Add ConfigResolver with shared singleton pattern

packages/smithy-core/src/smithy_core/config/__init__.py

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+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+from .resolver import ConfigResolver
+
+__all__ = [
+    "ConfigResolver",
+]

packages/smithy-core/src/smithy_core/config/resolver.py

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+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+from collections.abc import Sequence
+from typing import Any
+
+from smithy_core.interfaces.config import ConfigSource
+
+
+class ConfigResolver:
+    """Resolves configuration values from multiple sources.
+
+    The resolver iterates through sources in precedence order, returning
+    the first non-None value found for a given configuration key.
+    """
+
+    def __init__(self, sources: Sequence[ConfigSource]) -> None:
+        """Initialize the resolver with sources in precedence order.
+
+        :param sources: List of configuration sources in precedence order. The first
+            source in the list has the highest priority. The list is copied to
+            prevent external modification.
+        """
+        self._sources = list(sources)
+
+    def get(self, key: str) -> tuple[Any, Any]:
+        """Resolve a configuration value from sources by iterating through them in precedence order.
+
+        :param key: The configuration key to resolve (e.g., 'retry_mode')
+
+        :returns: A tuple of (value, source_name). If no source provides a value,
+            returns (None, None).
+        """
+        for source in self._sources:
+            value = source.get(key)
+            if value is not None:
+                return (value, source.name)
+        return (None, None)

packages/smithy-core/tests/unit/config/__init__.py

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+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0

packages/smithy-core/tests/unit/config/test_resolver.py

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+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+from typing import Any
+
+from smithy_core.config.resolver import ConfigResolver
+
+
+class StubSource:
+    """A simple ConfigSource implementation for testing.
+
+    Returns values from a provided dictionary, or None if the key
+    is not present.
+    """
+
+    def __init__(self, source_name: str, data: dict[str, Any] | None = None):
+        self._name = source_name
+        self._data = data or {}
+
+    @property
+    def name(self) -> str:
+        return self._name
+
+    def get(self, key: str) -> Any | None:
+        return self._data.get(key)
+
+
+class TestConfigResolver:
+    def test_returns_value_from_single_source(self):
+        source = StubSource("environment", {"region": "us-west-2"})
+        resolver = ConfigResolver(sources=[source])
+
+        result = resolver.get("region")
+
+        assert result == ("us-west-2", "environment")
+
+    def test_returns_None_when_source_has_no_value(self):
+        source = StubSource("environment", {})
+        resolver = ConfigResolver(sources=[source])
+
+        result = resolver.get("region")
+
+        assert result == (None, None)
+
+    def test_returns_None_with_empty_source_list(self):
+        resolver = ConfigResolver(sources=[])
+
+        result = resolver.get("region")
+
+        assert result == (None, None)
+
+    def test_first_source_takes_precedence(self):
+        first_priority_source = StubSource("source_one", {"region": "us-east-1"})
+        second_priority_source = StubSource("source_two", {"region": "eu-west-1"})
+        resolver = ConfigResolver(
+            sources=[first_priority_source, second_priority_source]
+        )
+
+        result = resolver.get("region")
+
+        assert result == ("us-east-1", "source_one")
+
+    def test_skips_source_returning_none_and_uses_next(self):
+        empty_source = StubSource("source_one", {})
+        fallback_source = StubSource("source_two", {"region": "ap-south-1"})
+        resolver = ConfigResolver(sources=[empty_source, fallback_source])
+
+        result = resolver.get("region")
+
+        assert result == ("ap-south-1", "source_two")
+
+    def test_resolves_different_keys_from_different_sources(self):
+        instance = StubSource("source_one", {"region": "us-west-2"})
+        environment = StubSource("source_two", {"retry_mode": "adaptive"})
+        resolver = ConfigResolver(sources=[instance, environment])
+
+        region = resolver.get("region")
+        retry_mode = resolver.get("retry_mode")
+
+        assert region == ("us-west-2", "source_one")
+        assert retry_mode == ("adaptive", "source_two")
+
+    def test_returns_non_string_values(self):
+        source = StubSource(
+            "default",
+            {
+                "max_retries": 3,
+                "use_ssl": True,
+            },
+        )
+        resolver = ConfigResolver(sources=[source])
+
+        assert resolver.get("max_retries") == (3, "default")
+        assert resolver.get("use_ssl") == (True, "default")
+
+    def test_get_is_idempotent(self):
+        source = StubSource("environment", {"region": "us-west-2"})
+        resolver = ConfigResolver(sources=[source])
+
+        result1 = resolver.get("region")
+        result2 = resolver.get("region")
+        result3 = resolver.get("region")
+
+        assert result1 == result2 == result3 == ("us-west-2", "environment")
+
+    def test_treats_empty_string_as_valid_value(self):
+        source = StubSource("test", {"region": ""})
+        resolver = ConfigResolver(sources=[source])
+
+        value, source_name = resolver.get("region")
+
+        assert value == ""
+        assert source_name == "test"
+
+    def test_external_list_modifications_do_not_affect_resolver(self):
+        source1 = StubSource("environment", {"region": "us-west-2"})
+        source2 = StubSource("config", {"region": "eu-west-1"})
+        sources = [source1]
+
+        resolver = ConfigResolver(sources=sources)
+
+        # Modify the original list after resolver construction
+        sources.append(source2)
+        sources.clear()
+
+        # Resolver should use the original source
+        result = resolver.get("region")
+        assert result == ("us-west-2", "environment")