Hey is it possible to use context value as a default for Par

Question

Hey is it possible to use context value as a default for Parameter

Marwan Sarieddine · Answer

I don t think so but perhaps you can set null as the default and then try to fetch the parameter value if it is None then make use of prefect context i e ```import random from prefect import task Parameter Flow task name= Task A def task a x x = x or prefect context get x return x + 2 with Flow test as flow x = Parameter x default=None task a x ```

Zanie · Answer

Thanks for responding Marwan I think that s likely the best way to do this

Dotan Asselmann · Answer

Thank you This is great slightly smiling face

Pedro Machado · Answer

I am testing an approach subclassing the `Parameter` class I added another keyword argument that receives a callable that provides the default value if the parameter is null I was using the approach above before but since it s so common in our flows I wanted to eliminate the extra task The advantage of not having a separate task is that the Parameter value can be used in templating without having to pass it as an explicit dependency to every task that needs it You might argue that it s better to have an explicit data dependency but in my use case the parameter is a date that is used as part of the path of the stored results

Matt Drago · Answer

Snap < Pedro Machado> I just implemented `ContextDefaultParameter` and `ContextDefaultDateTimeParameter` last night to solve this issue myself I was planning on adding it to the Prefect Task Library at some point I need to add the error handling code as I just wanted to see if I could get it to work

Pedro Machado · Answer

Cool Matt How did you implement the interface

Matt Drago · Answer

```class ContextDefaultParameter Parameter def init self name str context name str tags Iterable str = None gt None super init name=name required=False tags=tags self context name = context name def run self gt Any value = super run if value is None value = prefect context self context name return value```

Matt Drago · Answer

Note I m note a native python coder there was much copy paste change going on here

Matt Drago · Answer

One thing I d want to add to this is raising an error if the context does not have the named field

Pedro Machado · Answer

Since you are accessing context with instead of `get` I should already raise an exception if the key is missing

Matt Drago · Answer

Oh yeah I forgot about the `get` function Even using `get` though i think it should raise an error if the context key could not be found

Pedro Machado · Answer

Here s what I was playing with I don t love the name ```class ParameterWithDynamicDefault Parameter def init self name str required bool = True default callable Callable JSONSerializableParameterValue = None tags Iterable str = None gt None if default callable and not callable default callable raise ValueError f default callable must be a callable Got type default callable instead self default callable = default callable required = False if self default callable else required super init name required=required tags=tags def run self gt Any value = super run if value is None value = self default callable return value```

Matt Drago · Answer

Names are tough wink

Matt Drago · Answer

That s a different angle than I was thinking The way that I read this functionality is like I ll use a value that the flow is called with but if it s not supplied I ll run this function to calculate a value

Pedro Machado · Answer

Correct I am using the scheduled start time from the context to compute a relative start date if one is not provided

Matt Drago · Answer

I m doing a similar use case use the scheduled start time if the date to use is not supplied

Matt Drago · Answer

This is how I use it `extract date = ContextDefaultDateTimeParameter name= extract date context name= scheduled start time `

Pedro Machado · Answer

Got it I have to use some complex date offset logic and I wanted that encapsulated it in the `Parameter` so I could use it in `target` or `location` templating

Matt Drago · Answer

Nice I can see use cases for both approaches