SAR-55 Add option to use static tag string

This is useful for the Tango SKABaseDevice, where the only tag is the
device name.

ska_logging/__init__.py

@@ -2,12 +2,22 @@
 
 """Module init code."""
 
-__all__ = ("configure_logging", "get_default_formatter")
+__all__ = (
+    "configure_logging",
+    "get_default_formatter",
+    "SkaLoggingError",
+    "SkaLoggingTagsFormatError",
+)
 __author__ = "Anton Joubert"
 __email__ = "ajoubert+ska@ska.ac.za"
 
 
-from .configuration import configure_logging, get_default_formatter
+from .configuration import (
+    configure_logging,
+    get_default_formatter,
+    SkaLoggingError,
+    SkaLoggingTagsFormatError,
+)
 
 
 # BEGIN VERSION CHECK

ska_logging/configuration.py

@@ -7,6 +7,14 @@ import logging.config
 import time
 
 
+class SkaLoggingError(Exception):
+    """Base class for all SKA Logger exceptions."""
+
+
+class SkaLoggingTagsFormatError(SkaLoggingError):
+    """Invalid format for the 'tags' field string."""
+
+
 class _UTCFormatter(logging.Formatter):
     converter = time.gmtime
 
@@ -38,6 +46,8 @@ _FORMAT_STR_WITH_TAGS = (
     "%(message)s"
 )
 
+_INVALID_TAG_CHARS = ("|", "%")
+
 _LOGGING_CONFIG = {
     "version": 1,
     "disable_existing_loggers": False,
@@ -113,17 +123,33 @@ def get_default_formatter(tags=False):
     Parameters
     ----------
 
-    tags : bool, optional
-        If true, then include the "tags" field in the format string.  This requires
+    tags : bool or str, optional
+        If boolean, then treated as a toggle:
+          - True:  include the "tags" field in the format string.  This requires
                    a tags filter to be linked to the corresponding handler.
+          - False:  exclude the "tags" field from the format string.
+        If string, then it is a static tag.  Instead of using a logging filter, the
+        formatter will just use this static string for the "tags" field directly.
 
     Returns
     -------
     logging.Formatter
         A new default formatter.
 
+    Raises
+    ------
+    SkaLoggingTagsFormatError:
+        If the static tags string has an invalid format.
+
     """
-    if tags:
+    if isinstance(tags, str):
+        invalid_chars = [c for c in _INVALID_TAG_CHARS if c in tags]
+        if invalid_chars:
+            raise SkaLoggingTagsFormatError(
+                "Invalid char(s) {} in tags: {!r}".format(invalid_chars, tags)
+            )
+        format_str = _FORMAT_STR_WITH_TAGS.replace("%(tags)s", tags)
+    elif tags:
         format_str = _FORMAT_STR_WITH_TAGS
     else:
         format_str = _FORMAT_STR_NO_TAGS

tests/test_configuration.py

@@ -9,7 +9,7 @@ import pytest
 
 import ska_logging.configuration
 
-from ska_logging import configure_logging, get_default_formatter
+from ska_logging import configure_logging, get_default_formatter, SkaLoggingTagsFormatError
 
 
 @pytest.fixture
@@ -165,7 +165,7 @@ class TestConfigureLogging:
         assert get_named_handler(logger, "test")
 
 
-class TestGetFormatter:
+class TestGetDefaultFormatter:
     """Tests for :func:`~ska_logging.configuration.get_default_formatter`."""
 
     def test_default_no_tags(self):
@@ -173,16 +173,29 @@ class TestGetFormatter:
         assert isinstance(formatter, ska_logging.configuration._UTCFormatter)
         assert formatter._fmt == ska_logging.configuration._FORMAT_STR_NO_TAGS
 
-    def test_get_without_tags(self):
+    def test_get_tags_disabled(self):
         formatter = get_default_formatter(tags=False)
         assert isinstance(formatter, ska_logging.configuration._UTCFormatter)
         assert formatter._fmt == ska_logging.configuration._FORMAT_STR_NO_TAGS
 
-    def test_get_with_tags(self):
+    def test_get_tags_enabled(self):
         formatter = get_default_formatter(tags=True)
         assert isinstance(formatter, ska_logging.configuration._UTCFormatter)
         assert formatter._fmt == ska_logging.configuration._FORMAT_STR_WITH_TAGS
 
+    def test_get_tags_static_string(self):
+        formatter = get_default_formatter(tags="test-key:test-value")
+        assert isinstance(formatter, ska_logging.configuration._UTCFormatter)
+        tags_format = ska_logging.configuration._FORMAT_STR_WITH_TAGS
+        expected_format = tags_format.replace("%(tags)s", "test-key:test-value")
+        assert formatter._fmt == expected_format
+
+    def test_get_tags_invalid_static_string(self):
+        with pytest.raises(SkaLoggingTagsFormatError):
+            get_default_formatter(tags="no|pipes|allowed")
+        with pytest.raises(SkaLoggingTagsFormatError):
+            get_default_formatter(tags="no%percentage%symbols%allowed")
+
 
 class TestOverride:
     """Tests for :func:`~ska_logging.configuration._override`.