fix: handle extra SSM parameters (#11)

* bugfix: handle extra SSM parameters

Avoid issues of `extra fields not permitted (type=value_error.extra)` when there are non-relevant
params stored in SSM.

* Fixes

* Fix

* Flake8 fix

* Flake8 fix again

* Cleanup docs

pydantic_ssm_settings/settings.py

@@ -22,6 +22,11 @@ def customise_sources(
         file_secret_settings: SecretsSettingsSource,
     ) -> Tuple[SettingsSourceCallable, ...]:
 
+        ssm_settings = AwsSsmSettingsSource(
+            ssm_prefix=file_secret_settings.secrets_dir,
+            env_nested_delimiter=env_settings.env_nested_delimiter,
+        )
+
         return (
             init_settings,
             env_settings,
@@ -30,5 +35,5 @@ def customise_sources(
             # about unexpected arguments. `secrets_dir` comes from `_secrets_dir`,
             # one of the few special kwargs that Pydantic will allow:
             # https://github.com/samuelcolvin/pydantic/blob/45db4ad3aa558879824a91dd3b011d0449eb2977/pydantic/env_settings.py#L33
-            AwsSsmSettingsSource(ssm_prefix=file_secret_settings.secrets_dir),
+            ssm_settings,
         )

pydantic_ssm_settings/source.py

@@ -1,13 +1,16 @@
 import os
 import logging
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, Optional
+from typing import TYPE_CHECKING, Any, Dict, Mapping, Optional, Tuple
 
 from botocore.exceptions import ClientError
 from botocore.client import Config
 import boto3
 
-from pydantic import BaseSettings, typing
+from pydantic import BaseSettings
+from pydantic.typing import StrPath, get_origin, is_union
+from pydantic.utils import deep_update
+from pydantic.fields import ModelField
 
 if TYPE_CHECKING:
     from mypy_boto3_ssm.client import SSMClient
@@ -16,11 +19,20 @@
 logger = logging.getLogger(__name__)
 
 
+class SettingsError(ValueError):
+    pass
+
+
 class AwsSsmSettingsSource:
-    __slots__ = ("ssm_prefix",)
+    __slots__ = ("ssm_prefix", "env_nested_delimiter")
 
-    def __init__(self, ssm_prefix: Optional[typing.StrPath]):
-        self.ssm_prefix: Optional[typing.StrPath] = ssm_prefix
+    def __init__(
+        self,
+        ssm_prefix: Optional[StrPath],
+        env_nested_delimiter: Optional[str] = None,
+    ):
+        self.ssm_prefix: Optional[StrPath] = ssm_prefix
+        self.env_nested_delimiter: Optional[str] = env_nested_delimiter
 
     @property
     def client(self) -> "SSMClient":
@@ -31,38 +43,124 @@ def client_config(self) -> Config:
         timeout = float(os.environ.get("SSM_TIMEOUT", 0.5))
         return Config(connect_timeout=timeout, read_timeout=timeout)
 
-    def __call__(self, settings: BaseSettings) -> Dict[str, Any]:
-        """
-        Returns lazy SSM values for all settings.
-        """
-        secrets: Dict[str, Optional[Any]] = {}
-
-        if self.ssm_prefix is None:
-            return secrets
-
-        secrets_path = Path(self.ssm_prefix)
+    def load_from_ssm(self, secrets_path: Path, case_sensitive: bool):
 
         if not secrets_path.is_absolute():
             raise ValueError("SSM prefix must be absolute path")
 
         logger.debug(f"Building SSM settings with prefix of {secrets_path=}")
 
+        output = {}
         try:
             paginator = self.client.get_paginator("get_parameters_by_path")
             response_iterator = paginator.paginate(
                 Path=str(secrets_path), WithDecryption=True
             )
 
-            output = {}
             for page in response_iterator:
                 for parameter in page["Parameters"]:
                     key = Path(parameter["Name"]).relative_to(secrets_path).as_posix()
-                    output[key] = parameter["Value"]
-            return output
+                    output[key if case_sensitive else key.lower()] = parameter["Value"]
 
         except ClientError:
             logger.exception("Failed to get parameters from %s", secrets_path)
-            return {}
+
+        return output
+
+    def __call__(self, settings: BaseSettings) -> Dict[str, Any]:
+        """
+        Returns SSM values for all settings.
+        """
+        d: Dict[str, Optional[Any]] = {}
+
+        if self.ssm_prefix is None:
+            return d
+
+        ssm_values = self.load_from_ssm(
+            secrets_path=Path(self.ssm_prefix),
+            case_sensitive=settings.__config__.case_sensitive,
+        )
+
+        # The following was lifted from https://github.com/samuelcolvin/pydantic/blob/a21f0763ee877f0c86f254a5d60f70b1002faa68/pydantic/env_settings.py#L165-L237  # noqa
+        for field in settings.__fields__.values():
+            env_val: Optional[str] = None
+            for env_name in field.field_info.extra["env_names"]:
+                env_val = ssm_values.get(env_name)
+                if env_val is not None:
+                    break
+
+            is_complex, allow_json_failure = self._field_is_complex(field)
+            if is_complex:
+                if env_val is None:
+                    # field is complex but no value found so far, try explode_env_vars
+                    env_val_built = self._explode_ssm_values(field, ssm_values)
+                    if env_val_built:
+                        d[field.alias] = env_val_built
+                else:
+                    # field is complex and there's a value, decode that as JSON, then
+                    # add explode_env_vars
+                    try:
+                        env_val = settings.__config__.json_loads(env_val)
+                    except ValueError as e:
+                        if not allow_json_failure:
+                            raise SettingsError(
+                                f'error parsing JSON for "{env_name}"'
+                            ) from e
+
+                    if isinstance(env_val, dict):
+                        d[field.alias] = deep_update(
+                            env_val, self._explode_ssm_values(field, ssm_values)
+                        )
+                    else:
+                        d[field.alias] = env_val
+            elif env_val is not None:
+                # simplest case, field is not complex, we only need to add the
+                # value if it was found
+                d[field.alias] = env_val
+
+        return d
+
+    def _field_is_complex(self, field: ModelField) -> Tuple[bool, bool]:
+        """
+        Find out if a field is complex, and if so whether JSON errors should be ignored
+        """
+        if field.is_complex():
+            allow_json_failure = False
+        elif (
+            is_union(get_origin(field.type_))
+            and field.sub_fields
+            and any(f.is_complex() for f in field.sub_fields)
+        ):
+            allow_json_failure = True
+        else:
+            return False, False
+
+        return True, allow_json_failure
+
+    def _explode_ssm_values(
+        self, field: ModelField, env_vars: Mapping[str, Optional[str]]
+    ) -> Dict[str, Any]:
+        """
+        Process env_vars and extract the values of keys containing
+        env_nested_delimiter into nested dictionaries.
+
+        This is applied to a single field, hence filtering by env_var prefix.
+        """
+        prefixes = [
+            f"{env_name}{self.env_nested_delimiter}"
+            for env_name in field.field_info.extra["env_names"]
+        ]
+        result: Dict[str, Any] = {}
+        for env_name, env_val in env_vars.items():
+            if not any(env_name.startswith(prefix) for prefix in prefixes):
+                continue
+            _, *keys, last_key = env_name.split(self.env_nested_delimiter)
+            env_var = result
+            for key in keys:
+                env_var = env_var.setdefault(key, {})
+            env_var[last_key] = env_val
+
+        return result
 
     def __repr__(self) -> str:
         return f"AwsSsmSettingsSource(ssm_prefix={self.ssm_prefix!r})"