[core] Support sequence of strings for Formatter fmt

docs/changelog.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - 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.
+- Supports sequences of strings (e.g. lists and tuples) of field names for Formatter `fmt`.
 
 ### Changed
 - Rename `pythonjsonlogger.core.LogRecord` and `log_record` arguemtns to avoid confusion / overlapping with `logging.LogRecord`. [#38](https://github.com/nhairs/python-json-logger/issues/38)

src/pythonjsonlogger/core.py

@@ -128,7 +128,7 @@ class BaseJsonFormatter(logging.Formatter):
     # pylint: disable=too-many-arguments,super-init-not-called
     def __init__(
         self,
-        fmt: Optional[str] = None,
+        fmt: Optional[Union[str, Sequence[str]]] = None,
         datefmt: Optional[str] = None,
         style: str = "%",
         validate: bool = True,
@@ -145,11 +145,11 @@ def __init__(
     ) -> None:
         """
         Args:
-            fmt: string representing fields to log
+            fmt: String format or `Sequence` of field names of fields to log.
             datefmt: format to use when formatting `asctime` field
-            style: how to extract log fields from `fmt`
+            style: how to extract log fields from `fmt`. Ignored if `fmt` is a `Sequence[str]`.
             validate: validate `fmt` against style, if implementing a custom `style` you
-                must set this to `False`.
+                must set this to `False`. Ignored if `fmt` is a `Sequence[str]`.
             defaults: a dictionary containing default fields that are added before all other fields and
                 may be overridden. The supplied fields are still subject to `rename_fields`.
             prefix: an optional string prefix added at the beginning of
@@ -181,23 +181,34 @@ def __init__(
         - `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.
+        - `fmt` now supports sequences of strings (e.g. lists and tuples) of field names.
         """
         ## logging.Formatter compatibility
         ## ---------------------------------------------------------------------
         # Note: validate added in 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 style == "," or not validate:
-            self._style = style
-            self._fmt = fmt
+        if fmt is None or isinstance(fmt, str):
+            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
 
-        else:
-            raise ValueError(f"Style must be one of: {','.join(logging._STYLES.keys())}")
+            elif style == "," or not validate:
+                self._style = style
+                self._fmt = fmt
+
+            else:
+                raise ValueError(f"Style must be one of: {','.join(logging._STYLES.keys())}")
+
+            self._required_fields = self.parse()
+
+        # Note: we do this check second as string is still a Sequence[str]
+        elif isinstance(fmt, Sequence):
+            self._style = "__sequence__"
+            self._fmt = str(fmt)
+            self._required_fields = list(fmt)
 
         self.datefmt = datefmt
 
@@ -219,7 +230,6 @@ def __init__(
         self.reserved_attrs = set(reserved_attrs if reserved_attrs is not None else RESERVED_ATTRS)
         self.timestamp = timestamp
 
-        self._required_fields = self.parse()
         self._skip_fields = set(self._required_fields)
         self._skip_fields.update(self.reserved_attrs)
         self.defaults = defaults if defaults is not None else {}
@@ -282,12 +292,18 @@ def parse(self) -> List[str]:
             # (we already (mostly) check for valid style names in __init__
             return []
 
-        if isinstance(self._style, str) and self._style == ",":
-            # TODO: should we check that there are no empty fields?
-            # If yes we should do this in __init__ where we validate other styles?
-            # Do we drop empty fields?
-            # etc
-            return [field.strip() for field in self._fmt.split(",") if field.strip()]
+        if isinstance(self._style, str):
+            if self._style == "__sequence__":
+                raise RuntimeError("Must not call parse when fmt is a sequence of strings")
+
+            if self._style == ",":
+                # TODO: should we check that there are no empty fields?
+                # If yes we should do this in __init__ where we validate other styles?
+                # Do we drop empty fields?
+                # etc
+                return [field.strip() for field in self._fmt.split(",") if field.strip()]
+
+            raise ValueError(f"Style {self._style!r} is not supported")
 
         if isinstance(self._style, logging.StringTemplateStyle):
             formatter_style_pattern = STYLE_STRING_TEMPLATE_REGEX

tests/test_formatters.py

@@ -184,6 +184,32 @@ def test_comma_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
     return
 
 
+@pytest.mark.parametrize("class_", ALL_FORMATTERS)
+def test_sequence_list_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
+    env.set_formatter(class_(["levelname", "message", "filename", "lineno", "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_sequence_tuple_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
+    env.set_formatter(class_(("levelname", "message", "filename", "lineno", "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_defaults_field(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
     env.set_formatter(class_(defaults={"first": 1, "second": 2}))