────────────────────────────────────
+
+class TestValidateActivation:
+    """Regression tests for validate_activation.
+
+    Previously the function accessed ``function.__code__.co_argcount``
+    directly, which raised ``AttributeError`` on CPython builtins (which
+    lack a ``__code__`` attribute) even though the isinstance check admitted
+    ``BuiltinFunctionType``. It now uses ``inspect.signature`` + ``bind``,
+    matching ``validate_aggregation``.
+    """
+
+    def test_accepts_plain_function(self):
+        def f(z):
+            return z
+        validate_activation(f)  # should not raise
+
+    def test_accepts_lambda(self):
+        validate_activation(lambda z: z)  # should not raise
+
+    def test_accepts_builtin_with_introspectable_signature(self):
+        # This is the regression test for the original bug.
+        # Before the fix, validate_activation(abs) raised
+        # AttributeError: 'builtin_function_or_method' object has no
+        # attribute '__code__'.
+        validate_activation(abs)  # should not raise
+
+    def test_accepts_builtin_without_introspectable_signature(self):
+        # CPython's ``max`` and ``min`` raise ValueError from
+        # inspect.signature. The fallback should accept them.
+        validate_activation(max)
+        validate_activation(min)
+
+    def test_rejects_two_arg_function_with_invalid_activation_function(self):
+        def f(a, b):
+            return a + b
+        with pytest.raises(InvalidActivationFunction):
+            validate_activation(f)
+
+    def test_rejects_zero_arg_function_with_invalid_activation_function(self):
+        def f():
+            return 0.0
+        with pytest.raises(InvalidActivationFunction):
+            validate_activation(f)
+
+    def test_rejects_two_arg_builtin_with_invalid_activation_function(self):
+        # divmod has an introspectable signature (x, y, /) — should be
+        # rejected cleanly, not propagate a TypeError or AttributeError.
+        with pytest.raises(InvalidActivationFunction):
+            validate_activation(divmod)
+
+    def test_rejects_non_callable_with_invalid_activation_function(self):
+        with pytest.raises(InvalidActivationFunction):
+            validate_activation(42)
+        with pytest.raises(InvalidActivationFunction):
+            validate_activation("not a function")
+
+
 # ──────────────────────────────────────────────────────────────────────
 # Aggregation: deprecation warning on __getitem__ (lines 98-100)
 # ──────────────────────────────────────────────────────────────────────