Resolve special stenzas lazily with no side-effects on the body

kopf/reactor/invocation.py

@@ -11,6 +11,7 @@
 from typing import Callable
 
 from kopf import config
+from kopf.structs import dicts
 
 
 async def invoke(
@@ -37,9 +38,9 @@ async def invoke(
         kwargs.update(
             type=event['type'],
             body=event['object'],
-            spec=event['object'].setdefault('spec', {}),
-            meta=event['object'].setdefault('metadata', {}),
-            status=event['object'].setdefault('status', {}),
+            spec=dicts.DictView(event['object'], 'spec'),
+            meta=dicts.DictView(event['object'], 'metadata'),
+            status=dicts.DictView(event['object'], 'status'),
             uid=event['object'].get('metadata', {}).get('uid'),
             name=event['object'].get('metadata', {}).get('name'),
             namespace=event['object'].get('metadata', {}).get('namespace'),
@@ -54,9 +55,9 @@ async def invoke(
             new=cause.new,
             patch=cause.patch,
             logger=cause.logger,
-            spec=cause.body.setdefault('spec', {}),
-            meta=cause.body.setdefault('metadata', {}),
-            status=cause.body.setdefault('status', {}),
+            spec=dicts.DictView(cause.body, 'spec'),
+            meta=dicts.DictView(cause.body, 'metadata'),
+            status=dicts.DictView(cause.body, 'status'),
             uid=cause.body.get('metadata', {}).get('uid'),
             name=cause.body.get('metadata', {}).get('name'),
             namespace=cause.body.get('metadata', {}).get('namespace'),

kopf/structs/dicts.py

@@ -2,7 +2,8 @@
 Some basic dicts and field-in-a-dict manipulation helpers.
 """
 import collections.abc
-from typing import Any, Union, MutableMapping, Mapping, Tuple, List, Text, Iterable, Optional
+from typing import (Any, Union, MutableMapping, Mapping, Tuple, List, Text,
+                    Iterable, Iterator, Optional)
 
 FieldPath = Tuple[str, ...]
 FieldSpec = Union[None, Text, FieldPath, List[str]]
@@ -124,3 +125,43 @@ def walk(
             yield from walk(obj, nested=nested)
     else:
         yield objs  # NB: not a mapping, no nested sub-fields.
+
+
+class DictView(Mapping[Any, Any]):
+    """
+    A lazy resolver for the "on-demand" dict keys.
+
+    This is needed to have ``spec``, ``status``, and other special fields
+    to be *assumed* as dicts, even if they are actually not present.
+    And to prevent their implicit creation with ``.setdefault('spec', {})``,
+    which produces unwanted side-effects (actually adds this field).
+
+    >>> body = {}
+    >>> spec = DictView(body, 'spec')
+
+    >>> spec.get('field', 'default')
+    ... 'default'
+
+    >>> body['spec'] = {'field': 'value'}
+
+    >>> spec.get('field', 'default')
+    ... 'value'
+
+    """
+
+    def __init__(self, __src: Mapping[Any, Any], __path: FieldSpec = None):
+        super().__init__()
+        self._src = __src
+        self._path = parse_field(__path)
+
+    def __repr__(self):
+        return repr(dict(self))
+
+    def __len__(self) -> int:
+        return len(resolve(self._src, self._path, {}, assume_empty=True))
+
+    def __iter__(self) -> Iterator[Any]:
+        return iter(resolve(self._src, self._path, {}, assume_empty=True))
+
+    def __getitem__(self, item: Any) -> Any:
+        return resolve(self._src, self._path + (item,))

tests/invocations/test_callbacks.py

@@ -2,7 +2,7 @@
 import traceback
 
 import pytest
-from asynctest import Mock, MagicMock
+from asynctest import MagicMock
 
 from kopf.reactor.invocation import invoke, is_async_fn
 
@@ -129,24 +129,21 @@ async def test_async_detection(fn, expected):
 @syncasyncparams
 async def test_stacktrace_visibility(fn, expected):
     stack_trace_marker = STACK_TRACE_MARKER  # searched by fn
-    cause = Mock()
-    found = await invoke(fn, cause=cause)
+    found = await invoke(fn)
     assert found is expected
 
 
 @fns
 async def test_result_returned(fn):
     fn = MagicMock(fn, return_value=999)
-    cause = Mock()
-    result = await invoke(fn, cause=cause)
+    result = await invoke(fn)
     assert result == 999
 
 
 @fns
 async def test_explicit_args_passed_properly(fn):
     fn = MagicMock(fn)
-    cause = Mock()
-    await invoke(fn, 100, 200, cause=cause, kw1=300, kw2=400)
+    await invoke(fn, 100, 200, kw1=300, kw2=400)
 
     assert fn.called
     assert fn.call_count == 1
@@ -163,7 +160,9 @@ async def test_explicit_args_passed_properly(fn):
 @fns
 async def test_special_kwargs_added(fn):
     fn = MagicMock(fn)
-    cause = MagicMock(body={'metadata': {'uid': 'uid', 'name': 'name', 'namespace': 'ns'}})
+    cause = MagicMock(body={'metadata': {'uid': 'uid', 'name': 'name', 'namespace': 'ns'},
+                            'spec': {'field': 'value'},
+                            'status': {'info': 'payload'}})
     await invoke(fn, cause=cause)
 
     assert fn.called
@@ -173,14 +172,14 @@ async def test_special_kwargs_added(fn):
     assert fn.call_args[1]['cause'] is cause
     assert fn.call_args[1]['event'] is cause.event
     assert fn.call_args[1]['body'] is cause.body
-    assert fn.call_args[1]['spec'] is cause.body['spec']
-    assert fn.call_args[1]['meta'] is cause.body['metadata']
-    assert fn.call_args[1]['status'] is cause.body['status']
+    assert fn.call_args[1]['spec'] == cause.body['spec']
+    assert fn.call_args[1]['meta'] == cause.body['metadata']
+    assert fn.call_args[1]['status'] == cause.body['status']
     assert fn.call_args[1]['diff'] is cause.diff
     assert fn.call_args[1]['old'] is cause.old
     assert fn.call_args[1]['new'] is cause.new
     assert fn.call_args[1]['patch'] is cause.patch
     assert fn.call_args[1]['logger'] is cause.logger
-    assert fn.call_args[1]['uid'] is cause.body['metadata']['uid']
-    assert fn.call_args[1]['name'] is cause.body['metadata']['name']
-    assert fn.call_args[1]['namespace'] is cause.body['metadata']['namespace']
+    assert fn.call_args[1]['uid'] == cause.body['metadata']['uid']
+    assert fn.call_args[1]['name'] == cause.body['metadata']['name']
+    assert fn.call_args[1]['namespace'] == cause.body['metadata']['namespace']