PYTHON-3787 add types to max_staleness_selectors.py (#1276)

pymongo/max_staleness_selectors.py

@@ -26,17 +26,22 @@ When there is no known primary, a secondary S's staleness is estimated with:
 
 where "SMax" is the secondary with the greatest lastWriteDate.
 """
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
 
 from pymongo.errors import ConfigurationError
 from pymongo.server_type import SERVER_TYPE
 
+if TYPE_CHECKING:
+    from pymongo.server_selectors import Selection
 # Constant defined in Max Staleness Spec: An idle primary writes a no-op every
 # 10 seconds to refresh secondaries' lastWriteDate values.
 IDLE_WRITE_PERIOD = 10
 SMALLEST_MAX_STALENESS = 90
 
 
-def _validate_max_staleness(max_staleness, heartbeat_frequency):
+def _validate_max_staleness(max_staleness: int, heartbeat_frequency: int) -> None:
     # We checked for max staleness -1 before this, it must be positive here.
     if max_staleness < heartbeat_frequency + IDLE_WRITE_PERIOD:
         raise ConfigurationError(
@@ -53,7 +58,7 @@ def _validate_max_staleness(max_staleness, heartbeat_frequency):
         )
 
 
-def _with_primary(max_staleness, selection):
+def _with_primary(max_staleness: int, selection: Selection) -> Selection:
     """Apply max_staleness, in seconds, to a Selection with a known primary."""
     primary = selection.primary
     sds = []
@@ -75,7 +80,7 @@ def _with_primary(max_staleness, selection):
     return selection.with_server_descriptions(sds)
 
 
-def _no_primary(max_staleness, selection):
+def _no_primary(max_staleness: int, selection: Selection) -> Selection:
     """Apply max_staleness, in seconds, to a Selection with no known primary."""
     # Secondary that's replicated the most recent writes.
     smax = selection.secondary_with_max_last_write_date()
@@ -98,7 +103,7 @@ def _no_primary(max_staleness, selection):
     return selection.with_server_descriptions(sds)
 
 
-def select(max_staleness, selection):
+def select(max_staleness: int, selection: Selection) -> Selection:
     """Apply max_staleness, in seconds, to a Selection."""
     if max_staleness == -1:
         return selection