Name the source of each config option in diagnose

.changesets/name-the-source-of-each-config-option.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: change
+---
+
+The `appsignal diagnose` report now names where each configuration option's value came from, so an option that is not what you set says what set it instead. An option holding a value from more than one source lists each one with the value it holds, in the order they are merged.

src/appsignal/cli/diagnose.py

@@ -8,11 +8,11 @@
 from argparse import ArgumentParser
 from pathlib import Path
 from sys import stderr
-from typing import Any
+from typing import Any, cast
 
 from ..__about__ import __version__
 from ..agent import Agent
-from ..config import Config
+from ..config import SOURCE_ORDER, Config
 from ..push_api_key_validator import PushApiKeyValidator
 from ..transmitter import transmit
 from .command import AppsignalCLICommand
@@ -311,12 +311,36 @@ def _configuration_information(self) -> None:
         print("Configuration")
 
         for key in self.config.options:
-            print(f"  {key}: {self.config.options[key]!r}")  # type: ignore
+            value = self.config.options[key]  # type: ignore
+            print(f"  {key}: {value!r}{self._config_sources_label(key)}")
 
         print()
         print("Read more about how the diagnose config output is rendered")
         print("https://docs.appsignal.com/python/command-line/diagnose.html")
 
+    # Names where an option's value came from, so that somebody looking at an
+    # option that is not what they set can see what set it instead. An option
+    # left at its default says nothing, because there is no source to point at.
+    def _config_sources_label(self, option: str) -> str:
+        sources = cast(dict, self.config.sources)
+        names = [name for name in SOURCE_ORDER if option in sources[name]]
+
+        if names == ["default"]:
+            return ""
+
+        if len(names) == 1:
+            return f" (Loaded from: {names[0]})"
+
+        # More than one source holds the option, so each is listed with the
+        # value it holds, in the order they are merged. The last one wins.
+        width = max(len(name) for name in names) + 1
+        lines = ["", "    Sources:"]
+        for name in names:
+            label = f"{name}:".ljust(width)
+            lines.append(f"      {label} {sources[name][option]!r}")
+
+        return "\n".join(lines)
+
     def _validation_information(self) -> None:
         validation_report: Any = self.report["validation"]
         print("Validation")

src/appsignal/config.py

@@ -73,6 +73,13 @@ class Sources(TypedDict):
     environment: Options
 
 
+# The configuration sources, in the order they are merged, so the last one
+# holding an option is the one whose value it takes. Both the merge and the
+# diagnose report walk this, so neither can disagree with the other about
+# where a value came from.
+SOURCE_ORDER: list[str] = ["default", "system", "environment", "initial"]
+
+
 class Config:
     valid: bool
     sources: Sources
@@ -142,11 +149,10 @@ def __init__(self, options: Options | None = None) -> None:
             initial=without_none_overrides(options or Options(), system),
             environment=Config.load_from_environment(),
         )
+        sources = cast(dict, self.sources)
         final_options = Options()
-        final_options.update(self.sources["default"])
-        final_options.update(self.sources["system"])
-        final_options.update(self.sources["environment"])
-        final_options.update(self.sources["initial"])
+        for source in SOURCE_ORDER:
+            final_options.update(sources[source])
         self.options = final_options
         self._validate()
 

tests/cli/test_diagnose.py

@@ -116,3 +116,29 @@ def test_diagnose_with_missing_paths(mocker, capfd):
 
     out, err = capfd.readouterr()
     assert "Exists?: False" in out
+
+
+def test_diagnose_names_where_a_value_came_from(mocker, capfd):
+    os.environ["APPSIGNAL_APP_ENV"] = "production"
+    os.environ["APPSIGNAL_HOST_ROLE"] = "worker"
+
+    main(["diagnose", "--no-send-report"])
+
+    out, err = capfd.readouterr()
+    # One source holds it, so the option just says which.
+    assert "  host_role: 'worker' (Loaded from: environment)" in out
+    # The default holds a value too, so both are listed with what they hold.
+    assert (
+        "  environment: 'production'\n"
+        "    Sources:\n"
+        "      default:     'development'\n"
+        "      environment: 'production'\n"
+    ) in out
+
+
+def test_diagnose_says_nothing_about_an_option_left_at_its_default(mocker, capfd):
+    main(["diagnose", "--no-send-report"])
+
+    out, err = capfd.readouterr()
+    assert "  log: 'file'\n" in out
+    assert "  log: 'file' (Loaded from" not in out