diff --git a/pydra/engine/helpers_file.py b/pydra/engine/helpers_file.py
index f15e2028..39f38785 100644
--- a/pydra/engine/helpers_file.py
+++ b/pydra/engine/helpers_file.py
@@ -535,11 +535,11 @@ def is_local_file(f):
     return f.type is File and "container_path" not in f.metadata
 
 
-def is_existing_file(f):
+def is_existing_file(value):
     """ checking if an object is an existing file"""
-    if not f:
+    if value == "":
         return False
     try:
-        return Path(f).exists()
+        return Path(value).exists()
     except TypeError:
         return False

error returning numpy arrays with more than one element in workflow output
version: pydra master

the key piece appears to be the fact that a numpy array is being returned with more than 1 element. i.e the error goes away if the output of the function is changed to `np.array([val])` or `np.array([val, val]).tolist()` 

i think this is the same error that @nicolocin ran into.

<details> <summary> error </summary>

```shell
$ python multiout.py 
Traceback (most recent call last):
  File "multiout.py", line 24, in <module>
    sub(runnable=wf)
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/submitter.py", line 57, in __call__
    self.loop.run_until_complete(self.submit_workflow(runnable, rerun=rerun))
  File "/Users/satra/software/miniconda3/envs/mee-voice/lib/python3.8/asyncio/base_events.py", line 612, in run_until_complete
    return future.result()
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/submitter.py", line 71, in submit_workflow
    await workflow._run(self, rerun=rerun)
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/core.py", line 869, in _run
    save(odir, result=result, task=self)
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers.py", line 117, in save
    result = copyfile_workflow(wf_path=task_path, result=result)
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers.py", line 129, in copyfile_workflow
    new_value = _copyfile_single_value(wf_path=wf_path, value=value)
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers.py", line 138, in _copyfile_single_value
    return [_copyfile_single_value(wf_path, val) for val in value]
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers.py", line 138, in <listcomp>
    return [_copyfile_single_value(wf_path, val) for val in value]
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers.py", line 143, in _copyfile_single_value
    elif is_existing_file(value):
  File "/Users/satra/software/nipy-repo/pydra/pydra/engine/helpers_file.py", line 540, in is_existing_file
    if not f:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
</details>

To replicate run this code
<details> <summary> code </summary>

```python
import numpy as np
import pydra
import typing as ty
import os

@pydra.mark.task
@pydra.mark.annotate({"return": {"b": ty.Any}})
def test_multiout(val):
    return np.array([val, val])

if __name__ == "__main__":
    cache_dir = os.path.join(os.getcwd(), 'cache3')
    wf = pydra.Workflow("", input_spec=['val'], val=[0, 1, 2],
                        cache_dir=cache_dir)
    wf.add(test_multiout(name="mo",
                         val=wf.lzin.val,
                         cache_dir=cache_dir))
    wf.mo.split('val').combine('val')
    wf.set_output([("b", wf.mo.lzout.b)])

    with pydra.Submitter(plugin="cf", n_procs=2) as sub:
        sub(runnable=wf)
    results = wf.result(return_inputs=True)
```
</details>
diff --git a/pydra/engine/tests/test_numpy_examples.py b/pydra/engine/tests/test_numpy_examples.py
new file mode 100644
index 00000000..55884ac2
--- /dev/null
+++ b/pydra/engine/tests/test_numpy_examples.py
@@ -0,0 +1,35 @@
+import numpy as np
+import typing as ty
+import importlib
+import pytest
+
+from ..submitter import Submitter
+from ..core import Workflow
+from ...mark import task, annotate
+
+if importlib.util.find_spec("numpy") is None:
+    pytest.skip("can't find numpy library", allow_module_level=True)
+
+
+@task
+@annotate({"return": {"b": ty.Any}})
+def arrayout(val):
+    return np.array([val, val])
+
+
+def test_multiout(plugin):
+    """ testing a simple function that returns a numpy array"""
+    wf = Workflow("wf", input_spec=["val"], val=[0, 1, 2])
+    wf.add(arrayout(name="mo", val=wf.lzin.val))
+    wf.mo.split("val").combine("val")
+
+    wf.set_output([("array", wf.mo.lzout.b)])
+
+    with Submitter(plugin=plugin, n_procs=2) as sub:
+        sub(runnable=wf)
+
+    results = wf.result(return_inputs=True)
+
+    assert results[0] == {"wf.val": [0, 1, 2]}
+    for el in range(3):
+        assert np.array_equal(results[1].output.array[el], np.array([el, el]))

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest
3.7
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==24.2.0
backcall==0.2.0
beautifulsoup4==4.13.3
black==23.3.0
bleach==6.0.0
boutiques==0.5.27
cachetools==5.5.2
certifi @ file:///croot/certifi_1671487769961/work/certifi
cfgv==3.3.1
charset-normalizer==3.4.1
ci-info==0.3.0
click==8.1.8
cloudpickle==2.2.1
codecov==2.1.13
comm==0.1.4
coverage==7.2.7
debugpy==1.7.0
decorator==5.1.1
defusedxml==0.7.1
distlib==0.3.9
entrypoints==0.4
etelemetry==0.3.1
exceptiongroup==1.2.2
execnet==2.0.2
fastjsonschema==2.21.1
filelock==3.12.2
frozendict==2.4.6
identify==2.5.24
idna==3.10
importlib-metadata==6.7.0
importlib-resources==5.12.0
iniconfig==2.0.0
ipykernel==6.9.2
ipython==7.34.0
ipython-genutils==0.2.0
ipywidgets==8.1.5
jedi==0.19.2
Jinja2==3.1.6
json5==0.9.16
jsonschema==4.17.3
jupyter==1.1.1
jupyter-client==7.1.2
jupyter-console==6.4.4
jupyter-contrib-core==0.4.2
jupyter-contrib-nbextensions==0.5.1
jupyter-highlight-selected-word==0.2.0
jupyter-latex-envs==1.4.6
jupyter-nbextensions-configurator==0.4.1
jupyter_core==4.12.0
jupyterlab==2.3.2
jupyterlab-pygments==0.2.2
jupyterlab-server==1.2.0
jupyterlab_widgets==3.0.13
lxml==5.3.1
MarkupSafe==2.1.5
matplotlib-inline==0.1.6
mistune==3.0.2
mypy-extensions==1.0.0
nbclient==0.7.4
nbconvert==7.6.0
nbformat==5.8.0
nest-asyncio==1.6.0
nodeenv==1.9.1
notebook==5.7.8
numpy==1.21.6
packaging==24.0
pandocfilters==1.5.1
parso==0.8.4
pathspec==0.11.2
pexpect==4.9.0
pickleshare==0.7.5
pkgutil_resolve_name==1.3.10
platformdirs==4.0.0
pluggy==1.2.0
pre-commit==2.21.0
prometheus-client==0.17.1
prompt_toolkit==3.0.48
psutil==7.0.0
ptyprocess==0.7.0
-e git+https://github.com/nipype/pydra.git@acc90101af22637ff71139224441eacd08b14452#egg=pydra
Pygments==2.17.2
PyLD==2.0.4
pyrsistent==0.19.3
pytest==7.4.4
pytest-cov==4.1.0
pytest-env==1.0.1
pytest-rerunfailures==13.0
pytest-xdist==3.5.0
python-dateutil==2.9.0.post0
PyYAML==6.0.1
pyzmq==26.2.1
requests==2.31.0
Send2Trash==1.8.3
simplejson==3.20.1
six==1.17.0
soupsieve==2.4.1
tabulate==0.9.0
termcolor==2.3.0
terminado==0.13.3
tinycss2==1.2.1
tomli==2.0.1
tornado==4.5.3
traitlets==5.9.0
typed-ast==1.5.5
typing_extensions==4.7.1
urllib3==2.0.7
virtualenv==20.26.6
wcwidth==0.2.13
webencodings==0.5.1
widgetsnbextension==4.0.13
zipp==3.15.0

name: pydra
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2022.12.7=py37h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=22.3.1=py37h06a4308_0
  - python=3.7.16=h7a1cb2a_0
  - readline=8.2=h5eee18b_0
  - setuptools=65.6.3=py37h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.38.4=py37h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - attrs==24.2.0
      - backcall==0.2.0
      - beautifulsoup4==4.13.3
      - black==23.3.0
      - bleach==6.0.0
      - boutiques==0.5.27
      - cachetools==5.5.2
      - cfgv==3.3.1
      - charset-normalizer==3.4.1
      - ci-info==0.3.0
      - click==8.1.8
      - cloudpickle==2.2.1
      - codecov==2.1.13
      - comm==0.1.4
      - coverage==7.2.7
      - debugpy==1.7.0
      - decorator==5.1.1
      - defusedxml==0.7.1
      - distlib==0.3.9
      - entrypoints==0.4
      - etelemetry==0.3.1
      - exceptiongroup==1.2.2
      - execnet==2.0.2
      - fastjsonschema==2.21.1
      - filelock==3.12.2
      - frozendict==2.4.6
      - identify==2.5.24
      - idna==3.10
      - importlib-metadata==6.7.0
      - importlib-resources==5.12.0
      - iniconfig==2.0.0
      - ipykernel==6.9.2
      - ipython==7.34.0
      - ipython-genutils==0.2.0
      - ipywidgets==8.1.5
      - jedi==0.19.2
      - jinja2==3.1.6
      - json5==0.9.16
      - jsonschema==4.17.3
      - jupyter==1.1.1
      - jupyter-client==7.1.2
      - jupyter-console==6.4.4
      - jupyter-contrib-core==0.4.2
      - jupyter-contrib-nbextensions==0.5.1
      - jupyter-core==4.12.0
      - jupyter-highlight-selected-word==0.2.0
      - jupyter-latex-envs==1.4.6
      - jupyter-nbextensions-configurator==0.4.1
      - jupyterlab==2.3.2
      - jupyterlab-pygments==0.2.2
      - jupyterlab-server==1.2.0
      - jupyterlab-widgets==3.0.13
      - lxml==5.3.1
      - markupsafe==2.1.5
      - matplotlib-inline==0.1.6
      - mistune==3.0.2
      - mypy-extensions==1.0.0
      - nbclient==0.7.4
      - nbconvert==7.6.0
      - nbformat==5.8.0
      - nest-asyncio==1.6.0
      - nodeenv==1.9.1
      - notebook==5.7.8
      - numpy==1.21.6
      - packaging==24.0
      - pandocfilters==1.5.1
      - parso==0.8.4
      - pathspec==0.11.2
      - pexpect==4.9.0
      - pickleshare==0.7.5
      - pkgutil-resolve-name==1.3.10
      - platformdirs==4.0.0
      - pluggy==1.2.0
      - pre-commit==2.21.0
      - prometheus-client==0.17.1
      - prompt-toolkit==3.0.48
      - psutil==7.0.0
      - ptyprocess==0.7.0
      - pydra==0.5
      - pygments==2.17.2
      - pyld==2.0.4
      - pyrsistent==0.19.3
      - pytest==7.4.4
      - pytest-cov==4.1.0
      - pytest-env==1.0.1
      - pytest-rerunfailures==13.0
      - pytest-xdist==3.5.0
      - python-dateutil==2.9.0.post0
      - pyyaml==6.0.1
      - pyzmq==26.2.1
      - requests==2.31.0
      - send2trash==1.8.3
      - simplejson==3.20.1
      - six==1.17.0
      - soupsieve==2.4.1
      - tabulate==0.9.0
      - termcolor==2.3.0
      - terminado==0.13.3
      - tinycss2==1.2.1
      - tomli==2.0.1
      - tornado==4.5.3
      - traitlets==5.9.0
      - typed-ast==1.5.5
      - typing-extensions==4.7.1
      - urllib3==2.0.7
      - virtualenv==20.26.6
      - wcwidth==0.2.13
      - webencodings==0.5.1
      - widgetsnbextension==4.0.13
      - zipp==3.15.0
prefix: /opt/conda/envs/pydra

pydra/engine/tests/test_numpy_examples.py::test_multiout[cf]
pydra/engine/helpers_file.py
pydra/engine/helpers_file.py