Merge pull request #265 from djarecka/fix/isexistingfile

small fix to is_existing_file (follow-up to #255 and #257)

Author: djarecka

pydra/engine/helpers_file.py

@@ -537,7 +537,7 @@ def is_local_file(f):
 
 def is_existing_file(value):
     """ checking if an object is an existing file"""
-    if value is "":
+    if isinstance(value, str) and value == "":
         return False
     try:
         return Path(value).exists()

pydra/engine/tests/test_node_task.py

@@ -991,6 +991,7 @@ def test_task_state_6a(plugin):
         assert odir.exists()
 
 
+@pytest.mark.flaky(reruns=2)  # when dask
 def test_task_state_comb_1(plugin_dask_opt):
     """ task with the simplest splitter and combiner"""
     nn = fun_addtwo(name="NA").split(a=[3, 5], splitter="a").combine(combiner="a")
@@ -1279,6 +1280,7 @@ def test_task_state_comb_order():
 # Testing caching for tasks with states
 
 
+@pytest.mark.flaky(reruns=2)  # when dask
 def test_task_state_cachedir(plugin_dask_opt, tmpdir):
     """ task with a state and provided cache_dir using pytest tmpdir"""
     cache_dir = tmpdir.mkdir("test_task_nostate")