mark lazy mode deprecated and add logger.lazy() for lazy format value

loguru/__init__.pyi

@@ -25,6 +25,8 @@ from typing import (
     overload,
 )
 
+import typing_extensions
+
 if sys.version_info >= (3, 6):
     from typing import Awaitable
 else:
@@ -47,6 +49,7 @@ else:
 
 _T = TypeVar("_T")
 _F = TypeVar("_F", bound=Callable[..., Any])
+_P = typing_extensions.ParamSpec("_P")
 ExcInfo = Tuple[Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]]
 
 class _GeneratorContextManager(ContextManager[_T], Generic[_T]):
@@ -364,5 +367,7 @@ class Logger:
     def log(__self, __level: Union[int, str], __message: Any) -> None: ...  # noqa: N805
     def start(self, *args: Any, **kwargs: Any) -> int: ...
     def stop(self, *args: Any, **kwargs: Any) -> None: ...
+    @staticmethod
+    def lazy(fn: Callable[_P, Any], *args: _P.args, **kwargs: _P.kwargs) -> Any: ...
 
 logger: Logger

loguru/_logger.py

@@ -117,7 +117,6 @@
 else:
     from pathlib import PurePath as PathLike
 
-
 Level = namedtuple("Level", ["name", "no", "color", "icon"])  # noqa: PYI024
 
 start_time = aware_now()
@@ -234,6 +233,13 @@ class Logger:
     """
 
     def __init__(self, core, exception, depth, record, lazy, colors, raw, capture, patchers, extra):
+        if lazy:
+            warnings.warn(
+                "lazy mode is deprecated, use `LazyValue` for deferred value",
+                stacklevel=2,
+                category=DeprecationWarning,
+            )
+
         self._core = core
         self._options = (exception, depth, record, lazy, colors, raw, capture, patchers, extra)
 
@@ -2137,3 +2143,27 @@ def stop(self, *args, **kwargs):
             stacklevel=2,
         )
         return self.remove(*args, **kwargs)
+
+    @staticmethod
+    def lazy(fn, *args, **kwargs):
+        """Make fn lazy evaluated.
+
+        For example, fn will only be called if debug level is enabled.
+
+        ```python
+        logger.debug("hello {}", logger.lazy(fn, *...))
+        ```
+        """
+        return LazyValue(fn, *args, **kwargs)
+
+
+class LazyValue:
+    __slots__ = ("args", "fn", "kwargs")
+
+    def __init__(self, fn, *args, **kwargs):
+        self.fn = fn
+        self.args = args
+        self.kwargs = kwargs
+
+    def __format__(self, format_spec: str):
+        return format(self.fn(*self.args, **self.kwargs), format_spec)

pyproject.toml

@@ -99,6 +99,8 @@ filterwarnings = [
   'error',
   # Mixing threads and "fork()" is deprecated, but we need to test it anyway.
   'ignore:.*use of fork\(\) may lead to deadlocks in the child.*:DeprecationWarning',
+  # deprecated message for users
+  'ignore:lazy mode is deprecated.*',
 ]
 testpaths = ["tests"]
 

tests/test_lazy.py

@@ -0,0 +1,9 @@
+from loguru import logger
+
+
+def test_lazy_value(writer):
+    logger_bound = logger.bind(a=0)
+    logger_bound.add(writer, format="{message}")
+    logger_bound.info("hello {}", logger.lazy(str, 1))
+
+    assert writer.read() == "hello 1\n"

tests/test_type_hinting.py

@@ -1,8 +1,10 @@
 import sys
 
 import mypy.api
+import pytest
 
 
+@pytest.mark.skipif(sys.version_info < (3, 6), reason="typing supported is added in py36")
 def test_mypy_import():
     # Check stub file is valid and can be imported by Mypy.
     # There exist others tests in "typesafety" subfolder but they aren't compatible with Python 3.5.