Container component example fails because placeholder dereferences None

System information

Have I specified the code to reproduce the issue (Yes, No): Yes
Environment in which the code is executed (e.g., Local(Linux/MacOS/Windows), Interactive Notebook, Google Cloud, etc): MacOS
TensorFlow version: tensorflow==2.4.2
TFX Version: tfx==0.30.1
Python version: Python 3.7.7

Python dependencies (from pip freeze output):

absl-py==0.12.0
aiohttp==3.7.4.post0
apache-beam==2.30.0
appnope==0.1.2
argon2-cffi==20.1.0
astunparse==1.6.3
async-generator==1.10
async-timeout==3.0.1
attrs==20.3.0
avro-python3==1.9.2.1
backcall==0.2.0
bleach==3.3.0
cachetools==4.2.2
certifi==2021.5.30
cffi==1.14.5
chardet==4.0.0
chex==0.0.8
click==7.1.2
colorama==0.4.4
crcmod==1.7
cycler==0.10.0
debugpy==1.3.0
decorator==5.0.9
defusedxml==0.7.1
dill==0.3.1.1
dm-tree==0.1.6
docker==4.4.4
docopt==0.6.2
entrypoints==0.3
fastavro==1.4.2
fasteners==0.16.3
flatbuffers==1.12
flax==0.3.4
future==0.18.2
gast==0.3.3
google-api-core==1.30.0
google-api-python-client==1.12.8
google-apitools==0.5.31
google-auth==1.32.1
google-auth-httplib2==0.1.0
google-auth-oauthlib==0.4.4
google-cloud-aiplatform==0.7.1
google-cloud-bigquery==2.20.0
google-cloud-bigtable==1.7.0
google-cloud-core==1.7.1
google-cloud-datastore==1.15.3
google-cloud-dlp==1.0.0
google-cloud-language==1.3.0
google-cloud-pubsub==1.7.0
google-cloud-spanner==1.19.1
google-cloud-storage==1.40.0
google-cloud-videointelligence==1.16.1
google-cloud-vision==1.0.0
google-crc32c==1.1.2
google-pasta==0.2.0
google-resumable-media==1.3.1
googleapis-common-protos==1.53.0
grpc-google-iam-v1==0.12.3
grpcio==1.32.0
grpcio-gcp==0.2.2
h5py==2.10.0
hdfs==2.6.0
httplib2==0.19.1
idna==2.10
importlib-metadata==3.10.1
importlib-resources==5.2.0
ipykernel==6.0.1
ipython==7.25.0
ipython-genutils==0.2.0
ipywidgets==7.6.3
jax==0.2.16
jaxlib==0.1.68
jedi==0.18.0
Jinja2==2.11.3
joblib==0.14.1
jsonschema==3.2.0
jupyter-client==6.1.12
jupyter-core==4.7.1
jupyterlab-pygments==0.1.2
jupyterlab-widgets==1.0.0
Keras-Preprocessing==1.1.2
keras-tuner==1.0.1
kiwisolver==1.3.1
kubernetes==11.0.0
Markdown==3.3.4
MarkupSafe==2.0.1
matplotlib==3.4.2
matplotlib-inline==0.1.2
mistune==0.8.4
ml-metadata==0.30.0
ml-pipelines-sdk==0.30.1
msgpack==1.0.2
multidict==5.1.0
nbclient==0.5.3
nbconvert==6.1.0
nbformat==5.1.3
nest-asyncio==1.5.1
notebook==6.4.0
numpy==1.19.5
oauth2client==4.1.3
oauthlib==3.1.1
opt-einsum==3.3.0
optax==0.0.9
packaging==20.9
pandas==1.3.0
pandocfilters==1.4.3
parso==0.8.2
pexpect==4.8.0
pickleshare==0.7.5
Pillow==8.3.1
portpicker==1.4.0
presto-python-client==0.7.0
prometheus-client==0.11.0
promise==2.3
prompt-toolkit==3.0.19
proto-plus==1.19.0
protobuf==3.17.3
ptyprocess==0.7.0
pyarrow==2.0.0
pyasn1==0.4.8
pyasn1-modules==0.2.8
pybind11==2.6.2
pycparser==2.20
pydot==1.4.2
Pygments==2.9.0
pymongo==3.11.4
pyparsing==2.4.7
pyrsistent==0.18.0
python-dateutil==2.8.1
pytz==2021.1
PyYAML==5.4.1
pyzmq==22.1.0
requests==2.25.1
requests-oauthlib==1.3.0
rsa==4.7.2
scikit-learn==0.23.2
scipy==1.7.0
Send2Trash==1.7.1
six==1.15.0
slackclient==2.9.3
tabulate==0.8.9
tensorboard==2.5.0
tensorboard-data-server==0.6.1
tensorboard-plugin-wit==1.8.0
tensorflow==2.4.2
tensorflow-cloud==0.1.16
tensorflow-data-validation==0.30.0
tensorflow-datasets==4.3.0
tensorflow-estimator==2.4.0
tensorflow-hub==0.9.0
tensorflow-metadata==0.30.0
tensorflow-model-analysis==0.30.0
tensorflow-serving-api==2.4.1
tensorflow-text==2.4.3
tensorflow-transform==0.30.0
termcolor==1.1.0
terminado==0.10.1
terminaltables==3.1.0
testpath==0.5.0
tflite-support==0.1.0
tfx==0.30.1
tfx-bsl==0.30.0
threadpoolctl==2.1.0
toolz==0.11.1
tornado==6.1
tqdm==4.61.2
traitlets==5.0.5
typing-extensions==3.7.4.3
uritemplate==3.0.1
urllib3==1.26.6
wcwidth==0.2.5
webencodings==0.5.1
websocket-client==0.59.0
Werkzeug==2.0.1
widgetsnbextension==3.5.1
wrapt==1.12.1
yarl==1.6.3
zipp==3.5.0

Note: I get the same issue when using tensorflow==2.3.1 and tfx==0.26.3.

Describe the current behavior The download-grep-print example pipeline fails because the url placeholder dereferences None, leading to a missing URL. I'm using this example as inspiration to write my own container-based component, and I'm having similar issues.

Describe the expected behavior The url placeholder should dereference to a string, e.g. "https://raw.githubusercontent.com/karpathy/char-rnn/370cbcd/data/tinyshakespeare/input.txt" when running the pipeline defined in the example's test.

Standalone code to reproduce the issue components: https://github.com/tensorflow/tfx/blob/v0.30.1/tfx/examples/custom_components/container_components/download_grep_print_pipeline.py pipeline: https://github.com/tensorflow/tfx/blob/v0.30.1/tfx/examples/custom_components/container_components/download_grep_print_pipeline_on_beam_test.py

Install tfx[examples] or copy the components and pipeline definitions. Run the run_pipeline_on_beam() defined in the pipeline file.

Name of your Organization (Optional) Twitter

Other info / logs

...

WARNING:absl:Dereferenced None during placeholder evaluation. Ignoring.
WARNING:absl:Placeholder=key: "url"

INFO:absl:Container spec: {'image': 'google/cloud-sdk:278.0.0', 'command': ['sh', '-exc', '\n          url="$0"\n          output_data_uri="$1"/data  # TODO(b/150515270) Remove when fixed.\n          output_data_path=$(mktemp)\n\n          # Running the main code\n          wget "$0" -O "$output_data_path" || curl "$0" > "$output_data_path"\n\n          # Getting data out of the container\n          gsutil cp "$output_data_path" "$output_data_uri"\n        ', None, '/Users/codesue/tfx_root/pipelines/download-grep-print-pipelin/DownloadFromHttp/data/9'], 'args': []}
INFO:absl:Docker platform config: {}
INFO:absl:Docker: + url=

INFO:absl:Docker: + output_data_uri=/Users/codesue/tfx_root/pipelines/download-grep-print-pipelin/DownloadFromHttp/data/9/data

INFO:absl:Docker: + mktemp

INFO:absl:Docker: + output_data_path=/tmp/tmp.ApsavivuJH

INFO:absl:Docker: + wget  -O /tmp/tmp.ApsavivuJH

INFO:absl:Docker: : 7: : wget: not found

INFO:absl:Docker: + curl 

INFO:absl:Docker: curl: (3) URL using bad/illegal format or missing URL

...

RuntimeError: Container exited with error code "3" [while running 'Run[DownloadFromHttp]']

tensorflow / tfx

Container component example fails because placeholder dereferences None #4001