Add TypeVar for Job creation (#173)

sisyphus/job.py

@@ -18,7 +18,7 @@
 import sys
 import time
 import traceback
-from typing import List, Iterator
+from typing import List, Iterator, Type, TypeVar
 
 from sisyphus import block, tools
 from sisyphus.task import Task
@@ -61,12 +61,15 @@ def job_finished(path):
     )
 
 
+T = TypeVar("T", bound="Job")
+
+
 class JobSingleton(type):
 
     """Meta class to ensure that every Job with the same hash value is
     only created once"""
 
-    def __call__(cls, *args, **kwargs):
+    def __call__(cls: Type[T], *args, **kwargs) -> T:
         """Implemented to ensure that each job is created only once"""
         try:
             if "sis_tags" in kwargs:
@@ -109,6 +112,7 @@ def __call__(cls, *args, **kwargs):
         else:
             # create new object
             job = super(Job, cls).__new__(cls)
+            assert isinstance(job, Job)
             job._sis_tags = tags
 
             # store _sis_id
@@ -175,7 +179,7 @@ def get_lock(cls):
             Job._lock_storage.append(multiprocessing.Lock())
         return Job._lock_storage[Job._lock_index]
 
-    def __new__(cls, *args, **kwargs):
+    def __new__(cls: Type[T], *args, **kwargs) -> T:
         # Make sure unpickled jobs stay singletons
         assert len(args) == 1 and len(kwargs) == 0 and isinstance(args[0], str)
         sis_cache_key = args[0]