748: handle obj.get(k, default) call pattern for backward compatibility

pyxform/survey_element.py

@@ -4,6 +4,7 @@
 
 import json
 import re
+import warnings
 from collections.abc import Callable, Generator, Iterable, Mapping
 from itertools import chain
 from typing import TYPE_CHECKING, Optional
@@ -35,6 +36,7 @@
 )
 SURVEY_ELEMENT_EXTRA_FIELDS = ("_survey_element_xpath",)
 SURVEY_ELEMENT_SLOTS = (*SURVEY_ELEMENT_FIELDS, *SURVEY_ELEMENT_EXTRA_FIELDS)
+_GET_SENTINEL = object()
 
 
 class SurveyElement(Mapping):
@@ -53,6 +55,22 @@ def __hash__(self):
     def __getitem__(self, key):
         return self.__getattribute__(key)
 
+    def get(self, key, default=_GET_SENTINEL):
+        try:
+            return self.__getattribute__(key)
+        except AttributeError:
+            # Sentinel used rather than None since caller may write `default=None`.
+            if default is _GET_SENTINEL:
+                raise
+            warnings.warn(
+                "The `obj.get(key, default)` usage pattern will be removed in a future "
+                "version of pyxform. Please check the object type to ensure the "
+                "attribute will exist, or use `hasattr(obj, key, default)` instead.",
+                DeprecationWarning,
+                stacklevel=2,  # level 1 = here, 2 = caller.
+            )
+            return default
+
     @staticmethod
     def get_slot_names() -> tuple[str, ...]:
         """Each subclass must provide a list of slots from itself and all parents."""

tests/test_survey_element.py

@@ -0,0 +1,67 @@
+import warnings
+from unittest import TestCase
+
+from pyxform.survey_element import SurveyElement
+
+
+class TestSurveyElementMappingBehaviour(TestCase):
+    def tearDown(self):
+        # Undo the warnings filter set in the below test.
+        warnings.resetwarnings()
+
+    def test_get_call_patterns_equivalent_to_base_dict(self):
+        """Should find that, except for deprecated usage, SurveyElement is dict-like."""
+        # To demonstrate how dict normally works using same test cases.
+        _dict = {"name": "test", "label": None}
+        # getattr
+        self.assertEqual("default", getattr(_dict, "foo", "default"))
+        # defined key, no default
+        self.assertEqual("test", _dict.get("name"))
+        # defined key, with default
+        self.assertEqual("test", _dict.get("name", "default"))
+        # defined key, with None value
+        self.assertEqual(None, _dict.get("label"))
+        # defined key, with None value, with default
+        self.assertEqual(None, _dict.get("label", "default"))
+        # undefined key, with default
+        self.assertEqual("default", _dict.get("foo", "default"))
+        # undefined key, with default None
+        self.assertEqual(None, _dict.get("foo", None))
+        # other access patterns for undefined key
+        self.assertEqual(None, _dict.get("foo"))
+        with self.assertRaises(AttributeError):
+            _ = _dict.foo
+        with self.assertRaises(KeyError):
+            _ = _dict["foo"]
+
+        elem = SurveyElement(name="test")
+        # getattr
+        self.assertEqual("default", getattr(elem, "foo", "default"))
+        # defined key, no default
+        self.assertEqual("test", elem.get("name"))
+        # defined key, with default
+        self.assertEqual("test", elem.get("name", "default"))
+        # defined key, with None value
+        self.assertEqual(None, elem.get("label"))
+        # defined key, with None value, with default
+        self.assertEqual(None, elem.get("label", "default"))
+        # undefined key, with default
+        with self.assertWarns(DeprecationWarning) as warned:
+            self.assertEqual("default", elem.get("foo", "default"))
+        # Warning points to caller, rather than survey_element or collections.abc.
+        self.assertEqual(__file__, warned.filename)
+        # undefined key, with default None
+        with self.assertWarns(DeprecationWarning) as warned:
+            self.assertEqual(None, elem.get("foo", None))
+        # Callers can disable warnings at module-level.
+        warnings.simplefilter("ignore", DeprecationWarning)
+        with warnings.catch_warnings(record=True) as warned:
+            elem.get("foo", "default")
+        self.assertEqual(0, len(warned))
+        # other access patterns for undefined key
+        with self.assertRaises(AttributeError):
+            _ = elem.get("foo")
+        with self.assertRaises(AttributeError):
+            _ = elem.foo
+        with self.assertRaises(AttributeError):
+            _ = elem["foo"]