Add PEP 561 marker

setup.cfg

@@ -3,3 +3,52 @@
 # 3. If at all possible, it is good practice to do this. If you cannot, you
 # will need to generate wheels for each Python version that you support.
 python-tag=py3
+
+[mypy]
+
+# For details on each flag, please see the mypy documentation at:
+# https://mypy.readthedocs.io/en/stable/config_file.html#config-file
+
+# Import Discovery
+mypy_path = src
+namespace_packages = true
+
+# Disallow dynamic typing
+disallow_any_unimported = true
+disallow_any_expr = false
+disallow_any_decorated = true
+disallow_any_explicit = true
+disallow_any_generics = false
+disallow_subclassing_any = true
+
+# Untyped definitions and calls
+disallow_untyped_calls = false
+disallow_untyped_defs = false
+disallow_incomplete_defs = true
+check_untyped_defs = true
+disallow_untyped_decorators = true
+
+# None and Optional handling
+no_implicit_optional = true
+
+# Configuring warnings
+warn_redundant_casts = true
+warn_unused_ignores = true
+warn_no_return = true
+warn_return_any = true
+warn_unreachable = true
+
+# Miscellaneous strictness flags
+implicit_reexport = true
+strict_equality = true
+
+# Configuring error messages
+show_error_context = true
+show_column_numbers = true
+show_error_codes = true
+pretty = true
+show_absolute_path = true
+
+# Miscellaneous
+warn_unused_configs = true
+verbosity = 0

src/pythonjsonlogger/jsonlogger.py

@@ -9,20 +9,22 @@
 import traceback
 import importlib
 
+from typing import Dict, Union, List, Tuple
+
 from inspect import istraceback
 
 from collections import OrderedDict
 
 # skip natural LogRecord attributes
 # http://docs.python.org/library/logging.html#logrecord-attributes
-RESERVED_ATTRS = (
+RESERVED_ATTRS: Tuple[str, ...] = (
     'args', 'asctime', 'created', 'exc_info', 'exc_text', 'filename',
     'funcName', 'levelname', 'levelno', 'lineno', 'module',
     'msecs', 'message', 'msg', 'name', 'pathname', 'process',
     'processName', 'relativeCreated', 'stack_info', 'thread', 'threadName')
 
 
-def merge_record_extra(record, target, reserved):
+def merge_record_extra(record: logging.LogRecord, target: Dict, reserved: Union[Dict, List]) -> Dict:
     """
     Merges extra attributes from LogRecord object into target dictionary
 
@@ -138,7 +140,7 @@ def _str_to_fn(self, fn_as_str):
         module = importlib.import_module(path)
         return getattr(module, function)
 
-    def parse(self):
+    def parse(self) -> List[str]:
         """
         Parses format string looking for substitutions
 
@@ -155,7 +157,11 @@ def parse(self):
             formatter_style_pattern = re.compile(r'%\((.+?)\)s', re.IGNORECASE)
         else:
             raise ValueError('Invalid format: %s' % self._fmt)
-        return formatter_style_pattern.findall(self._fmt)
+
+        if self._fmt:
+            return formatter_style_pattern.findall(self._fmt)
+        else:
+            return []
 
     def add_fields(self, log_record, record, message_dict):
         """
@@ -220,6 +226,7 @@ def format(self, record):
             # Python2.7 doesn't have stack_info.
             pass
 
+        log_record: Dict
         try:
             log_record = OrderedDict()
         except NameError:

src/pythonjsonlogger/py.typed

@@ -0,0 +1 @@
+# PEP-561 marker. https://mypy.readthedocs.io/en/latest/installed_packages.html

tox.ini

@@ -11,4 +11,8 @@ python =
   3.10: py310
 
 [testenv]
-commands = python -m unittest discover
+deps =
+  mypy
+commands =
+  python -m unittest discover
+  mypy src