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fix(event_sources): implement Mapping protocol on DictWrapper for better interop with existing middlewares #1516

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Sep 28, 2022
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fix(event_sources): implement Mapping protocol on DictWrapper for better interop with existing middlewares #1516

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ISSUE-1503: ISSUE-1503: ISSUE-1503: fix: add Mapping abc and missing …
Tankanow Sep 12, 2022
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Tankanow Sep 28, 2022
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10 changes: 8 additions & 2 deletions aws_lambda_powertools/utilities/data_classes/common.py
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Original file line number Diff line number Diff line change
@@ -1,9 +1,9 @@
import base64
import json
from typing import Any, Dict, Optional
from typing import Any, Dict, Iterator, Mapping, Optional


class DictWrapper:
class DictWrapper(Mapping):
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@Tankanow Tankanow Sep 13, 2022 •
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NOTE: I thought for a long time whether or not to explicitly add the Mapping protocol here or to simply implement the missing dunder methods and allow duck-typing to carry the day.

In the end, I decided it was better to explicitly add the Mapping protocol because, all things being equal, I think it is bad for subclasses of this class to break that protocol. It is good for mypy to warn when an implementer chooses to break the protocol. It should require an explicit type: ignore comment when choosing to break the protocol.

There is one existing class, StreamRecord, that breaks the protocol. There are no subclasses of StreamRecord and it is used only within the context of a DynamoDBRecord object. Because there is only one instance of this type breakage, and it is so localized, I marked StreamRecord.keys as type: ignore and added a comment and some tests to indicate that this class explicitly breaks the Mapping protocol.

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In older days I'd ask to remove it as we're breaking it in StreamRecord - I'd take this as an instance of Practicality Beats Purity.

It's highly unlikely a customer will use .keys to get the keys of a StreamRecord dict given the intent. However, if we do receive any customer complaint we'll consider removing the subclassing.

"""Provides a single read only access to a wrapper dict"""

def __init__(self, data: Dict[str, Any]):
Expand All @@ -19,6 +19,12 @@ def __eq__(self, other: Any) -> bool:

return self._data == other._data

def __iter__(self) -> Iterator:
return iter(self._data)

def __len__(self) -> int:
return len(self._data)

def get(self, key: str, default: Optional[Any] = None) -> Optional[Any]:
return self._data.get(key, default)

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4 changes: 3 additions & 1 deletion aws_lambda_powertools/utilities/data_classes/dynamo_db_stream_event.py
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Expand Up @@ -182,8 +182,10 @@ def approximate_creation_date_time(self) -> Optional[int]:
item = self.get("ApproximateCreationDateTime")
return None if item is None else int(item)

# This override breaks the Mapping protocol of DictWrapper, it's left here for backwards compatibility with
# a 'type: ignore' comment. This is currently the only subclass of DictWrapper that breaks this protocol.
@property
def keys(self) -> Optional[Dict[str, AttributeValue]]:
def keys(self) -> Optional[Dict[str, AttributeValue]]: # type: ignore
"""The primary key attribute(s) for the DynamoDB item that was modified."""
return _attribute_value_dict(self._data, "Keys")

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31 changes: 31 additions & 0 deletions tests/functional/test_data_classes.py
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Expand Up @@ -74,6 +74,7 @@
AttributeValueType,
DynamoDBRecordEventName,
DynamoDBStreamEvent,
StreamRecord,
StreamViewType,
)
from aws_lambda_powertools.utilities.data_classes.event_source import event_source
Expand Down Expand Up @@ -101,6 +102,19 @@ def message(self) -> str:
assert DataClassSample(data1).raw_event is data1


def test_dict_wrapper_imlements_mapping():
class DataClassSample(DictWrapper):
pass

data = {"message": "foo1"}
dcs = DataClassSample(data)
assert len(dcs) == len(data)
assert list(dcs) == list(data)
assert dcs.keys() == data.keys()
assert list(dcs.values()) == list(data.values())
assert dcs.items() == data.items()


def test_cloud_watch_dashboard_event():
event = CloudWatchDashboardCustomWidgetEvent(load_event("cloudWatchDashboardEvent.json"))
assert event.describe is False
Expand Down Expand Up @@ -617,6 +631,23 @@ def test_dynamo_attribute_value_type_error():
print(attribute_value.get_type)


def test_stream_record_keys_with_valid_keys():
attribute_value = {"Foo": "Bar"}
sr = StreamRecord({"Keys": {"Key1": attribute_value}})
assert sr.keys == {"Key1": AttributeValue(attribute_value)}


def test_stream_record_keys_with_no_keys():
sr = StreamRecord({})
assert sr.keys is None


def test_stream_record_keys_overrides_dict_wrapper_keys():
data = {"Keys": {"key1": {"attr1": "value1"}}}
sr = StreamRecord(data)
assert sr.keys != data.keys()


def test_event_bridge_event():
event = EventBridgeEvent(load_event("eventBridgeEvent.json"))

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