🐛 fix: merging of two similarly keyed mappings (#11)

config/common/__init__.py

@@ -2,12 +2,24 @@
 from collections import abc
 from typing import Any, Dict, List, Mapping, Optional, Type, TypeVar
 
+from deepmerge import Merger
+
 from config.errors import ConfigurationOverrideError
 
 T = TypeVar("T")
 
+merger = Merger(
+    type_strategies=[
+        (list, ["append"]),
+        (dict, ["merge"]),
+        (set, ["union"]),
+    ],
+    fallback_strategies=["override"],
+    type_conflict_strategies=["override"],
+)
+
 
-def apply_key_value(obj, key, value):
+def apply_key_value(obj: Mapping[str, Any], key: str, value: Any) -> Mapping[str, Any]:
     key = key.strip("_:.")  # remove special characters from both ends
     for token in (":", "__", "."):
         if token in key:
@@ -52,27 +64,33 @@ def apply_key_value(obj, key, value):
                     )
 
                 try:
-                    sub_property[index] = value
+                    sub_property[index] = merger.merge(sub_property[index], value)
                 except IndexError:
                     raise ConfigurationOverrideError(
                         f"Invalid override for mutable sequence {key}; "
                         f"assignment index out of range"
                     )
             else:
                 try:
-                    sub_property[last_part] = value
+                    if isinstance(sub_property, abc.Mapping):
+                        sub_property[last_part] = merger.merge(
+                            sub_property.get(last_part),
+                            value,
+                        )
+                    else:
+                        sub_property[last_part] = value
                 except TypeError as type_error:
                     raise ConfigurationOverrideError(
                         f"Invalid assignment {key} -> {value}; {str(type_error)}"
                     )
 
             return obj
 
-    obj[key] = value
+    obj[key] = merger.merge(obj.get(key), value)
     return obj
 
 
-def merge_values(destination, source):
+def merge_values(destination: Mapping[str, Any], source: Mapping[str, Any]) -> None:
     for key, value in source.items():
         apply_key_value(destination, key, value)
 

pyproject.toml

@@ -21,7 +21,7 @@ classifiers = [
 ]
 keywords = ["configuration", "root", "management", "strategy", "settings"]
 
-dependencies = ["tomli; python_version < '3.11'", "python-dotenv~=1.0.0"]
+dependencies = ["deepmerge~=1.1.0", "tomli; python_version < '3.11'", "python-dotenv~=1.0.0"]
 
 [project.optional-dependencies]
 yaml = ["PyYAML"]

requirements.txt

@@ -4,6 +4,7 @@ black==22.10.0
 build==0.10.0
 click==8.1.3
 coverage==6.5.0
+deepmerge==1.1.0
 flake8==5.0.4
 iniconfig==1.1.1
 isort==5.10.1

tests/test_configuration.py

@@ -1,5 +1,6 @@
 import os
 from typing import Any, Dict
+from uuid import uuid4
 
 import pkg_resources
 import pytest
@@ -343,11 +344,13 @@ def test_to_dictionary_method_after_applying_env():
     # in this test, environmental variables with TEST_ prefix are used
     # to override values from a previous source
 
-    os.environ["TEST_b__c__d"] = "200"
-    os.environ["TEST_a__0"] = "3"
+    prefix = str(uuid4())
+
+    os.environ[f"{prefix}_b__c__d"] = "200"
+    os.environ[f"{prefix}_a__0"] = "3"
     builder = ConfigurationBuilder(
         MapSource({"a": [1, 2, 3], "b": {"c": {"d": 100}}}),
-        EnvVars("TEST_"),
+        EnvVars(f"{prefix}_"),
     )
 
     config = builder.build()
@@ -368,3 +371,16 @@ def test_overriding_sub_properties():
     assert config.a.b.c == 200
     assert config.a.b2 == "foo"
     assert config.a2 == "oof"
+
+
+def test_deep_merges_of_mappings():
+    builder = ConfigurationBuilder(
+        MapSource({"a": {"b": {"c": 100}}, "a2": "oof"}),
+        MapSource({"a": {"b": {"d": 200}}}),
+    )
+
+    config = builder.build()
+
+    assert config.a.b.c == 100
+    assert config.a.b.d == 200
+    assert config.a2 == "oof"