test: Detect cython method signature variations in lxml < 5

Positional arguments in lxml 4.x cython class methods don't have default values; they are only available since 5.0, thus check accordingly.

test-rt/_testutils/common.py

@@ -3,6 +3,10 @@
 from types import ModuleType
 from typing import NamedTuple
 
+from lxml.etree import LXML_VERSION
+
+is_lxml_4x = LXML_VERSION < (5, 0)
+
 
 class FilePos(NamedTuple):
     file: str

test-rt/_testutils/decorator.py

@@ -1,12 +1,23 @@
 import functools
-from inspect import Signature, _ParameterKind, signature
+from inspect import Parameter, Signature, _ParameterKind, signature, isclass
 from typing import Any, Callable, ParamSpec, Sequence
 
 import pytest
+from .common import is_lxml_4x
 
 _P = ParamSpec("_P")
 
 
+# Very crude and probably won't work on many corner cases,
+# but enough for here so far
+def is_cython_class_method(func: Callable[..., Any]) -> bool:
+    glob = getattr(func, 'func_globals', None)
+    if not glob:
+        return False
+    parent = func.__qualname__.rsplit('.', 1)[0]
+    return isclass(glob[parent])
+
+
 def signature_tester(
     func_to_check: Callable[..., Any],
     param_data: Sequence[tuple[str, _ParameterKind, Any] | None],
@@ -17,16 +28,24 @@ def wrapped(*args: _P.args, **kw: _P.kwargs) -> None:
             sig = signature(func_to_check)
             param = list(sig.parameters.values())
             assert len(param) == len(param_data)
+
+            # For lxml < 5, args in class methods never contain
+            # default values (.__defaults__ property is empty).
+            # This is probably due to older cython
+            # compiler which doesn't support that yet
+            no_default = False
+            if is_lxml_4x and is_cython_class_method(func_to_check):
+                no_default = True
+
             for i in range(len(param_data)):
-                if param_data[i] is None:
+                if (p := param_data[i]) is None:
                     continue
-                # fmt: off
-                assert (
-                    param[i].name,
-                    param[i].kind,
-                    param[i].default
-                ) == param_data[i]
-                # fmt: on
+                assert param[i].name == p[0]
+                assert param[i].kind == p[1]
+                if no_default:
+                    assert param[i].default == Parameter.empty
+                else:
+                    assert param[i].default == p[2]
             f(*args, **kw)
 
         return wrapped

test-rt/test_etree_element.py

@@ -112,10 +112,8 @@ def test_sequence_modify(self, xml_tree: _ElementTree) -> None:
     @_testutils.signature_tester(_Element.index, (
         None,
         ("child", Parameter.POSITIONAL_OR_KEYWORD, Parameter.empty),
-        ("start", Parameter.POSITIONAL_OR_KEYWORD,
-            None if LXML_VERSION >= (5, 0) else Parameter.empty),
-        ("stop" , Parameter.POSITIONAL_OR_KEYWORD,
-            None if LXML_VERSION >= (5, 0) else Parameter.empty),
+        ("start", Parameter.POSITIONAL_OR_KEYWORD, None           ),
+        ("stop" , Parameter.POSITIONAL_OR_KEYWORD, None           ),
     ))
     # fmt: on
     def test_method_index(self, xml_tree: _ElementTree) -> None:
@@ -281,8 +279,7 @@ def test_method_extend(self, xml_tree: _ElementTree) -> None:
     # fmt: off
     @_testutils.signature_tester(_Element.clear, (
         None,
-        ("keep_tail", Parameter.POSITIONAL_OR_KEYWORD,
-            False if LXML_VERSION >= (5, 0) else Parameter.empty),
+        ("keep_tail", Parameter.POSITIONAL_OR_KEYWORD, False),
     ))
     # fmt: on
     def test_method_clear(self, xml_tree: _ElementTree) -> None: