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I have a question about using `Parameter` in `Shel...
# prefect-community
p
I have a question about using 
Parameter
in 
ShellTask
According to the docs, 
Parameter
is a type of 
Task
, and I'm guessing it needs to get the method 
run
called on it before it's usable. I'm guessing that this method is called when you pass the parameter to a function with the 
@task
decorator? I have a shell task, which I'd like to pass an object (or a string) which is created using a 
Parameter
object. Normally Shell tasks seem to be defined in the flow block. When I try to run a shell task, in a flow, with arguments that are dependent on a parameter, it fails with something like:
Copy code
Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/macmenaminpe/code/prefect/pdb_flow/parameterized_flow.py", line 45, in <module>
    num_lines = s_task(command=f"wc -l {job.pdb_fp} > {job.job_dir}num_lines.txt")
  File "/home/macmenaminpe/.local/lib/python3.8/site-packages/prefect/tasks/core/function.py", line 68, in __getattr__
    raise AttributeError(f"'FunctionTask' object has no attribute {k}")
AttributeError: 'FunctionTask' object has no attribute pdb_fp
So, I guess the two questions are: • how do I "get at" the input val of a Parameter in a flow (or do I not do this, and always pass that param out to a 
@task
and let it get handled there • how do I create a shellTask that takes args
c
Hi @Philip MacMenamin - you should treat Parameters like normal tasks. So for example:
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from prefect import task, Parameter

@task
def my_parametrized_task(p):
    print(p)

with Flow("example") as flow:
    param = Parameter("p", default=42)
    my_parametrized_task(param)

flow.run()
flow.run(p="new value")
For the shell task specifically, I suggest customizing it to your needs via subclassing:
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class MyTask(ShellTask):
    def run(self, custom_param):
        # do stuff with custom_param
       return super().run(**kwargs) # pass whatever kwargs you want to the parent run method
and then add this to your flow in the standard way
p
ok - so for the first question, you always pass the param to a task for it to be used. You never use it within the flow block.
And, ok, subclass out the shellTask class to call it with args. Got it. Thanks!
class MvTask(ShellTask):
def run(self, custom_param):
# do stuff with custom_param
return super().run(command = f"util_name {custom_param}")
c
yup that looks good to me
p
so if I wanted to pass a string to the shell task, I set the command this way?
ok, thanks
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