typing: make BindableLogger._context read-only

Nobody is writing into it except a deprecated threadlocal API.

Author: hynek

src/structlog/stdlib.py

@@ -537,10 +537,6 @@ class AsyncBoundLogger:
     #: synchronously occasionally.
     sync_bl: BoundLogger
 
-    # Blatant lie, we use a property for _context. Need this for Protocol
-    # though.
-    _context: Context
-
     _executor = None
     _bound_logger_factory = BoundLogger
 
@@ -565,11 +561,9 @@ def __init__(
         )
         self._loop = asyncio.get_running_loop()
 
-    # We have to ignore the type because we've already declared it to ensure
-    # we're a BindableLogger.
     # Instances would've been correctly recognized as such, however the class
     # not and we need the class in `structlog.configure()`.
-    @property  # type: ignore[no-redef]
+    @property
     def _context(self) -> Context:
         return self.sync_bl._context
 

src/structlog/threadlocal.py

@@ -127,7 +127,7 @@ def as_immutable(logger: TLLogger) -> TLLogger:
             processors=logger._processors,  # type: ignore[attr-defined]
             context={},
         )
-        bl._context = ctx
+        bl._context = ctx  # type: ignore[misc]
 
         return bl
     except AttributeError:

src/structlog/typing.py

@@ -136,7 +136,8 @@ class BindableLogger(Protocol):
     .. versionadded:: 20.2.0
     """
 
-    _context: Context
+    @property
+    def _context(self) -> Context: ...
 
     def bind(self, **new_values: Any) -> Self: ...