feat(logger): pretty-print JSON when POWERTOOLS_DEV is set

aws_lambda_powertools/logging/formatter.py

@@ -111,9 +111,15 @@ def __init__(
             Key-value to be included in log messages
 
         """
+
         self.json_deserializer = json_deserializer or json.loads
         self.json_default = json_default or str
-        self.json_serializer = json_serializer or partial(json.dumps, default=self.json_default, separators=(",", ":"))
+        self.json_indent = (
+            4 if os.getenv("AWS_SAM_LOCAL", "").lower() == "true" else None
+        )  # indented json serialization when in AWS SAM Local
+        self.json_serializer = json_serializer or partial(
+            json.dumps, default=self.json_default, separators=(",", ":"), indent=self.json_indent
+        )
 
         self.datefmt = datefmt
         self.use_datetime_directive = use_datetime_directive

docs/core/logger.md

@@ -555,6 +555,9 @@ As parameters don't always translate well between them, you can pass any callabl
 --8<-- "examples/logger/src/bring_your_own_json_serializer.py"
 ```
 
+???+ info
+    When your code runs in [AWS SAM local invoke](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-cli-command-reference-sam-local-invoke.html) (marked by the `AWS_SAM_LOCAL` env var), Logger's default `json.dumps` will apply indentation by four spaces.
+
 ## Testing your code
 
 ### Inject Lambda Context

tests/functional/test_logger_powertools_formatter.py

@@ -1,6 +1,7 @@
 """aws_lambda_logging tests."""
 import io
 import json
+import os
 import random
 import string
 import time
@@ -288,3 +289,22 @@ def test_log_formatting(stdout, service_name):
 
     # THEN the formatting should be applied (NB. this is valid json, but hasn't be parsed)
     assert log_dict["message"] == '["foo bar 123 [1, None]", null]'
+
+
+def test_log_json_indent_default(stdout, service_name, monkeypatch):
+    # GIVEN a logger with default settings while NOT in AWS SAM Local
+    if "AWS_SAM_LOCAL" in os.environ:
+        monkeypatch.delenv(name="AWS_SAM_LOCAL")
+    logger = Logger(service=service_name, stream=stdout)
+    logger.info("Test message")
+    # THEN the json should not be indented at all (using four blank spaces)
+    assert " " * 4 not in stdout.getvalue()
+
+
+def test_log_json_indent_aws_sam_local(stdout, service_name, monkeypatch):
+    # GIVEN a logger with default settings while in AWS SAM Local
+    monkeypatch.setenv(name="AWS_SAM_LOCAL", value="true")
+    logger = Logger(service=service_name, stream=stdout)
+    logger.info("Test message")
+    # THEN the json should contain indentation (of four blank spaces)
+    assert " " * 4 in stdout.getvalue()