fix: Resolve lint and test failures in CI

- Remove unused `_singleton_connection` import in __init__.py (F401)
- Remove unused `os` import in test_thread_safe.py (F401)
- Remove unused `Callable` import in connection.py (F401)
- Fix mock_cache fixture: `cache` → `download_path` for 2.0 settings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/__init__.py

@@ -130,8 +130,8 @@ def conn(
     ThreadSafetyError
         If thread_safe mode is enabled.
     """
-    from .instance import _singleton_connection, _check_thread_safe, _global_config
     import datajoint.instance as instance_module
+    from pydantic import SecretStr
 
     _check_thread_safe()
 
@@ -140,19 +140,18 @@ def conn(
         # Use provided values or fall back to config
         host = host if host is not None else _global_config.database.host
         user = user if user is not None else _global_config.database.user
-        password = password if password is not None else _global_config.database.password
-        if password is not None and hasattr(password, 'get_secret_value'):
-            password = password.get_secret_value()
+        raw_password = password if password is not None else _global_config.database.password
+        password = raw_password.get_secret_value() if isinstance(raw_password, SecretStr) else raw_password
         port = _global_config.database.port
         use_tls = use_tls if use_tls is not None else _global_config.database.use_tls
 
         if user is None:
             from .errors import DataJointError
-            raise DataJointError(
-                "Database user not configured. Set dj.config['database.user'] or pass user= argument."
-            )
+
+            raise DataJointError("Database user not configured. Set dj.config['database.user'] or pass user= argument.")
         if password is None:
             from .errors import DataJointError
+
             raise DataJointError(
                 "Database password not configured. Set dj.config['database.password'] or pass password= argument."
             )
@@ -250,6 +249,7 @@ def FreeTable(conn_or_name, full_table_name: str | None = None) -> _FreeTable:
         # Called as FreeTable(conn, "db.table") - use provided connection
         return _FreeTable(conn_or_name, full_table_name)
 
+
 # =============================================================================
 # Lazy imports — heavy dependencies loaded on first access
 # =============================================================================

src/datajoint/instance.py

@@ -194,8 +194,7 @@ def _check_thread_safe() -> None:
     """
     if _load_thread_safe():
         raise ThreadSafetyError(
-            "Global DataJoint state is disabled in thread-safe mode. "
-            "Use dj.Instance() to create an isolated instance."
+            "Global DataJoint state is disabled in thread-safe mode. " "Use dj.Instance() to create an isolated instance."
         )
 
 
@@ -221,9 +220,8 @@ def _get_singleton_connection() -> Connection:
 
         host = _global_config.database.host
         user = _global_config.database.user
-        password = _global_config.database.password
-        if password is not None:
-            password = password.get_secret_value()
+        raw_password = _global_config.database.password
+        password = raw_password.get_secret_value() if raw_password is not None else None
         port = _global_config.database.port
         use_tls = _global_config.database.use_tls
 

tests/unit/test_thread_safe.py

@@ -1,7 +1,5 @@
 """Tests for thread-safe mode functionality."""
 
-import os
-
 import pytest