CLI: fix bug in the CliFormatter log utility

The `format` method assumed that the `prefix` attribute would always
exist, which is not always this case. This would result in an uncaught
exception when a message would get logged. This went unnoticed since the
utility was not individually tested. Additionally, messages with literal
percentage signs would also except because it would always be
interpolated with the `record.args` but if that was an empty tuple, a
`TypeError` would be raised.

aiida/cmdline/utils/log.py

@@ -51,7 +51,15 @@ def format(record):
         except KeyError:
             fg = 'white'
 
-        if record.prefix:
+        try:
+            prefix = record.prefix
+        except AttributeError:
+            prefix = None
+
+        if prefix:
             return f'{click.style(record.levelname.capitalize(), fg=fg, bold=True)}: {record.msg % record.args}'
 
-        return f'{record.msg % record.args}'
+        if record.args:
+            return f'{record.msg % record.args}'
+
+        return record.msg

tests/cmdline/utils/test_log.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+###########################################################################
+# Copyright (c), The AiiDA team. All rights reserved.                     #
+# This file is part of the AiiDA code.                                    #
+#                                                                         #
+# The code is hosted on GitHub at https://github.com/aiidateam/aiida-core #
+# For further information on the license, see the LICENSE.txt file        #
+# For further information please visit http://www.aiida.net               #
+###########################################################################
+"""Tests for the :mod:`aiida.cmdline.utils.log` module."""
+import logging
+
+from aiida.cmdline.utils import log
+
+
+def test_cli_formatter():
+    """Test the ``CliFormatter.format`` method for a plain message.
+
+    Note that if it contains percentage signs but no arguments, it should not try to convert it.
+    """
+    message = 'message'
+    record = logging.LogRecord('name', logging.INFO, 'pathname', 0, message, (), 'exc_info')
+    assert log.CliFormatter.format(record) == message
+
+
+def test_cli_formatter_no_args():
+    """Test the ``CliFormatter.format`` method for a message with percentage signs but no args."""
+    message = 'PID    MEM %    CPU %  started'
+    record = logging.LogRecord('name', logging.INFO, 'pathname', 0, message, (), 'exc_info')
+    assert log.CliFormatter.format(record) == message
+
+
+def test_cli_formatter_args():
+    """Test the ``CliFormatter.format`` method for a message with a single argument."""
+    record = logging.LogRecord('name', logging.INFO, 'pathname', 0, 'Some %s', ('value',), 'exc_info')
+    assert log.CliFormatter.format(record) == 'Some value'
+
+
+def test_cli_formatter_prefix():
+    """Test the ``CliFormatter.format`` method for a message with a single argument."""
+    record = logging.LogRecord('name', logging.INFO, 'pathname', 0, 'Some %s', ('value',), 'exc_info')
+    record.prefix = True
+    assert log.CliFormatter.format(record) == '\x1b[34m\x1b[1mInfo\x1b[0m: Some value'