Split main into functions for easier testing

src/CSET/__init__.py

@@ -26,7 +26,44 @@
 
 
 def main():
-    """CLI entrypoint."""
+    """CLI entrypoint.
+
+    Handles argument parsing, setting up logging, top level error capturing,
+    and execution of the desired subcommand.
+    """
+    parser = setup_argument_parser()
+    cli_args = sys.argv[1:] + shlex.split(os.getenv("CSET_ADDOPTS", ""))
+    args, unparsed_args = parser.parse_known_args(cli_args)
+    setup_logging(args.verbose)
+
+    # Down here so runs after logging is setup.
+    logging.debug("CLI Arguments: %s", cli_args)
+
+    if args.subparser is None:
+        print("Please choose a command.", file=sys.stderr)
+        parser.print_usage()
+        sys.exit(127)
+
+    try:
+        # Execute the specified subcommand.
+        args.func(args, unparsed_args)
+    except ArgumentError as err:
+        # Error message for when needed template variables are missing.
+        print(err, file=sys.stderr)
+        parser.print_usage()
+        sys.exit(127)
+    except Exception as err:
+        # Provide slightly nicer error messages for unhandled exceptions.
+        print(err, file=sys.stderr)
+        # Display the time and full traceback when debug logging.
+        logging.debug("An unhandled exception occurred.")
+        if logging.root.isEnabledFor(logging.DEBUG):
+            raise
+        sys.exit(1)
+
+
+def setup_argument_parser() -> argparse.ArgumentParser:
+    """Create argument parser for CSET CLI."""
     parser = argparse.ArgumentParser(
         prog="cset", description="Convective Scale Evaluation Tool"
     )
@@ -134,59 +171,43 @@ def main():
     )
     parser_recipe_id.set_defaults(func=_recipe_id_command)
 
-    cli_args = sys.argv[1:] + shlex.split(os.getenv("CSET_ADDOPTS", ""))
-    args, unparsed_args = parser.parse_known_args(cli_args)
+    return parser
 
-    # Setup logging.
-    logging.captureWarnings(True)
-    loglevel = calculate_loglevel(args)
-    logger = logging.getLogger()
-    logger.setLevel(min(loglevel, logging.INFO))
-    stderr_log = logging.StreamHandler()
-    stderr_log.addFilter(lambda record: record.levelno >= loglevel)
-    stderr_log.setFormatter(logging.Formatter("%(asctime)s %(levelname)s %(message)s"))
-    logger.addHandler(stderr_log)
 
-    # Down here so runs after logging is setup.
-    logging.debug("CLI Arguments: %s", cli_args)
-
-    if args.subparser is None:
-        print("Please choose a command.", file=sys.stderr)
-        parser.print_usage()
-        sys.exit(127)
-
-    try:
-        # Execute the specified subcommand.
-        args.func(args, unparsed_args)
-    except ArgumentError as err:
-        logging.error(err)
-        parser.print_usage()
-        sys.exit(3)
-
-
-def calculate_loglevel(args) -> int:
-    """Calculate the logging level to apply.
+def setup_logging(verbosity: int):
+    """Configure logging level, format and output stream.
 
     Level is based on verbose argument and the LOGLEVEL environment variable.
     """
+    logging.captureWarnings(True)
+
+    # Calculate logging level.
     try:
         name_to_level = logging.getLevelNamesMapping()
     except AttributeError:
         # logging.getLevelNamesMapping() is python 3.11 or newer. Using
         # implementation detail for older versions.
         name_to_level = logging._nameToLevel
     # Level from CLI flags.
-    if args.verbose >= 2:
-        loglevel = logging.DEBUG
-    elif args.verbose == 1:
-        loglevel = logging.INFO
+    if verbosity >= 2:
+        cli_loglevel = logging.DEBUG
+    elif verbosity == 1:
+        cli_loglevel = logging.INFO
     else:
-        loglevel = logging.WARNING
-    return min(
-        loglevel,
-        # Level from environment variable.
-        name_to_level.get(os.getenv("LOGLEVEL"), logging.ERROR),
-    )
+        cli_loglevel = logging.WARNING
+    # Level from environment variable.
+    env_loglevel = name_to_level.get(os.getenv("LOGLEVEL"), logging.ERROR)
+    loglevel = min(cli_loglevel, env_loglevel)
+
+    # Configure the root logger.
+    logger = logging.getLogger()
+    # Record everything at least INFO for the log file.
+    logger.setLevel(min(loglevel, logging.INFO))
+    stderr_log = logging.StreamHandler()
+    # Filter stderr log to just what is requested.
+    stderr_log.addFilter(lambda record: record.levelno >= loglevel)
+    stderr_log.setFormatter(logging.Formatter("%(asctime)s %(levelname)s %(message)s"))
+    logger.addHandler(stderr_log)
 
 
 def _bake_command(args, unparsed_args):