[core] Add support for comma format (#54)

Fixes #15

### Test Plan
- Unit tests

Author: nhairs

docs/changelog.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Support `DictConfigurator` prefixes for `rename_fields` and `static_fields`. [#45](https://github.com/nhairs/python-json-logger/pull/45)
   - Allows using values like `ext://sys.stderr` in `fileConfig`/`dictConfig` value fields.
+- Support comma seperated lists for Formatter `fmt` (`style=","`) e.g. `"asctime,message,levelname"` [#15](https://github.com/nhairs/python-json-logger/issues/15)
+  - Note that this style is specific to `python-json-logger` and thus care should be taken not to pass this format to other logging Formatter implementations.
 
 ### Changed
 - Rename `pythonjsonlogger.core.LogRecord` and `log_record` arguments to avoid confusion / overlapping with `logging.LogRecord`. [#38](https://github.com/nhairs/python-json-logger/issues/38)

src/pythonjsonlogger/core.py

@@ -175,23 +175,31 @@ def __init__(
         - Renaming fields now preserves the order that fields were added in and avoids adding
           missing fields. The original behaviour, missing fields have a value of `None`, is still
           available by setting `rename_fields_keep_missing` to `True`.
+
+        *Added in 4.0*:
+
+        - `fmt` now supports comma seperated lists (`style=","`). Note that this style is specific
+          to `python-json-logger` and thus care should be taken to not to pass this format to other
+          logging Formatter implementations.
         """
         ## logging.Formatter compatibility
         ## ---------------------------------------------------------------------
-        # Note: validate added in 3.8, defaults added in 3.10
+        # Note: validate added in python 3.8, defaults added in 3.10
         if style in logging._STYLES:
             _style = logging._STYLES[style][0](fmt)  # type: ignore[operator]
             if validate:
                 _style.validate()
             self._style = _style
             self._fmt = _style._fmt
 
-        elif not validate:
+        elif style == "," or not validate:
             self._style = style
             self._fmt = fmt
 
+            # TODO: Validate comma format
+
         else:
-            raise ValueError(f"Style must be one of: {','.join(logging._STYLES.keys())}")
+            raise ValueError("Style must be one of: '%{$,'")
 
         self.datefmt = datefmt
 
@@ -271,6 +279,12 @@ def parse(self) -> List[str]:
         Returns:
             list of fields to be extracted and serialized
         """
+        if self._fmt is None:
+            return []
+
+        if isinstance(self._style, str) and self._style == ",":
+            return [field.strip() for field in self._fmt.split(",") if field.strip()]
+
         if isinstance(self._style, logging.StringTemplateStyle):
             formatter_style_pattern = STYLE_STRING_TEMPLATE_REGEX
 
@@ -285,10 +299,7 @@ def parse(self) -> List[str]:
         else:
             raise ValueError(f"Style {self._style!r} is not supported")
 
-        if self._fmt:
-            return formatter_style_pattern.findall(self._fmt)
-
-        return []
+        return formatter_style_pattern.findall(self._fmt)
 
     def serialize_log_record(self, log_data: LogData) -> str:
         """Returns the final representation of the data to be logged

tests/test_formatters.py

@@ -158,12 +158,22 @@ def test_default_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]
 
 @pytest.mark.parametrize("class_", ALL_FORMATTERS)
 def test_percentage_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
-    env.set_formatter(
-        class_(
-            # All kind of different styles to check the regex
-            "[%(levelname)8s] %(message)s %(filename)s:%(lineno)d %(asctime)"
-        )
-    )
+    # Note: We use different %s styles in the format to check the regex correctly collects them
+    env.set_formatter(class_("[%(levelname)8s] %(message)s %(filename)s:%(lineno)d %(asctime)"))
+
+    msg = "testing logging format"
+    env.logger.info(msg)
+    log_json = env.load_json()
+
+    assert log_json["message"] == msg
+    assert log_json.keys() == {"levelname", "message", "filename", "lineno", "asctime"}
+    return
+
+
+@pytest.mark.parametrize("class_", ALL_FORMATTERS)
+def test_comma_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
+    # Note: we have double comma `,,` to test handling "empty" names
+    env.set_formatter(class_("levelname,,message,filename,lineno,asctime,", style=","))
 
     msg = "testing logging format"
     env.logger.info(msg)